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How do they work?  Theory and practice.
Spark plug threads, repairs, Helicoils.
Tightening torques. Gaps.
Antiseize compound ......or not?
Spark plug caps.
Resistor & non-resistor spark plugs.
Heat-Range chart; equivalent numbers.
Copyright, 2013, R. Fleischer

How do spark plugs work?

You could search on the internet for something like 'how do spark plugs work', and 'spark plug heat ranges' if you wanted to.

Spark plugs were invented a very long time ago. Most historian agree that the invention was in 1839; but that its first use in a gas engine dated to 1860. The basic design has not changed much since then.

A rather complete article is here:

The basic idea is that a spark plug, with its insulation and two metal places for a spark to jump between them, ignites the air-fuel mixture coming from the carburetor (on Airheads), after the mixture is compressed by the piston movement. The actual ignition of the mixture is by something called ionization, but I'm going to avoid getting into that nerdy subject here; although if you are curious about the subject, you might be interested in how it may be used in some modern cars and motorcycles, see near the end of this article:

A spark plug consists of a sort-of cylindrical body of metal with a hole through it, that has a ceramic material as a high voltage capable insulator in the middle, and a metal rod goes through that ceramic material. Electricity from the ignition coil is applied to one end of the metal rod, and the electricity has such a high voltage that even under the pressure of the gas-fuel mixture in the combustion chamber, the spark will jump from the rod (the central electrode) to a projection from the bottom of the spark plug metal body (called the ground electrode).

The white central insulator must be hot enough to allow such as oil and other things to burn-off during normal engine operation. If the ceramic insulator does not get hot enough, the insulator will accumulate contaminates which will carbonize and eventually conduct electricity and the spark plug will be shorted (sometimes called 'fouling') and not deliver a proper spark whose purpose is to ignite the fuel-air mixture. In the days of leaded gasoline, the lead would slowly 'poison' the insulator, and that is why spark plugs did not last as long as they do today. Many a modification to the spark plug design, and to ignition circuits, were made to try to overcome the fouling problem and/or improve the mixture ignition. Higher and higher powered ignitions were one of the methods used for improving mixture ignition in more powerful engines, particularly as compression ratios increased more and more.

The temperature of the burning gas varies with throttle position, fuel richness or leanness, design of the camshaft that operates the valves, the compression ratio, and other things, including even the shape of the combustion chamber. For each engine design, there is an optimum spark plug 'heat range'. The heat range means that over a certain range of temperatures, the spark plug will work properly, and not fail due to accumulating electrically conductive deposits; nor, fail through being so hot that the insulator begins to break-down.

Manufacturers of spark plugs use their own heat range numbers, & charts of equivalents, or near equivalents, are easy to find; a pertinent one is in this article, considerably further down.  In some numbering systems the spark plug number is more informative than in others, although in all the numbering systems in use the number is not any absolute number, relating to a specific temperature.  For an example of a system of somewhat more informative numbering, The BOSCH W5 is a spark plug that, itself, runs cooler, than a Bosch W6, or W7, in the same engine. The reason a cooler spark plug may be used is because the gas temperature is likely higher, but there can be many factors possible, including the type of metal in the cylinder head (in our case, an aluminum alloy), where in the head the spark plug is located, in our situation, a swirling motion of the incoming gases strikes the spark plug in a certain way, etc. Lots of ETC. Even the torque to which the spark plug is tightened will have an effect on heat transfer.  The main control over the spark plug heat range specification is the length of the insulator inside the spark plug. The longer the insulator, the more the tip end runs hotter, for a variety of reasons.

For any given spark plug, there are spark plugs by other manufacturer's that work almost identically. Some may cover a slightly different middle value of temperatures; or, a slightly different and wider or narrower effective heat range (even when charts say they are equivalent-to-xxxxxx). Generally, the charts are more than adequate enough. Popular equivalents are such as the NGK brand, and there are numerous others.

In general, the earlier Airheads had a higher effective compression ratio, and the mixture tended to burn a bit hotter due to the CR and to the jetting, etc. The time it took to burn the mixture is also a factor, but that is way beyond purposes of this article. Thus, for the earlier engines, a colder rated spark plug was required. Re-stating: Spark plug heat range values needed to be hotter for the later Airheads; thus, instead of W5, the engine may require W6 or W7. Another way to think about this is that the earlier Airheads tended to have more heat in the combustion chamber, so a colder spark plug was required.

If you change the jetting in your carburetor, that will affect the spark plug. Sometimes on this LIST you will see a recommendation for a slight change in needle clip position, or size of a jet, which improve engine performance. Usually the spark plug already specified will be OK.

BMW specified a certain heat range for the spark plug in your engine. The recommendation will be accurate for normal use of the motorcycle. Serious problems can occur if radical changes in spark plugs and/or jetting are made.

The above photo is of three types of 'extended nose' spark plugs, 3/4" reach, 14 mm threads.  Extended nose means that the center white insulator extends beyond the end of the threads. The left-most spark plug is appropriate for Airheads; it has a single ground electrode and a standard sized center electrode and has the mentioned extended nose. The middle spark plug, Bosch XR7LDC, has two ground electrodes, & can be used in Classic K bikes. Classic K bikes can also use X5D, X5DC, XR5DC.  There must be resistor spark plug caps/wires to safely use non-resistor spark plugs in a K bike.  The XLR7DC spark plug with dual grounding electrodes, introduced during the K1200 production, works well with prior K bikes.  The middle & right-most spark plug has even more ground electrodes & versions can have standard or a fine wire (much less diameter) center electrodes.  Those fine wire spark plugs have no use in Airheads & not in Classic K bikes if the spark plugs have more than 2 grounding electrodes.   The particular combustion chamber designs tend to shroud the spark plug with the fuel-air mixture, hence these statements.  Failure to heed this advice can result in occasional misfiring, even if not felt (but can reduce fuel mileage), or worse.

NOTE:  Do not try to re-gap the XR7LDC spark plug, even when the spark plug is brand-new, as it is a special plug, and the gaps are preset, and would be difficult to change anyway.

NOTE:  You can re-gap single ground electrode spark plugs when brand new, and never run, onlyBMW says to not re-gap spark plugs.  BMW did not explain that advice; and, BMW should have added this:  ".....after they have been in use".  That is what BMW means & I am in total agreement.     After any spark plug has been "in use" the metal structure of the bendable ground electrode changes.   If you then try to re-gap the spark plug you weaken the ground electrode.  In quite rare instances they have broken off, causing engine damage.   In some instances the insulator has been damaged during re-gapping, and that can create sharp hot spots which can cause serious combustion problems.   Re-gapping was very commonly done on old cars, and bikes.  Do not do it, on the rare chance of a ground electrode breakage or damaging the insulator tip. "In use" here means that the engine has been run.   This is especially so after the engine has been run under load.

As miles accumulate, the electrodes gap increases.  How fast this happens is somewhat affected by the engine running leaner than richer. Mostly, the change happens due to the spark causing electrode erosion.  It happens less if there is a resistor spark plug or a resistor in the spark plug cap.  Use of higher power levels and some types of gasoline's also accelerates gap wear.  Gap wear is usually acceptable until the gap becomes very excessive, at which time marginal ignition systems or engines running very lean or very rich might begin to misfire, leading to poor fuel mileage.  Fine wire tipped precious metal (center electrode) spark plugs are less likely to have this problem with advancing miles & the spark will find it easier on them to jump to the ground electrode(s), increasing the life a bit more, but fine wire spark plugs may not work all that well in some engines.  I am OK with you trying them, but only if they have single ground electrodes, if used on Airheads.

The mileage it takes to get to a point where you notice problems will vary considerably with a wide range of things affecting, and affected by, the spark plug gap wear, such as spark plug heat range, caps resistance, actual mixture burning conditions, oil usage, type of ignition system, whether resistor spark plugs or not, etc.   Spark plugs last much longer now, at a minimum doubled, due to the change to unleaded fuels.  In the past, leaded fuels would eventually cause electrical shorts/leakage problems at the center insulator ...and it was common to replace spark plugs at 10K-20K miles; or, to clean them by sand-blasting, & replace them at the next service interval.   Do not expect spark plugs on your BMW Airhead motorcycle to last as long as spark plugs on modern fuel injected cars.

For the Airheads, worn gaps approaching even .040" are often still usable ...but spark plugs should be replaced sooner, especially if the grounding electrode gets quite thin (usually a dished thinness shape), as they can weaken.   Decades ago we Wrenches would file the ground electrode, sand-blast the plugs, set gaps, etc.    These days, the gap can be checked, but do not adjust it.  Check plug base color; check central insulator color and appearance. Replace the spark plug at reasonable intervals.

Spark plugs screw into cylinder head threaded holes.  These male & female threads are exposed to combustion heat/pressure & also to byproducts and additives of gasoline & oil.  These things tend to carbonize, & can accumulate in the threads.  Combustion is not the same in all Airheads, some have a fair amount of burnt oil or carbonized oil, & other combustion products accumulating in the threads.  Worn threads tend to accumulate more hard carbon. The carbon can act like an abrasive.  It also can act like a cement!

If reinstalling a used spark plug (never re-gap one) I like to clean the threads if the spark plug requires more than finger pressure to screw in (not including final torqueing).  If the threads in the head have the look of excessive deposits I may go so far as to rotate the engine to open the exhaust valve, & blow compressed air into the intake area as I clean the threads (wearing protective goggles).  A greased tap can work, but then you have to degrease the threads and I dislike that taps will tend to cut head metal, not just the carbon. I mostly use brass gun cleaning brushes, rotating them during cleaning.  I've seen brand-new spark plug threads that were not made all that nicely. Poor spark plug threads will damage the head threads.  Poor spark plug threads have been identified on spark plugs made in various countries.  Poor spark plug threads can damage the cylinder head threads.  A product called Color-Tune is reported to damage threads.

Aluminum cylinder heads can be damaged easily if one is not careful.  There is a limit to abuse from over-tightening a spark plug, or removing & replacing a spark plug; this is even moreso when the cylinder head spark plug threads are full of combustion byproducts & carbon.  Carbon (from such as oil burning, or, the very bad use of oil or grease on the threads) can get very hard, & very abrasive.  It can cause the spark plugs to injure the cylinder head threads during removal. It is not a good idea to put much torque on a spark plug when attempting to remove it and it is found quite tight; in that case first try heating the cylinder head, as the metal expansion may be enough to avoid damage.  Running the engine is a good way to do that.

Some BMW motorcycles prior to the /5 series, I will just call them /2 bikes, had some metal alloy & casting problems, including soft alloys & voids.  Even with very careful torqueing with the best antiseize compounds of the day, we still saw occasional problems.

I've seen poor metals & poor castings even bring about failure of factory installed 'cast-in-place' steel threads inserts, & heat-cool cycling on those early inserts caused problems.  For our /5 & later Airheads, the metal in the heads, & the castings, are very considerably better.

Many folks refuse to use torque wrenches, & some who do or do not, still manage to over-torque spark plugs.  I have seen someone over-torque a spark plug & he was using a torque wrench & it was set properly.  His problem was not realizing that many clicker torque wrenches tend to be hard to detect the click if the setting is fairly low on the wrench's total capacity. This tends to be especially so on such as 100+ maximum foot-pound wrenches which might have a rather subtle click when set low, say around 12 foot-pounds to 18 foot-pounds.   Know your wrench.  If you don't know what the click feels like at various settings, and/or are not all that familiar with torque wrenches, especially yours, then put the square end into your bench vise, use soft jaws or protective metal, & test the feel required for various torque settings, that is, feel & listen for that click. Do this in the tightening direction.  Cheap" beam-type torque wrenches (not meaning the dial indicator commercial types) may be very inaccurate ...& should not be used for critical items.  Same caution for cheap tubular clickers.  You can check the calibration close enough, yourself, see my article on torque wrenches.

Abused, the cylinder head spark plug hole threads will eventually start distorting & pulling out if they don't immediately rip out. While Helicoils or welding & re-machining, all do work OK, & can even be superior when done correctly, such repairs are usually avoidable if one takes some extra care in the first place.

I don't like to see extra places for sharp bits of carbon combustion products to adhere to.  They create 'hot red glowing places' in the cylinder heads from using inserts & can cause pre-ignition problems.

I recommend against Time-serts, which can change the spark plug heat value.

I have installed Helicoils, & also have welded up the spark plug thread areas & drilled & re-tapped them ...of course the heads have to come off for that to remove the head in any fix anyway ....although one can use the official Helicoil Spark Plug Thread repair kit, with well-greased parts, & with the exhaust valves open so you can blow chips out with an air hose.   You can do a good job without removing the head, in most instances.

It is critical that drilling & tapping be done squarely. >>>>Absolutely and always use a drill guide & tap guide! MORE information on installing Helicoils, head on, or head off the engine, is later in this article.

DO NOT USE GRAPHITE PRODUCTS FOR COATING SPARK PLUG THREADS.  A corrosive type of reaction is possible on the aluminum head.

Anti-seize compounds (never use graphite products on steel going into aluminum):

The use of antiseize compound at spark plug threads is somewhat controversial, even among some experienced Airhead mechanics ...mostly I think this is due to some spark plug manufacturer's literature; & perhaps some is due to misconceptions. Some is due to faulty belief in old-wives tales, ...or, possibly, bad experiences from those ham-fisted about torquing things. Yes, some professional wrenches are concerned about you, a non-pro, abusing things, & it is true that use of antiseize could (not will) make it somewhat easier for abuse.

Some manufacturer's, trying to play it safer (?) have, over many years, changed from recommending anti-seize, to not recommending it, or just ignoring the use.   An argument can be made for not using anti-seize, as someone not knowing the stuff was applied, might then not compensate by reducing the required torque.   I just cannot see any other argument against anti-seize use that is valid ...and I can provide a lot of reasons to use the stuff.

Karle Seyfert, an expert who writes for MOTOR, an automotive repair industry publication, had an extensive article in MOTOR in March 2013.   That article, & some controversy & comments by one well-known 'guru' in the BMW motorcycle repair business, at approximately that time on an Internet mailing list, prompted me to add more commentary to this part of the article you are reading.  I have not changed my own practice, intent, nor recommendations; but I will give you more information on what automobile manufacturer's are publishing/recommending! ...and some of "why".

Some experienced mechanics believe that if antiseize is used it is 'easier' to over-torque & damage threads.  That is probably true, but it is true only if the person is not adjusting torque lower, to compensate for the use of anti-seize compound; or, just being more ham-fisted. Another reason is that many engine makers have the torque specifications on the high side of what they should really be, particularly for aluminum cylinder heads.  Sometimes gross errors in publications from long long ago were carried over, and never checked.

It is my strong belief that you should be using a torque wrench, and, lowering the official torque value (which is without antiseize compound unless specifically so-stated).   I believe the factory torque settings (some, anyway) are too high ...even if anti-seize compound is not used, or never has been, on the engine.

Since popular anti-seize compounds act like a lubricant, the applied torque (if the book value means no antiseize) must be reduced ...about 15% reduction, is typically ~correct for spark plugs into aluminum, on Airheads.  I believe the oft-quoted 'up to 1/3; or, up to 30%, is too great a reduction.   Note that these percentages are for original published values of 16-18 ftlbs, to be reduced by 15%.

Some types of spark plugs, such as taper seat types (not used on Airheads!), are not supposed to be used with regular anti-seize compounds, as they are degree-tightened & so the manufacturer typically makes a blanket statement not to use anti-seize compounds, applying to all their spark plugs, rather than providing full information which can be more complicated.  Other manufacturers say OK, & give the needed lower torque values.  Some manufacturer's do not mention use of anti-seize at all, & some say you must use it!

Some manufacturer's say they are concerned with heat conduction; that the heat range of the spark plug is or could be changed by the use of anti-seize compound. That is possible, particularly if the torque used is not correct, but with torque being reasonable, at worst case the effect is quite small, otherwise none, & even that would very seldom happen, and would have no effect on your BMW engine ...&  I believe any such concerns should be ignored. What those manufacturer's do not tell you is that their biggest concerns are for considerably loose spark plugs.  I have not seen any problem in real life situations if the torque is set even close to proper. I tested for this, with various torques & test runs on a bike, reading the spark plugs, and it took a substantial amount of torque decrease to increase the spark plug heating, and for overtorquing there was no change at all.

I have used some sort of anti-seize compound on my own various bikes' spark plugs. I have over 650,000 miles on just my own BMW's.  I used it on my customer's bikes, and on other engines besides BMW.   No problems, ever!

I am in favor of using anti-seize compound on spark plug threads.  I believe that if official torque values are reduced (assuming originally specified without anti-seize compound), & you use the anti-seize compound, you are less likely to have pulled thread problems, heat range will be more consistent, & there will be less problems with hard particles of carbon that deposit themselves into the cylinder head threads, which, otherwise, can cause excessive wear every time you R/R a spark plug.

Antiseize compounds vary in characteristics, but I have found that all commonly available ones that I have tested, which includes the silvery colored Permatex anti-seize commonly found at autoparts stores, are OK at the spark plugs. Caution:  Using oil or WD40 will tend to eventually result in hard carbon deposits in the threads, & thus tend to wear the threads & tend to change the effective torque.  Do not use oil or WD40!

What do the manufacturer's say; what are some problems seen, etc. :

Some manufacturer's recommend "penetrating oil" for tight plugs during removal.   Most fail to tell you to very thoroughly remove that oil before installing a new spark plug.  It is also not easy to get penetrating oil into the spark plug threads with the head on the engine ...but if you can safely unscrew the spark plug some, then you can use a thin oil with the head in place.  A mixture of acetone & automatic transmission oil, ~50-50 works better than all penetrating oils many of us have tested!

Once anti-seize compound is ever used, it tends to work its way into the aluminum head metal.   From that point on, it is best to always use anti-seize compound. Do not use graphite types as they can cause corrosion problems.

Some manuals, such as later Clymers, etc., may tell you to put a dab of anti-seize compound (often specifying aluminum type) on the spark plug threads before installation; but, these books may forget to tell you to reduce the torque.

Some spark plugs seem to have a bit less diameter or other irregularities with the threads that can promote carbon or wear.   Thread & quality control problems have been reported on Champion & other spark plugs.  It has been reported even on Bosch plugs, if not made in Germany.  Overall, German Bosch tolerances seem to be best.  NGK makes a good product & I have never seen a problem with NGK plug manufacturing.   Bosch may plate the threads, which helps avoid use of anti-seize, if you are so inclined.  NGK plates some spark plug bodies, & on some threads they put a coating or special treatment; see below.  I still use anti-seize on all spark plugs with standard non-tapered seats. 

****The latest NGK literature, specifically mentions using anti-seize compound, in that it should be used on their non-plated plugs.  As I noted above, I use it on all spark plugs for our BMW motorcycle engines.

If you were to access the factory repair manuals for many bikes & cars, you would find that they are not consistent with use of anti-seize compound at the spark plugs.  Some say you must, some don't say anything, some say you must not.  I've already mentioned some of this, & regarding taper seat plugs, etc.  Now, we get into this more deeply.

2010 Honda Pilot:  Honda says to apply antiseize compound.

2007 Subaru Legacy:  gives a torque rating 'without oil on the spark plugs, new plugs'.  If lubricated (does not specify type of lubricant) they say to reduce torque by 'approxmately 1/3 of that specified' to 'avoid overstressing'.

Chevy Cobalt says not to use antiseize ....because if you do, you will damage the cylinder head if you overtorque. Chevrolet does not mention that overtorquing anyplace is likely to cause damage.

Service information for many vehicles not mentioned here were checked.  BMW cars, Cadillac, Dodge, Ford, Hyundai, Mazda, Mercedes-Benz, Porsche, Toyota, VW, Volvo.  All these gave specific information on torque, but none mentioned anti-seize.  I could, obviously, not check for every model, every year ...but, I looked at numerous relatively late model vehicles, & while I did look at older cast iron head engine vehicles, I concentrated on aluminum heads.  BMW, motorcycle division, is surprisingly quiet.

NGK, the spark plug manufacturer, gives the strongest advice against using anti-seize ...but, in a way, they also at the same time give very strong advice about why you should use anti-seize least on their spark plugs.    They have an entire technical bulletin on the subject.  Condensing that bulletin for you, ... they do not want the compound used on plated plugs (in one place they say all their plugs are like that now ...not true, but good enough), & then they say not to use anti-seize compound on initial installation.   Then they stop talking about it, & confusingly say that all their plugs are coated with 'special trivalent zinc-chromate shell plating' which is designed to prevent both corrosion & seizure, eliminating need for compounds or lubricants.  They do not separate threads from the body in the discussion, but it can be implied from the prior sentence.

You must read what NGK says & how, very carefully ....because apparently, NGK does not want anyone to remove a spark plug unless a new one is then installed. Yes, no reusing of a spark plug even if only removed for inspection!

NGK makes it difficult to get the information, but here is their information, in my words.  Read this very carefully:  If you insist on re-installing an NGK spark plug, even if never actually having run the engine, you must use anti-seize compound. ... Because:  Once you unscrew the plug, the NGK super-thin 'plating' coating is, in essence, partially removed permanently, each time removes more, & the plug can then have galling & other problems.   NGK says nothing directly about what happens to that plating coating, which is another story entirely.  It could be damage-causing, or; well, anything.  There is simply no further information.    It is my belief that installing and removing the spark plug simply does, by friction in the threads, remove much of the super-thin-"plated" coating that NGK applied (at least to some plugs).

Autolite (another major supplier) spark plug sales information says its plugs are nickel-plated, & says anti-seize compounds 'can have a torque multiplying effect...'....but then says nothing else.   However, engineering books from Autolite say things that contradict its other books' information, & says you must use a thin-film coating of high-temperature nickel anti-seize ...and specifies, specifically, certain plugs & problems that occur when this is not done particular the long reach HT plugs (used on Ford 3 valve modular engines, etc...that have well-known plug removal problems).

There is considerably more to all the above things about anti-seize compound; which I abbreviated, so as to not bore you too much.

My advice:  Do use anti-seize compound, and use my recommended reduced torque values. Look at the proper book information.  BMW has had various torques, for the same 3/4 reach 14 mm plugs.  More later, herein.

I am fine with you using nickel type or copper type of anti-seize compounds.  Most anti-seize compounds contain one or the other or both, & do not specify.   They likely have copper (even if silvery in color), but don't say (which contradicts what a certain foul-mouthed maker of videos for BMW bike repairs says in his videos ....about a must to use a certain copper product).

Do not use graphite products, including graphite-based antiseize compounds,  for coating spark plug threads.   A corrosive type of reaction is possible on the aluminum head.

Amount of torque to use when installing spark plugs:

1.  You can, but should not, tighten spark plugs by hand without using a torque wrench, even after you have a good feel for it.   I recommend you always use a torque wrench!   I triply recommend it for bottom 1/2" reach spark plugs on dual-plugged modified Airheads!

2.  I've seen various forces needed to properly seat spark plug crush rings, because crush rings vary in how they are made and some are not easily removable from the spark plugs, so folks don't remove and replace them.  Contrary to many books and some advice by others, I am OK with not replacing the semi-captive type crush rings, if they are in good condition, & if you use a torque wrench!  Many books say 1/2 turn on new crush rings after the spark plug just barely is seated, amounts to proper seating to the head & crush ring; & they may also say 1/4 or 1/3 turn on used crush rings.  I say, do not depend on any of that!  I suggest you do not do it that way on your Airhead nor Classic K.  In fact, I recommend against those practices for any modern engine unless specified by the factory.

3.  I suggest, again, strongly, that you use a torque wrench.  Keep in mind that not only do you not want threads pulled, but you also want to maintain the rated spark plug heat value specification; and, proper torque for other reasons.

4. The torque values I supply are generally safe values, with new or used crush washers/rings ...when used with a drop of anti-seize compound spread thinly onto the clean spark plug threads (fingertip, or you can push the stuff very thinly into the threads using an old toothbrush). You need not cover every thread.  Do not get anti-seize compound onto the end of the spark plug area.

3/4" reach, 14 mm, used as top spark plugs, about 14-15 ftlbs.  This is the stock top spark plug size for Airheads.

1/2" reach, 14 mm, about 12-14 ft lbs.  This is a commonly used bottom spark plug size on dual-plug conversions.  This torque value is usually but not always enough to fully seat the washer and spark plug.   You don't want the plug too loose ...if it loosens and rattles out, that pulls/wears threads.   Some have installed smaller thread diameter spark plugs for the lower plugs.  I've had good results with 8-10 ftlbs for those.  Some manuals will show 8-12 ftlbs. in aluminum.

Dual Plugging information:
This paragraph assumes you are using anti-seize on the spark plug threads. Some have installed, for the lower spark plug in dual-plugged installations, a custom made thick aluminum spacer/washer that is  welded to the head.  Some just use such a spacer without welding.  The washer may be threaded if welded, or may not be, if just used as a spacer.  Some may have welded up the actual head material to be thicker, then drilled and threaded it.  The reason for these various things is that the lower head area does not have enough aluminum thickness to use the 3/4" reach type of spark plug (3/4" reach is used, stock, on the top spark plugs) without doing something; and, the spark plug is too long anyway.  The described methods are only done at the lower spark plug threaded hole.  The result is that the top & bottom spark plugs can both be 3/4" reach.  If you did not do something like these things, then your only choice is a 1/2" reach spark plug.   If you have installed a welded or not welded washer (~1/4" long/thick), unthreaded, and are using a 3/4" reach spark plug, then use torque value of 12-14 ftlbs.  If you have welded up the area, by adding material, or welded a ~1/4" washer to the area, and the threading covers the entire hole/washer/weld, then use my recommended 14-15 ftlbs.

5.  One is supposed to, in most considerably older literature, use a brand-new crush washer each time a plug is removed & replaced, but we all know that most of you won't, & many plugs have the washers fairly well captive & finding new crush washers is often frustrating.  Just be careful about torque.  Many years ago it was common for mechanics to install a fresh washer each time, and tighten with a common tubular wrench, not a torque wrench, and tighten until the crush washer was felt to crush & seat; & then increase a certain number of degrees; or by feel.  That was done on cast iron heads.  This can still be done on aluminum heads if you have a decent feel for it, but it is more dangerous to do it on aluminum heads.  Do use a small amount of antiseize, torque wrench, & proper torque.

6.  What about installing clean and dry (No anti-seize compound)??  With no anti-seize 'ever' used on your heads, or exceptionally clean threads, you can torque to 16+ footpounds (do not exceed 18) on the top plugs.  If you have the 1/2" reach 14 mm bottom plugs, you can go towards 13-14.

18 ftlbs maximum for clean & dry threads is my personal limit for 3/4" reach plugs, which are the standard top plugs for Airheads.  Clean & dry here, means no antiseize; or, none visible with a careful look-see.

Here is information from the official BMW Service Information sheet for the late Airheads; this is the torque value to set per BMW:
18.4 ftlbs (3/4" reach standard spark plugs)
.  Don't use that high a figure for threads with anti-seize compound.

The latest Bosch manuals, for 14 mm threads spark plugs in both aluminum & cast iron heads, recommend 7-15 ft lbs. That is not oiled nor using antiseize compound.   Unfortunately, no further information about the thread length is given.    MY opinion is that 7 is much too low.  Best you follow my advice given earlier.

Quite some years ago it was common was to see 14 mm spark plugs specified in plug manuals at 26-30 ftlbs (!!!) in cast iron; 18-22 ftlbs in aluminum. No allowance for short or longer thread types was usually shown. Do not use such values! ...You will ruin the threads in the head!

Removing spark plugs.
Repairing spark plug hole threads:

It is not uncommon to see damaged spark plug threaded holes in cylinder heads.   This comes from excessive torque, dirty carboned threads, etc.    If the threads are in need of repair, you may ....or may not able to 'reform' them, without installing a Helicoil.    Some experience & inspection with #1 eyeball, should tell you yes or no.

If you find a spark plugs that, upon some loosening, seems to tighten up appreciably, STOP!!

Avoid thread damage!!  Use a lubricating penetrant.   A homemade 50-50 mixture of ATF in acetone really is better than commercial penetrating oil products!   Apply the mixture several times at the junction of spark plug and head.  Allow to soak, even overnight if need be; & then remove the spark plug carefully to avoid damaging the threads.  This means, sometimes, using a tighten-loosen-tighten-loosen approach using more penetrant.   You will be initially using the penetrant without removing the head, but be careful, do not damage the threads by using too much removal torque;...take your time. Snowbum tries, once, to heat up the head (running the engine is best for this), and then tries spark plug removal with additional penetrant mixture. NO excessive loosening torque, no matter if hot, or oiled, or anything.  If the spark plug will not remove without damaging the threads, then it is your choice whether to remove the spark plug head on, or head off.    If the head is off the engine, Snowbum heats it to a decent sizzle temperature in his oven (that means around 212 to 230F).

How to clean up, prepare, and repair damaged threads, with the head being left in place on the engine:
(if just carboned, try using hand-rotary motion on a shotgun cleaning brush in both directions)

To prevent any chips from getting into the cylinder, there are various ways.  These include using a lot of grease on the tap (my method); filling the cylinder with shaving cream (messy ...& must be cleaned out later); using a vacuum cleaner in reverse to pressurize the cylinder from the intake with the exhaust valve open). ...or, a combination of things.

If you are installing a Helicoil with the head still on the cylinder, you may want to use the grease-the-tap method & the air pressure method, both at the same time & wear eye protection. This usually works well.  Note that it is critical that the threads be 100% degreased before installing the Helicoil, & the Helicoil may be locked in place by using Loctite red during its installation (do not forget to have the Helicoil below the surfaces!).   Allow the quite strong red Loctite to fully cure for a couple of days, then clean the threads thoroughly with a strong solvent, such as acetone or MEK, and a brush, and again, before installing the spark plug. Treat the helicoil and spark plug threads with a sparse amount of anti-seize compound.  Do not forget these steps of cleaning off the excess uncured Loctite! ...You do not want to Loctite your spark plugs into the Helicoil!  The heads get very hot & Loctite only gives a small amount of holding power when the head is hot, but it does help.  You should use a grade of Loctite specified for use at high temperatures.  For the latest part numbers, see:   In an emergency situation, or if you don't want to wait a day or more for the full Loctite cure, ....just clean the threads & install the Helicoil, & ride away.  I recommend the full cure.

It is very important to have a guide machined or purchased to allow exactly 90 drilling & a guide machined or purchased for the tap.  Guides ensure, if you use things properly, that your drilled hole & threads are dead square to the surface.  Tap handles often have a centering recess in one end, helpful with some jobs that can lay flat.  If the heads are off the bike you can use that handle feature in your drill press, with the head carefully positioned and clamp to the drill press table.  Done correctly, you do not need the guides if done on a drill press.

Here is a method to try if the threads are not too bad:
((I have also used this method when having to drill, thread, & Helicoiling, or, just reforming original threads))

Be sure the piston is not too near being fully outwards. When doing threads or Helicoil work, be sure to have the piston down from TDC (down from OT mark on flywheel), enough so that drills, taps, etc., will not strike the piston.  Do not go too far with this, or there will be lots more volume which takes too much air movement to blow out the chips.

Take a small piece of cloth, & roll it up to fit moderately tightly in the spark plug hole.  Soak in a slow evaporating penetrant, let sit overnight.  The next day, use a shotgun cleaning brass brush with a solvent to clean the threads; rotating both right and left and then left to back out the cloth or brush, carrying the debris.  Then use a good fresh sharp 14 mm tap that is quite well heavily greased. You can use white lead grease too, even anti-seize compound.  Be generous with the substance, you want it to also capture metal particles.  Keep the tap quite square to the surface, using a guide if needed.  Back the tap out often, cleaning and reforming a few threads at a time.   In many instances the tap will reform the threads, rather than cut them (it may do both).   I then thoroughly clean the threads, leaving the threads clean & dry.  Then I put a dab of anti-seize ...not much ... on the first 4 threads of the spark plug, measured from the spark plug electrodes end, before installing the new spark plug.  I then torque properly. Do not spread a lot of anti-seize onto plug threads.  You want to totally avoid getting any on the ceramic tip area, where it will electrically short-circuit the spark plug.

NOTE! ....If using the air flow pressure method to the intake port to blow out any chips, be sure to wear eye protection ...any nasty sharp chips could be flying out the spark plug hole. For that method, the exhaust valve should be open, so debris comes out the exhaust system.

Resistor caps:

I do NOT recommend resistor type spark plugs, from any manufacturer, for BMW Airhead motorcycles.  While 'resistor' plugs or resistor caps or resistor wire, reduce the current, thereby reducing Radio Frequency Interference (RFI), the use of resistor spark plugs in Airheads is not a good idea.

Some types of RFI can affect other things, such as electronic ignitions & electronic tachometers, sometimes in strange ways. I do not recommend zero ohms caps. 1000 ohm caps should not be used in anything but points ignition in our Airhead motorcycles. I do not recommend resistance wire.   In Airheads, resistance in the plug caps has several functions are a few ...with more following this paragraph.
(1) reduce the erosion (wear) of the electrodes in the spark plugs. 
(2) RFI to surrounding vehicles.   
(3) Protect against RFI being conducted into wiring & thereby getting to the Hall element in the canister on 1981+ models can destroy that part.
(4) Interference with the tachometer.  You will find the same cautions for various reasons with such as the Boyer Microdigital ignition, etc.

It is possible that added resistance can slightly increase the length of time the spark ionization effect lasts, allowing better igniting of the fuel-air mixture, up to a point, & then the energy level during part of the striking arc falls off, & performance suffers.  There are also arguments against that theory of spark (plasma) duration; and, if any, the current reduction may be said to offset it.  For all this theory one gets into electrical spark characteristics in some esoteric ways, so I won't go further with that here.

One discussion deals with resistors upsetting the relationship between specified coil secondary resistance and other coil characteristics, and the spark. I will mostly avoid that discussion.

There can be real problems with spark plugs that contain "resistors".  Some of these resistors are really not pure resistors, but are low resistance coils, acting like an electronic part called a choke (real term:  inductor).  It is possible they can damage the 1981+ ignitions for complex reasons.  Re-said, some 'resistor' spark plugs are not true resistor plugs ...they have a small coil of wire which reduces RFI by acting as an electronic 'choke' (inductor), and may be so wound to incorporate a capacitance effect as well.    The electronic ignition (and even the electronic tachometer) may not like this.   That same sort of thing was done with spiral-wrapped-metal-core ignition wire ....sold as Suppression Wire ...where it was not really resistance suppression wire. You can measure the plug, cap, or wires if curious ....with an ohmmeter.

A real resistor is incorporated into the stock or NGK aftermarket spark plug resistor-type caps; at least on the ones I have tested or torn apart.

Spark plug cap nominal values are 1000 ohms on early two-coil model Airheads prior to 1979 that had points ignition.  Later points bikes used 5000 ohm caps, but 1000 is OK to use for any points model Airhead.  BMW has shipped 5000 ohm caps on some motorcycles before 1979.  A change on only points models, from 5000 back to 1000 ohms might give some extra ignition performance in the highest rpm area ...near redline+- .....& possibly help in starting in cold weather & in other marginal situations, including old oil burning engines.    Use only the nominal value 5000 ohm caps on the electronics ignition models (1981+). The spark plug cap with its resistor does everything the pure resistor spark plugs do ....without the danger of injuring the 1981+ ignition by forgetting to use the resistor plugs when you decided to use non-resistor caps.  Don't use resistor carbon cables like cars sometimes do ...they give intermittent problems on Airheads.  If you install both resistor plugs & resistor caps, your spark can become too weak to reliably fire the mixture, particularly in later leaner-running bikes.   This has been seen with 5,000 ohm caps & resistor spark plugs being used at the same time.   I am aware that BMW does this on some Classic K bikes ...but, did you know the original specification was resistor caps, only?   I suspect BMW added resistor spark plugs to further reduce radiated radio interference.   On Classic K bikes I recommend resistor caps, & not resistor plugs, although they may seem to work OK ....until something gets marginal!

The 1981+ Airhead motorcycles have considerably more energy coming from the coil(s), & are designed for 5000 ohm caps ...BUT anything under a measured 2500 ohms is likely to damage the ignition on these 1981+ models.   Do not misconstrue my words here.  For the electronic ignition models, I recommend the 5K caps & that you do not use resistor plugs! ...although you may (or may not!) get away with using both without a degradation in performance.  Do not use 1000 ohm or zero ohm spark plug caps on 1981+ electronic ignition Airheads as you may risk damaging the ignition system, and the damage may not show up immediately.

For those who want originality, original type Beru metal cap spark plug caps; wires, and other bits, in assorted styles, are available from:

Bosch spark plug problems:

First Problem:
Bosch has been dropping the availability of non-resistor spark plugs.  I expect, per what Bosch technical support folks have told me, that eventually you will not be able to purchase Bosch spark plugs without resistors; except, perhaps, from BMW bike dealers.   Bosch has told me that the nominal value for the resistors in their resistor plugs is going to be 2000 to 6000 ohms.  Below ~2200 (and possibly higher) is not safe for the BMW 1981+ ignition.  At this time I can not recommend, & in fact I specifically caution against, using Bosch "resistor" plugs in 1981+ models, or any models you have converted to electronic ignition  ...unless you retain the proper 5000 ohm BMW resistor spark plug caps, or NGK 5000 ohm caps.  I do not like to see any resistor plugs at all in Airheads.

Second problem:
The worst thing about what Bosch did is that they ...purposely, or accidentally, ....sometimes ...put these resistor plugs in the older small cardboard boxes with wrong printing .  If you purchase a Bosch spark plug (they do make quality plugs! ...the German made ones anyway), check the box contents the number off the plug metal body itself, do not depend on the box numbers!

Third problem:
Bosch has gone to a 4 digit part number for spark plugs.  There is no good way for you to tell if the plug is a resistor type, or not, from just that number, unless you know that number means resistor type or not. The plug might have that number or an old number on the metal body.  There is more on this box problem later in this article!   Your BMW bike dealership, if it orders plugs from BMW, will probably have the non-resistor plugs, & with the old part numbers on them ....but, be sure.  I am not going to try to keep up with it all anymore.  Just look at the markings on the spark plug metal body. No "R" allowed in the part number if it is a conventional old-style Bosch number. You can also just use an ohmmeter.

Bosch sales (and perhaps other Bosch folks) may tell you that their resistor spark plugs are fine in engines that called for non-resistor plugs.  I suggest you should not believe them.

Miscl. notes ....and in-depth discussion on some things:

Non-projected nose spark plugs may foul and/or misfire when used in Airheads.  You might not feel it or hear it, but fuel mileage will decrease.  Using spark plugs that have an extended nose/tip is important for proper combustion on an Airhead and K bike engine.

Platinum or other precious metal tipped plugs, very especially with more than one surrounding ground electrode, are a waste of money & may not give reliable ignition ...they can occasionally misfire & even if you don't feel it in engine performance, they, too, can waste fuel.

Single ground electrode, even with a projected nose, platinum or other precious metal tipped  plugs are OK, but a waste of money & there is some indication that these 'fine electrode' plugs are not as good as the stock types.  Of course, this is contrary to manufacturer's advertising ...nothing new about this sort of thing.

BMW did engineering work to ensure that the incoming charge is swirled & passes 'just so' at the spark plug.  Save your money, do not buy specialty spark plugs. Avoid specialty dual, triple, or forked, and other similar special electrode(s) plugs.  Don't try to get more spark plug life by using special spark plugs of the type that have forked ground electrodes, more grounding electrodes, and fine wire or precious metal tipped center electrodes.  You will be replacing your spark plugs often enough, that the lower metal erosion wear on the specialty plugs is not any advantage, and some really do cause problems.  Wasting fuel by using the wrong spark plugs can pay for a lot of spark plugs.  In any event, with today's unleaded fuels, spark plugs last a lot longer, usually at least double the old 15,000 miles we all expected to replace the plugs at using leaded fuels.  I have seen plugs last longer than 30K on Airheads, and K bikes.

The latest NGK Iridium spark plugs are constructed with a double shelf, or call it a tiny step in the side of the central ceramic insulator, near the tip end.  This offers a secondary spark area, that also cleans, or helps clean, the center area.  Preliminary tests confirm NGK's claims to some extent.   I have yet to do dyno tests on any BMW bike with this type plug.  So far, no misfiring that I can figure out from the plug and a portable oscilloscope, and performance is at least equal to the stock plug.  This might end up being one specialty spark plug worth the use of....for some situations.

Where precious metal (iridium, platinum...) spark plugs, so far, have real value, is on engines that don't have the spark plugs replaced very often; and/or, that have especially powerful ignitions.  Decades ago there were almost no such ignitions, & the usage of long lasting precious metal center electrode spark plugs was for industrial pumping engines, etc.   More recently, especially in the nineties & to the present, many cars have manufacturer's recommend plug changes as late as 100K miles.   This is primarily due to the use of UNleaded fuels, & the exceptionally clean burning of modern fuel injected engines, coupled with cleaner burning oils and fuels.  Many modern cars run somewhat higher rpm, may have more sparks per mile, & many use very high powered ignition systems which can be harder on spark plug electrodes;  old type electrodes could erode away faster, widening the gap too much, so that is another reason for the more expensive plugs.  For those vehicles, I recommend you use the manufacturer's stated make and model of spark plug.

Those of you that no longer have or use leaded fuels, don't have a oil burning engine, and use full synthetic oils or quality part-synthetic, may find that your spark plugs could last 50K miles in your bike.  The plugs will usually work fine even up to 0.040" ...often even slightly larger gap ...on a Classic K bike. They can also work fine on Airheads with widening gaps from many miles of use.  Do not bend the ground electrode after any use.   Because spark plugs are inexpensive relative to fuel costs, I recommend 30,000 mile spark plug replacement for Airhead and Classic K bikes, but only if using unleaded fuel.

Some of the new spark plugs with two grounding electrodes come pre-gapped, & the gap is typically wider than your manual says to use, and the gap would be problematical to change.  Leave the gap alone.  An example is the plug that BMW started specifying for the K1200 models:   Bosch XLR7DC.  Those also seem to work OK in the older Classic K bikes with the stock 5K caps, where the original plug did not have a resistor, or, if it did have one. For longevity of the Hall sensors, I suggest you do have at least 5000 ohms in the combination of plug and cap, so be careful if you install aftermarket ignition wire and/or caps.

In the old days, before the change to unleaded fuel in the eighties, spark plugs had to be replaced at ~15,000 miles due to leaded fuel fouling.  Mechanics would often sand-blast the plugs in a special blasting machine, and get more life. I still have one of those sandblasters. Not a great idea for modern engines.    Leaded fuels are now long gone in the USA (except for some aviation fuels); and, the present additives, which do deposit on the spark plugs, are vastly less inclined to electrically foul the spark plugs.  Engines are running leaner, which means more heat, and the spark plugs are being specified to run slightly hotter to keep deposits reasonable, all these things keep the spark plug cleaner ...but eat spark plug metal a tad faster in some situations, and in others, less wear; and the ignition system output has a large effect on this, with wear being more of a problem with early high current low voltage situations. That is one of the two reasons for a resistor plug & resistor cap on some models.   Because modern engines are fuel-injected and burn as lean as possible and very close to optimum at nearly all times; they need powerful high voltage ignitions, well maybe better said as high energy ignition.  Most cars now have quite high performance ignition systems ...many have separate coils for each cylinder and/or fast rise times on the spark, and lots of Joule's (a measure of total energy in the spark).   These systems can kill.   Some cars have a coil located at each spark plug.   Spark duration due to resistance effects is not totally accepted by all folks or engineering nerds.  It is a complex subject, voltage and current for ignitions, and fun nerdy things like fast waveform rise time can be involved.

High powered ignitions can really eat electrodes, particularly if the resistance in the circuit is relatively low.  There are some cars that have a polarity reversing method in their ignition systems ...every other spark is reversed in polarity ...this helps a small amount with erosion, but not so good with ignition performance ...unless the energy is increased ...which is often done.  The net result is an improvement in lowering over-all electrode wear. There is also an effect on the sharpness of the electrodes, since sharper-edged electrodes will enhance the ability of the spark to begin.   There can be problems with some cars that use one coil per two cylinders, ...they need increased electrical energy.   There are even plasma detecting ignitions ...won't get into that here, but I put some literature on that subject in my website.  See item 18 here:

One method of reducing the rate of constantly but slowly increasing spark gap, due to spark/plasma erosion, is to use precious metals, which do not erode so quickly; that is, the precious metal lasts considerably longer, so gaps are more constant.  The precious metal plugs also typically have a smaller central electrode, leading to higher temperature of the center electrode being possible with somewhat easier spark initiation, plus easier spark jumping. Not bad Modern cars can be hard on spark plugs without these things.

Higher resistances, such as the use of 5K caps, are also helpful in reducing current (not voltage), which reduces metal erosion.  Higher resistances have some nerdy effects.  The total spark energy will decrease as the current is less. The coil energy takes longer to discharge fully, due to the resistance, the higher it is, the slower the coil discharges, but it needs to discharge fast enough to break-through deposits, yet deliver nearly all its energy, if possible, and these things work opposite engine RPM.  Konfuzzzed?  No, no going to get into it further.

I see no reason to pay premium prices for Airhead or BMW Classic K bike's spark plugs least not for Iridium or Platinum tips.   BUT ...if you are going to try for 50,000 miles on your plugs, you might be interested in trying them.  Some of the fine wire spark plugs have another feature that is hard to describe. NGK has been at the forefront on this recently, and I described it earlier.  The insulator at the tip has a taper and a L shelf.   This can improve the performance.  It is hard to do this with a large size central tip, so these are fine-wire precious metal spark plugs.  They seem to work well in K bikes.

There are reasons to use certain types of spark plugs.   In a Service Information Bulletin for the K1200, BMW went from the recommended Bosch XR5DC (Earliest prior K models had X5D, then X5DC, and then XR5D) to a dual ground electrode type XR7LDC.  The XR7LDC has been adopted by many BMW dealerships for prior K models.   These spark plugs have seemingly equivalent heat ranges in practice (in the K bike), never mind the 5 versus 7 here.  I do think the 7 is slightly hotter, by maybe half a step, in actual practice. This thin center & dual grounding electrode plug lasts longer, offers slightly better ignition under some types of conditions goes the theory.  Many will substitute NGK or other plugs, some are listed in the K bike section well below, and these have been substituted quite successfully.   That they might be wasting a teeny bit of fuel is typically not noticeable to the riders.  I am not sure they are wasting fuel, as it is difficult to measure except by an exhaust gas analyzer and an oscilloscope. The in-expensive 4163 spark plug is an example of this substituting. The XR7LDC comes pre-set for gap.  It is wider than you are used to.  Do not change the gap!  This spark plug does not fit Airheads!

There is a lot of snake oil hype regarding spark plugs.  Do not carry over my dual grounding electrode remarks to include all other engines.  It is a known fact that the type of combustion chamber coupled with the intake mixture direction, and location of the spark plug and valves, on the Airheads, is such that a dual electrode spark plug is worse than the stock single electrode.   This has to do with the pathway of the swirling mixture to and through/over the spark plug gap area.   The so-called split tail or dual grounding electrode plugs that are hyped, SplitFire, and others, are not of any help.   Worse ...for Airheads ...are the multiple grounding electrode shrouded centers ...almost always considerably worse than stock.   I have removed multiple shrouded plugs (even Bosch +4 Platinum's) from Airheads because they work lousily ...& I can quote other examples.

Indexing:   A technique called indexing is often used in racing (and a few stock cars too, like the Honda Insight), for various purposes, including reducing metal-to-metal interference in very high compression ratio engines; and, to point the spark plug gap at the intake valve.  Supposedly this will help with initiating rich mixtures, etc.  Yes, it helps.  No, it does not help on stock or near stock Classic K bikes and similarly stock or near stock Airheads.

The bottom line, for Airhead owners, with stock or near stock engines (moderately increased compression ratio or dual plugs is considered near stock by me), is to use single ground electrode, extended nose spark plugs, & use of the cheaper larger center electrode plugs works fine.  In other words, the stock spark plugs are fine!  Don't buy the hype!

****As noted earlier, on Airhead dual-plugged heads, it is somewhat common to use a lower cylinder spark plug that is one heat range step hotter, as for various reasons the lower spark plug may otherwise tend to carbon-up faster.  I've even seen 2 steps hotter used.

Very old Bosch numbers, used on some quite old BMW bikes:

W240T1:   1/2" reach, 14 mm threads, same as later W4AC.  This spark plug was used on the BMW singles & a few pre-Airhead twins.

W3CC; W4CC; W5CC:    3/4" reach, 14 mm threads version of the above plug, with various heat ranges.  Used on some twins.

W240R2:    3/4" reach, same as later W4C2.

W260T1:    1/2" reach, same as W3A1.

W260T2:    3/4" reach, same as later W4C1.

There is confusion over how these plugs were numbered & identified. The above is the best information I have at the moment ...but, see the chart below.

Bosch no longer has these very old type numbers even in their big thick catalogs.

Bosch has recently transitioned even their recent plugs to 4 digit numbers, see the chart below.

****In the plugs like the W230T30, ETC., as the "230" part of the spark plug number went higher in numerical order, the plug got colder, which is the opposite of the Bosch, newer, W numbers in the chart below.

Equivalents, Bosch & NGK:
(Snowbum has charts crossing over almost every make of spark plug, even old English types, even back into the 1920's...).

Sometimes spark plugs from one manufacturer are in-between a heat range of another manufacturer, or have a slightly wider heat range capability; thus you see the Bosch 6 & 7 series being covered by the NGK 6 series.  Spark plug heat ranges sometimes are not 'exact' when trying to cross one manufacturer's number with another.   Complicating this is that spark plugs were improved over the years, & a single part number may now cover a slightly wider heat range.  Usually these small differences are not very important.

Bosch WR plugs (not recommended for Airheads) may have the wrong gaps for Airheads!  Check the gaps!  Bosch plugs beginning with  WR  are resistor plugs!  They are not directly interchangeable with the spark plugs not having the "R" matter what Bosch or a salesman says!

Bosch plugs with a + sign at the ending might really be resistor plugs!! ...even if the part number does not have an R in it!

Do not use resistor plugs for Airheads.   If the part number on the spark plug body has an R in it, it is a resistor plug.  It is possible to be a resistor spark plug if there is a + symbol in the part number, as just noted. Some plug makers, including Bosch in its new numbering system (which I dislike, they offer no information by number as to heat range nor size) ...may not use R, or +, simply using an all numbers no letters part number!  If you insist on using resistor plugs because you are too lazy to wait for an order for the non-resistor types, & you also use the stock resistor caps, your ignition may be a bit weak, but I don't see any problems beyond, maybe, hard starting &/or occasional misfiring & wasting fuel ...all depending considerably on engine & electrical's condition.   You may not even notice any misfiring ...but your fuel mileage will or might suffer a little.  There is a chance of more combustion chamber deposits too.

****In the plugs like the W230T30, ETC., as the '230' part of the spark plug number went higher in numerical order, the plug got colder, which is the opposite of the Bosch, newer, W numbers in the chart below.

BOSCH, early


BC/DC Bosch

Latest Bosch number series

NGK, older

NGK with removable nut NGK withOUT removable nut


W5D W5DC 7591 BP7ES 1034 3995 W5 covers W225T30 & the colder W230T30; BMW  12-12-1-338-146. Be sure  the plug is not WR5DC
  W5B W5BC 7531 BP7HS 5111    
W200T30 W6D W6DC 7594 BP6ES


4007 Champion N-9YC is OK if you can't find NGK or BMW  12-12-1-338-145. Be sure the plug is not WR6DC
  W6B W6BC 7593 BP6HS 7331    
W175T30 W7D W7DC 7500 BP6ES 7333 4007 BMW sold  W7DC as 12-12-1-267-485. May be found as 12-12-9-062-594. Be sure the plug is NOT WR7DC.  Beru 14/7DU was BMW 12-12-1-265-595.  BMW also sold  Champion equivalent as 12-12-1-338-147.
  W7B W7BC 7597 BP6HS 7331    
  W8D W8DC 7505 BP5ES 7832 2140  
  W8B W8BC 7503 BP5HS 4111    

Higher numbers are colder.  That is, W230 is colder than a W175.

Higher numbers are hotter

 Plugs above ending in C cover a slightly wider heat range than the non-C & early Bosch to the left.  Higher numbers are hotter

Caution! Be sure the actual   plug, for your Airhead, no matter what the carton has printed on it, is a non-resistor.  See notes in this chart, & article.

Higher numbers are colder

    BMW dealerships & Independents may carry the non-resistor Bosch Spark Plugs.   Be sure that, for Airheads,  you do not get the WR  spark plugs, which have resistors.
    'BC' :  1/2" reach.            
    'DC' :  3/4" reach.          

Bosch has sometimes put resistor plugs in the small cardboard boxes that are marked for the non-resistor plugs.   Also reported to me was that the number on the box for non-resistor used to start as 0-241-; and Bosch may be putting resistor plugs into those boxes, with the boxes showing 0-242-.  My answer to this is to not trust what the box says it and see what is printed on the plug base metal.  I'm anal enough to bring an ohmmeter, just to be sure!

Here is a slightly enlarged picture showing what one Bosch box end looks like.  Do not trust anything on that box end ...look at the markings on the plug metal itself.  Use your ohmmeter too.

Spark plug gapping, checking, and/or adjusting):

Typical wires-type spark plug measuring and adjusting tools.

For gapping brand-new plugs for Airheads (spark plugs that have never ever been run in an engine), I suggest you use 0.6 mm, 0.024" (preferred); 0.7 mm, 0.028".  I am aware that some literature says that up to 0.9 mm (0.035") is OK for late eighties to 1995.   If these brand-new spark plugs need the gap reset, be very careful to not pressure, and thereby damage, the central insulator, as the slightest chip is cause to throw the spark plug in the trash can.

The place to put the bending pressure on the ground electrode is dangerously close to the central electrode/insulator;  Do not pressure the white insulator or central electrode.  Plug gap will open up as the miles accumulate & the ground electrode will probably dish a bit. Erosion/Dishing will be seen at the center electrode if the ignition coil is wrongly connected (wrong polarity at the spark plug due to that).   The gap should be measured with a round wire gauge (the same round dish tool that has the tiny fork to adjust the gap), as in the photo above. My quite old recommendation was that if plug gap was worn to over over 0.8 mm (0.032"), to throw them away. Part of that recommendation was that leaded-fuels were in use.   With unleaded fuels, if the spark plugs are working OK, no misfiring for example, and the gap has opened up to even as wide as 0.045", I am OK with that! ...just do not adjust the gap on any used or worn plug.  The reason is that ground electrodes in the spark plugs change their metal structure once heated; bending them can weaken them, and you do not want a ground electrode breaking off in the combustion chamber.  Note that the rate of gap change is dependent on several factors, including the spark plug cap resistance; so, gap change can vary, for the same mileage, between year, model, cap resistance, etc. In general, you are unlikely to see quite worn gaps.    Again:  Do not reset the gap on previously run spark plugs, unless you are willing to take the admittedly very small chance of the ground electrode coming loose during operation of the engine; or, that you might damage the ceramic insulator.   Due to leaded gasoline having gone bye-bye, spark plugs will last a lot longer than they used to. You also no longer have to clean/sandblast spark plugs, although you could.  In the leaded gas days, spark plugs in a clean burning engine would last 15K-20K at best. Double that is pretty common these days for Airheads, and even longer is common with higher quality oils, especially synthetic oils.   I am OK with running spark plugs for as long as they work well; providing that the gap has not worn too far (absolutely not over .045"); & the ground electrode is not dangerously thinned.

Spark Plugs for Classic K bikes (K1, K75, K100, K1100, all versions of these models):
(you can probably add the K1200 here too, see text well above, and here)

All these bikes use a 12 mm 3/4" reach spark plug.  These are available in both  5/8" & 11/16" hex. Keep that in mind as you might have only one spark plug wrench size.   Note that some tubular wrenches/sockets may not fit as the surrounding casting is quite close.

The spark plugs used on the Classic K bikes were specified, depending on the year of the manual, BMW SI's, etc, as:  X5D; X5DC; & XR5DC. The last update could be considered to be the XR7LDC, but it wasn't specified for the Classic K bikes, but for the K1200, but is usable on the earlier K models.  The XR7LDC has a dual grounding electrode.   "R" in a plug number means it is a resistor type.  Classic BMW K bikes had 5KΩ spark plug caps.  The K75 bikes had a gap in the spark plug lead, thus cannot be quite as easily measured for the spark plug cap resistance.   Except for the XR7LDC, the plugs should be gapped (bend the ground electrode only if the plug is 100% brand-new, never run in an engine) at 0.6-0.7 mm (0.024"-0.028").   The XR7LDC comes pre-gapped, wider than that just mentioned.  Do not change its gap. Be a bit cautious about what you may think you know about heat ranges.  An example is that the XR7LDC is nearly the same heat range as a X5DC (or the other 5DC models), in actual use in the K motorcycle.   It is perfectly OK to use non-resistor X5D or X5DC spark plugs; and I slightly prefer these, although gap wear is a wee bit faster.

NGK makes a 12 mm, 3/4" reach, 5/8" hex sparkplug.   Model DCPR7E, new number 3932, has removable terminals, tapered, shelf'd insulator, unsure of gap they come with.   It supposedly is a substitute for the DR7EIX, a fine wire iridium plug, #5686, that is substituted for the Bosch XR7LDC; the TIP lasts longer.  I can speculate that in oil burning engines it may be quite good.

NGK equivalents such as the D7EA (7912) and DR7EA (7839) also work OK in BMW Classic K-bikes.

There is absolutely nothing wrong with using the original non-resistor (or resistor), large electrode, inexpensive type of spark plugs in a Classic K bike.   The electronics are protected by the 5000 ohm spark plug caps.

Another spark plug that has been proven to work well, and is inexpensive, is the Autolite 4163.

When installing weather-resistant rubber covers (boots) over the spark plug wires where they fit over the neck of the spark plug caps, lightly coat the inside of the rubber cover with silicone dielectric grease so that the boots seal to the spark plug cap and to the cable.  Generally, do not use on the electric contacts or contacting area.  Use it on the rubber, for environmental sealing and ease of removal in the future.

You can coat the spark plug white ceramic outer area a tiny bit, if you want to, with silicone dielectric grease.  It makes the rubber part of the cap release easier, as in some instances the rubber will tear or otherwise stick to the spark plug insulator.

Coat (faintly) the inside surface of the rubber boots at the coils.   The application will make removing the boots easier, and prevent electrical contact corrosion from the elements, etc.  Generally, don't use dielectric grease on the metal fitting and bottom connection inside the coil tower(s).  Generally, if you want additional environmental protection inside the coil tower, use Caig DeOxit. 

Be sure that the brass electrical fitting end of the ignition wire is tight-fitting into the coil internal metal contact.  You can be fooled into thinking it fits fully into the coil tower fitting, as it may take pliers to force the fitting into the bottom contacting area of the coil.  It is an excellent idea to have the copper wires inside the ignition cable bent over and soldered to the brass wire fitting.  The brass should be lightly cleaned to shiny, before soldering is attempted.   Do not have solder where it might make the fitting larger in diameter, or it will not fit the coil.

In severe atmospheric conditions, or when deep water and other situations may be often encountered, first ensure that the internal copper wires, inside the ignition wire, are bend over and soldered to its end brass connector that fits into the coil.  Clean the brass.  Clean the inside of coil tower (the brass connection deep inside).  Then, ensure that the brass connector will fit quite tightly into the coil connection (It must go into that internal coil brass connection).  I recommend these things also for motorcycles that will be used in extremely high smog conditions.  Has lots of information, explained differently, but clearly.

Below is a chart of NGK spark plug caps which are popular with the Airhead community.  Commonly available is the LB01EP and LB01F; and LB05 series.

LB01E       new number 8011
LB01EP     new number 8328

LB05E       new number 8031
LB05EP    new number 8020
LB05FP    new number 8030
LB05F       new number 8051

NOTE!   You will likely not find all NGK spark plug cap models easily available to you.   The most common types available are the LB01 and LB05 type.  Note carefully before you purchase any, as to the next letter following.  That is, note the E and F above.  Stock BMW spark plug wires come with caps that have varied in style.  Some are metal covered, some fully molded, etc, and caps have been both a nominal 1000 ohms or nominal 5000 ohms ...but one thing is common to all the stock BMW Airhead caps:  they are designed to fit onto the threaded post of a spark plug.  There are spark plugs that do not have removable threaded caps, so watch out for those.  I suggest you do not change to permanent cap type of spark plugs.     Be sure that whatever spark plug cap you use is compatible to the spark plug itself.   Take into consideration that you might be using some sort of shorting device or adaptor when synchronizing carburetors, so that is also a consideration if you should change your type of plug or cap type.  NGK spark plug caps last a long time and I have almost never seen a failure of the resistor element inside them.

04/22/2003:  Clarifications & emphasis.
08/09/2003:  Clarity.
08/13/2004:  Minor editing for additional clarity, including more detailed information on previously anti-seized threads.
09/13/2005:  Add section on resistor plugs & wires, etc.
01/11/2007:  Update with latest information, and add the NGK chart.
03/25/2007:  Add equivalents, Bosch & NGK.
03/27/2007:  Rearrange order of last items on page, add more information on the Bosch Plugs problems.
09/16/2007:  Clean up.
01/06/2008:  Remove bad hyperlink to old Bosch plugs at
01/15/2008:  Totally revise the article, combining information on holes, etc., from Engine Internals, ETC.
10/01/2008:  Add section expanding on special spark plugs.
11/09/2008:  Add OLD Bosch numbers section.
01/18/2010:  Go through entire article, improve clarity; fix errors, clean up emphasis items, etc.
10/30/2010:  Add new NGK numbers.
10/31/2010:  Clean up article somewhat and add final note to the NGK information, in red.
01/01/2011:  Add really old Bosch numbers.
04/25/2011:  Add photo and description of the three plugs at the top of the page.
01/01/2012:  Add Euromotoelectics link.
02/16/2012:  Expand warnings about regapping.
03/31/2012:  Revise article, make table of equivalents, etc.  NO errors were found nor corrected on prior version.
undated, 2012:  remove indexing section.
04/01/2012:  Clean up more.  Clarify that is the exhaust valve, not both, that are to be open if not removing head for insert installation.
04/03/2012:  A bit more work on the table-chart, and eliminate duplications earlier.
04/07/2012:  Add more information on NGK spark plugs to the table-chart.
10/14/2012:  Add QR code, add language button, update Google Ad-Sense code.
12/10/2012:  Clean up article a fair amount.  Add Airhead gapping information in detail.
sometime in 2013:  Remove language button; coding was causing problems with SOME browsers.
03/14/2013:  Much deeper into the controversy over using, or not, antiseize compounds.  This was added as few understand that I have REAL data, & REAL industry information, that is not, seemingly, read by those who have FAULTY ideas on the subject.
09/17/2014:  Clean up article, mostly for use with smaller screens.
02/04/2015:  Clean up article once more, nothing substantial changed in values/methods.
08/23/2015:  Release two updates, combined, since March.
11/13/2015:  Clean up article presentation.  Meta updates. Add more comments to the chart on spark plugs, etc.
12/15/2015:  Final meta-data fix.  Clean up article, justifying to left, etc., for better presentation on smaller devices.
12/16/2015:  Clarify BMW part numbers and Bosch cautions.
12/25/2015:  Minor clarifications about torque values.
03/31/2016:  Update metcodes, layout, narrow tables ABP, fonts, colors, other improvements and more clarity.
11/24/2016:  Update metas, scripts, layout, excessive HTML.  Additional clarity on torques.  NGK caps/covers link is now dead, so remove link.
05/08/2017:  Clarify torque values and why.
08/21/2017:  Add section on how spark plugs work.  Clean up article for fewer font changes, fewer colors, fewer bolds, etc.

Copyright, 2016, R. Fleischer

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Last check/edit: Monday, August 21, 2017