Spark plug threads, repairs, Helicoils.
Tightening torques. Gaps.
ANTISEIZE...>> OR NOT?
Spark plug caps.
Resistor & non-resistor spark plugs.
Heat-Range chart; equivalent numbers.
© Copyright, 2013, R. Fleischer
Above photo is of three types of EXTENDED NOSE spark plugs, 3/4" reach, 14 mm. Extended nose means that the center white insulator extends beyond the end of the threads. ONLY THE LEFT-MOST spark plug is appropriate for Airheads; it has a SINGLE GROUND ELECTRODE AND A STANDARD SIZED CENTER ELECTRODE, WITH AN EXTENDED NOSE. The middle spark plug, Bosch XR7LDC, has two ground electrodes, & can be used in Classic K bikes; these bikes can also use X5D, X5DC, XR5DC. The XLR7DC spark plug with dual grounding electrodes, introduced during the K1200 production, works well with all prior K bikes. The middle & right-most spark plug has even more ground electrodes & versions can have standard or a fine wire center electrode. These have NO USE in Airheads & NOT in Classic K bikes if the spark plugs have more than 2 grounding electrodes.
NOTE: Do not try to re-gap the XR7LDC spark plug, even when brand-new. You can re-gap single ground electrode spark plugs when brand new only.
BMW says NOT
to re-gap spark plugs. BMW did not explain that advice very well.
They really 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 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.
Re-gapping WAS very commonly done on old cars, and bikes. DO NOT do this.
"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. This is usually acceptable until the gap becomes very excessive, at which time marginal systems or those 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 problem varies highly, with a wide range of things affecting the wear, such as spark plug heat range, caps resistance, actual mixture burning conditions, type of ignition system, whether resistor spark plugs or not, ETC. Spark plugs last MUCH longer now, due to the use of 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; 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), as they can weaken. In the OLD OLD days, we Wrenches would file the ground electrode, sand-blast the plugs, set gaps, etc. These days, nothing should be done but to check but not adjust the gap; check plug base color; check central insulator color and appearance.
Spark plugs screw into threads; these male & female threads are exposed to combustion
heat/pressure & also to byproducts 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 exhaust valve open, & blow compressed air into the intake area as I clean the threads. 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 for 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.
Aluminum cylinder heads can be damaged relatively easily if one is not careful. There is
a limit to abuse from over-tightening a spark plug, or removing & replacing a spark
plug 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. 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
Many folks simply refuse to use torque wrenches, & some who do or do not, 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. NOTE that "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....best 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 any possible chips out with an air hose. You CAN do a good job withOUT removing the head. 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!):
The use of antiseize is controversial, even among 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 a somewhat easier for abuse.
SOME manufacturer's, trying to play it safer (???) have changed from recommending anti-seize, to not recommending it, or just ignoring the use, over the years. A good 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, a 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, has prompted me to ADD more
commentary to this part of the article you are reading. I have NOT changed my own
practice, intent, or 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 errors in publications from long long ago were carried over, and never checked.
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 that book value means NO antiseize) MUST be reduced...about 20% reduction is typically about correct for spark plugs into aluminum.
SOME types of spark plugs, such as taper seat types, 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 & full information can be more
complicated. BUT: 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 is, 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, would have NO EFFECT on your BMW engine...and I believe any such concerns SHOULD BE IGNORED. What those manufacturer's do not tell you is that their concerns are for WAY 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 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 personally feel that if the 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 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!
Stuck/frozen, or other problem: 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!
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.
NOTE: 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 better than some other spark plug manufacturers. 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: it should be used on the UNplated plugs. I will get into this more deeply HERE:
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. 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 a bit 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 to NOT use antiseize....because if you DO, you will damage the cylinder head IF YOU OVERTORQUE.
Service information for a LOT of 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 cast iron head engine'd 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 a very strong advice, about why you SHOULD use anti-seize....at least on THEIR spark plugs. They have an entire technical bulletin on the subject. Condensing it here 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 the plating, & 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 the THREADS from the BODY, but it can be implied from that prior sentence, of course. 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.
NGK makes it very difficult to get the information, but here is THEIR information, 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 NO INFORMATION.
Autolite 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....in particular the long reach HT plugs (used on Ford 3 valve modular engines, etc...that have WELL-KNOWN plug removal problems).
There is a LOT MORE to all the above; 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 ...that is, they likely have copper, 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 "copper anti-seize').
DO NOT USE GRAPHITE PRODUCTS 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:
4. Since we all know that many of you are cheapskates & will not use fresh crush rings for
those plugs where they can be removed & replaced, the values I am giving here are
generally safe values, with new or used crush washers/rings...used with a drop of
anti-seize compound spread VERY THINLY onto the clean spark plug threads (fingers work
well for this, you can push the stuff very thinly into the threads... or use an old toothbrush). You need NOT cover every
thread....just a wee bit towards the electrodes end THREADS (combustion chamber ends),
pushed into the threads with a finger tip or brush...again, a WEE BIT....is enough. As the plug is screwed-in, the compound is spread into the rest of the threads. I personally coat, VERY thinly, all-around.
I use my fingers, or a tiny brush with fairly stiff bristles. The shortened-bristles end of a
common 'acid brush' or an old toothbrush, both are perfect if you don't want the stuff on your fingers. DO NOT get
anti-seize compound onto the end of the spark plug area.
Torques; with spark plug having a faint coating of anti-seize on its threads:
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 the commonly used bottom spark plug size on dual-plug conversions. This torque value is USUALLY...not always... enough to seat the washer and a tad more. Snug is right. You don't want the plug too loose...if it loosens and rattles out, that pulls/wears threads. Some have installed smaller spark plugs for the lower plugs. I've had good results with 8-10 ftlbs for those. Some manuals will show 8-12 in aluminum.
Some have installed a welded (to the head) custom-made thick aluminum spacer/washer, or just welded up the material to be thicker, ...either is only for at the lower spark plug threaded hole, that allows the top & bottom spark plugs to both be 3/4" reach. If you have this style of lower spark plug threaded area my recommendation is, as noted above, about 14-15 ftlbs; EXCEPT if the modification was by an unthreaded spacer, welded or not, then use 14 ftlbs.
With regard to the above various values: UPDATED torque figures, if any, MIGHT be found
in article 71B.
5. One is supposed to, in most 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...and/or by feel. That was done on CAST IRON HEADS. This can still be done, if you have a decent feel for it, but it is more dangerous to do on aluminum heads. My basic advice is, and I am hammering you on this with repeating it: wee bit of antiseize, torque wrench, proper torque.
What about installing Clean and Dry (NOT with 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 absolute 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.
OFFICIAL BMW Service Information sheet on the late Airheads; this is the torque value to set per BMW:
18.4 ftlbs (3/4" reach). Don't use that figure for threads with anti-seize compound.
The latest Bosch manuals, for 14 mm spark plugs in BOTH aluminum & cast iron heads, recommend 7-15 ft lbs. That is NOT oiled NOR antiseized!!! 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.
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!!!
Removing spark plugs:
Also: Repairing spark plug hole threads:
It is not uncommon to see damaged spark plug threaded holes in the cylinder heads. This comes from excessive torque, dirty carbone'd threads, etc. If the threads are in need of repair, you may....or may not....be 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 a SMALL AMOUNT of loosening, seems to
tighten up appreciably, STOP!!!!
Now is the time to try to 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;...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. Flood the threads with more penetrant if needed. In some instances the penetrant can be used without removing the head, but be careful, do not damage the threads by using too much removal torque;...take your time. Snowbum heats up the head (running the engine is best for this), and then tries spark plug removal. NO excessive torque, no matter if hot, or oiled, or anything. If the head is off the engine, Snowbum heats it to sizzle temperature in his oven (that means around 200°F.
How to clean up & prepare damaged threads; the head being left in place on the engine
(if just carboned, try using hand-rotary motion on a shotgun cleaning brush):
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, 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-on-tap method & the pressure method, both at the same time & do wear eye protection. That usually works quite 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 surface!). Allow the RED Loctite to FULLY cure for a couple of days, then clean the threads VERY thoroughly with a strong solvent, such as acetone or MEK, and a brush, BEFORE installing the spark plug (treat the helicoil AND spark plug threads with a thin amount of anti-seize compound). DO NOT FORGET THAT STEP OF CLEANING OFF THE EXCESS UNCURED LOCTITE!... You do NOT want to Loctite your spark plugs into the Helicoil! The heads get pretty hot & Loctite only gives a small amount of holding power when the head is hot. You can also use grades of Loctite for use in higher temperatures. For the latest part numbers, see: http://bmwmotorcycletech.info/chemicalsetc.htm. In an emergency situation, or if you don't want to wait a few days, ....just clean the threads & install the Helicoil, & ride away.
It is VERY important to have a guide machined or purchased to allow exactly 90° drilling AND a guide machined or purchased for the the tap.. to drill the hole properly and to start threads dead square to the surface, & continue with the tool dead square. 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 FLAT on the drill press table.
Here is a method to try if the threads are not too bad:
Be sure the piston is not too near being fully outwards. Take a small piece of cloth, & roll it up to fit moderately tightly in the spark plug hole. Soak in penetrant, let sit overnight. The next day, use a shotgun cleaning brass brush with a solvent or even the penetrant, to clean the threads. Then use a good fresh sharp 14 mm tap that is well-greased. Keep the tap quite square to the surface. Back the tap out often, tapping a couple or three threads at a time. Use a bit of White Lead or good cutting compound meant for aluminum materials on the tap during the thread reforming, which may involve some cutting. You can use anti-seize compound for this if you wish. 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 from the spark plug electrode tip 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 plug.
When doing threads or Helicoil work, be sure to have the piston down from TDC (down from OT mark), 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.NOTE!....If using the pressure method, be SURE to wear eye protection...sharp nasty chips WILL likely be flying out the spark plug hole.
I do not recommend resistor spark plugs from ANY manufacturer for BMW Airhead motorcycles. The reason to use 'Resistor' plugs or resistor caps or resistor wire is to reduce the current, thereby reducing Radio Frequency Interference (RFI). A side benefit is that the reduced current will DEcrease spark plug electrode wear. It is possible that another reason might have been spark duration time....as resistance can slightly increase the length of time the spark 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; then one gets into electrical spark characteristics in some esoteric ways, so I won't go further with that here. One final argument deals with resistors upsetting the relationship between specified COIL secondary resistance and the spark. There can be real problems with spark plugs that contain "resistors". Some of these resistors are really not anything but low resistance coils, acting like an electronic part called a choke (real term: inductor). This can INJURE the 1981+ ignitions. Resaid: SOME 'resistor' spark plugs are not true resistor plugs...they have a small coil of wire wound to act like an electronic 'choke' (inductor), and may be so wound to incorporate a capacitance effect as well. The electronic ignition may NOT like it, & I can speculate about some potential problems. That same sort of thing was done with spiral-wrapped ignition wire....sold as Suppression Wire, where it was NOT really resistance suppression wire. You can measure the plug or wires, etc, if curious....with an ohmmeter.
Suffice it to say that a real resistor is incorporated into the stock or NGK aftermarket spark plug resistor-type caps. Values are approximately 1000 ohms on early bikes with points ignition; and, 5000 ohms on some later points ignition bikes (1000 is OK for them), and ONLY 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.
Some types of RFI can affect other things, such as electronic ignitions & electronic tachometers, sometimes in strange ways. I do NOT recommend zero ohms leads/caps/plugs (over-all zero). In Airheads, the resistance in the plug caps have SEVERAL functions...here are a few...more following this paragraph. (1) reduce the erosion of the electrodes in the spark plugs. (2) RFI. (3) Protect against RFI being conducted into wiring & thereby getting to the Hall element in the canister on 1981+ models....it 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.
If one uses resistor plugs and non-resistor caps, there is a danger that you (or someone)
might install non-resistor plugs later on, injuring your electronic ignition. 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 at the same time. Yes, I am aware that BMW does it on some K
bikes...but, did you know the original spec was resistor CAPS, only?
On the POINTS airheads, especially the points models with two coils before 1979, the stock caps were about 1000 ohms in the earliest years, then later went to 5000 ohms. 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. Thus, 1000 ohm caps are OK, in my opinion, for POINTS models. I recommend the 1K caps for POINTS models.
The 1981+ models have more energy coming from the coil(s), & are designed for & easily handle the 5000 ohm caps...BUT anything under 2500 ohms is likely to damage the ignition on 1981+ models. Do not misconstrue my words here. For the electronic ignition models, I recommend the 5K caps & NOT to use resistor plugs!...although you MAY get away with using both, without a degradation in performance. Note the word MAY.
For those who want originality: Original type Beru metal cap spark plug caps; wires, and other bits, assorted styles, are available from: www.euromotoelectrics.com
Using spark plugs that have an extended nose/tip is important for proper combustion on an Airhead and K bike engine.
Those of you that no longer have or use leaded fuels, AND use FULL synthetic oils or quality part-synthetic), may well find that your spark plugs can last 50K miles
in your bike. DO NOT bend the ground electrode after use, the plugs will usually work fine
even up to even 0.040"..often even larger gap...on a Classic K bike.
Much of this is heavily edited from something I posted to the K-bmw list:
Where precious metal (iridium, platinum...) spark plugs have value is on vehicles that don't have the spark plugs replaced very often; and/or, that have quite powerful ignitions. Years & years 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 even 100K miles. This is primarily due to the use of UNleaded fuels, & the exceptionally clean burning of modern fuel injected engines. Many modern cars are running somewhat higher rpm, more sparks per mile, & using very high powered ignition systems....that can be harder on spark plug electrodes; they can erode away faster; another reason for the more expensive plugs. For these vehicles, I almost always recommend you use the manufacturer's stated make and model of spark plug.
I see no reason to pay premium prices for Airhead or BMW Classic K bike's spark plugs....at 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. 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 work well in K bikes. See prior section on NGK's iridium plugs.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), ....so never mind the 5 versus 7 here. I do think the 7 is SLIGHTLY hotter, by MAYBE half a step, in practice. This thin center & dual grounding electrode plug lasts longer, offers slightly better ignition under some types of conditions....so 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 are conventional electrode; for the somewhat special K bike plugs used on some models. That they MIGHT be wasting a teeny bit of fuel is not noticeable to them. I am not sure they ARE wasting fuel, as it is difficult to measure. The in-expensive 4163 spark plug is an example of this substituting.
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:
Bosch made these spark plugs, used on some bikes:
W240T1 was 1/2" reach, 14 mm threads, same as later W4AC. This plug was used on the BMW singles & a few pre-Airhead twins. The 3/4" reach, 14 mm threads version of this plug was used on those twins that took 3/4" reach spark plugs; typical numbers were W3CC; W4CC; W5CC.
W240R2 was 3/4" reach, same as later W4C2.
W260T1 was 1/2" reach, same as W3A1.
W260T2 was 3/4" reach, same as later W4C1.
After so many years 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 the 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, as the "230" went higher in number, the plug got COLDER.
Exactly the opposite of the newer Bosch W numbers in the chart below.
Equivalents, Bosch, NGK, etc:
Bosch number series
|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|
|4007||Champion N-9YC can work OK, if you cannot find NGK or BMW 12-12-1-338-145. BE SURE the plug is NOT WR6DC|
|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.|
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
Be careful with these numbers. Be SURE the ACTUAL plug, for your Airhead, no matter what the carton has printed on it, is a NON-resistor. See note in paragraph after this chart, and earlier in article.
Higher numbers are COLDER
|BMW dealerships & Independents may carry the NON-resistor Bosch Spark Plugs. Be sure that you do NOT get the WR..... spark plugs, which have resistors, for your Airhead.|
Bosch has, on occasion, put their
resistor plugs in the small cardboard boxes that are marked for
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...open it and see what is printed on the plug base metal. I'm anal enough to bring an ohmmeter!
Here is an 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. You might find this box contains a resistor plug.
Gapping brand-new plugs for Airheads (plugs have NEVER EVER BEEN RUN), I suggest you use 0.6 mm,
0.024" (preferred); ...to 0.7 mm, 0.028". I am aware of what some literature says & 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 damage the central insulator, the slightest chip is cause to throw the spark plug in the trash can. The place to put the bending pressure is close to the central electrode/insulator, but do NOT pressure the white insulator. Plug gap will open up as the miles accumulate & the ground electrode will probably dish a bit. Erosion/Dishing will be 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 try to adjust such a 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. DO NOT reset the gap on previously run spark plugs, unless you are willing to take the chance (admittedly VERY small chance) of the ground electrode coming loose during operation of the engine. 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 sandblast spark plugs to clean them.
In the leaded gas days, spark plugs in a clean burning engine would last 15K-20K at very best.
Double that is pretty common these days for Airheads, & some have gone 60K. I am OK with
running spark plugs for as long as they work well; PROVIDING that the gap has not worn too
far (not over .045"); & the ground electrode is NOT
Spark Plugs for Classic K bikes (K1, K75, K100, K1100, all versions of these models):(you can probably add the non-classic K1200 here too, see text well above, and here)
These bikes use a 12 mm 3/4" reach spark plug. These are 5/8" or 11/16" hex depending on whose plug you use. Keep THAT in mind as you might have only one spark plug wrench size, & a larger size may not fit well considering the size of the 'casting hole area' around the spark plug.
NGK equivalents such as the D7EA (7912) and DR7EA (7839) work fine in BMW
There is absolutely nothing wrong with using the original non-resistor, large electrode, inexpensive type of spark plugs in a Classic K bike!!! The electronics is protected by the 5000 ohm spark plug caps.
Another spark plug that has been proven to work well, and is INexpensive, is the
https://en.wikipedia.org/wiki/Spark_plugLots of information, explained differently, but clearly. Reference information: http://www.ngksparkplugs.com/docs/resistor_covers.pdf covers the below information and more.
Below is a chart on NGK spark plug caps, which are very popular with the Airhead community. Very 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 those NGK spark plug cap models available to you. The most common type available is 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 varied from a nominal 1000 ohms or 5000 ohms)....but one thing is common to ALL the STOCK BMW Airhead caps: they are designed to fit onto the THREADED post of the spark plug. There ARE spark plugs that do NOT have removable threaded caps, so watch out for those too. 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.
©Copyright, 2013, R. Fleischer
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Last edit of THIS page: Tuesday, April 26, 2016