SPARK PLUGS
Spark plug threaded holes.
Tightening torques. Gaps.
ANTISEIZE... OR NOT?
Spark plug caps.
Resistor & non-resistor spark plugs.

sparkplugs.htm-32

© Copyright, 2013, R. Fleischer

Above is a photo 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 side spark plug is appropriate for Airheads; it has a SINGLE GROUND ELECTRODE and a STANDARD SIZED CENTER ELECTRODE.
The middle spark plug has two ground electrodes, and is used on some Classic K bikes, as Bosch XR7LDC; these bikes can also use X5D, X5DC, XR5DC, etc.
The right-most spark plug has more than two ground electrodes and a fine wire center electrode (that is hidden in this photo).  It has no use in Airheads, and not in Classic K bikes either.
The XLR7DC spark plug with dual grounding electrodes, introduced during the K1200 production, works well with all prior K bikes.

BMW says to NOT re-gap spark plugs.  BMW did not explain this very well.  They really should have THIS added:  "AFTER they have been in use".  That IS what BMW means, and I am in total agreement.     After the plug has been in use, the structure of the GROUND electrode changes, and if you then try to re-gap the spark plug, you can WEAKEN the ground electrode, and in rare instances it has broken off and caused extensive damage.   Do NOT re-gap plugs after they have been in use!  IN USE here means that the engine has been run.

Aluminum cylinder heads can be damaged relatively easily if one is not careful.  There is a distinct limit to any abuse from over-tightening a spark plug, or removing and replacing a spark plug when the cylinder head spark plug threads are full of carbon.

Some BMW motorcycles PRIOR to the /5 series, airheads; let me just call them the /2 bikes, had some metal alloy and casting problems, including soft alloys and voids,  and even with very careful torqueing, with the best antiseize compounds of the day, still brought occasional problems. I've seen poor metals and poor castings bring about failure of the factory installed 'cast-in-place' steel threads inserts, and heat-cool cycling on those early inserts caused problems. For our /5 and later airheads, the metal in the heads, and the castings, are very considerably better.  Problems occur in several ways.  Many folks simply refuse to use torque wrenches, and some who do or do not, still over-torque the spark plugs.  I have seen someone over-torque a spark plug and he WAS using a torque wrench, and 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. SOME 75 or 80 or 100 footpound wrenches might have a rather subtle click when set much lower, say around 12 footpounds....cheaper ones possible even at higher settings.  As one example, I have personally seen such a wrench, 0-75 ft lb range, that had such a subtle click that someone pulled a cylinder stud...at a setting of ~26+ ftlbs. They did this right in front of me at a TechDay.  Know your wrench.  If you don't know what the click feels like at various settings, and are not all that familiar with torque wrenches, put the square end into your bench vise and test the feel required for various torque settings, ... feel and listen for that click.  NOTE that cheap beam-type torque wrenches (not meaning the dial indicator commercial types) may be very INaccurate....and should not be used for critical items.

Abused, the cylinder head threads will eventually start distorting and pulling out if they don't immediately rip out. While Helicoil or other types of inserts, or welding and re-machining, all do work OK, such repairs are usually avoidable if one takes some extra care in the first place.  However, if an insert IS installed, do consider problems if that is not done correctly.  Example:  I don't like to see extra places for sharp bits of carbon combustion products to adhere to (they create hot spots in the cylinder heads) from using inserts.  Sharp spots can create places for improper combustion...and PRE-IGNITION. 

I have installed Helicoils, and also have actually welded up the areas and drilled and re-tapped them, of course the heads have to come off for sure....best to remove the head in any fix anyway....although one CAN use the official Helicoil Spark Plug Thread repair kit, and with well-greased parts, and 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; but is not the best way to go, which is removal.  It is CRITICAL that the drilling and tapping be done squarely. ABSOLUTELY AND ALWAYS use a drill guide and tap guide!
 

MORE information on tapping and installing Helicoils, head on, or head off the engine, is later in this article.

Antiseize compounds:  

The use of antiseize is controversial, even among experienced Airhead mechanics...... mostly I think this is due to SOME spark plug manufacturer's literature; and perhaps some is due to misconceptions; and, frankly, some is due to faulty belief in old-wives tales, or, possibly, bad experiences, ETC.   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.  There is a lot to all this, and I will explain what is going on.  I expanded this section in March 2013. 

NOTE:   Karle Seyfert, one of the expert writers for MOTOR, an INDUSTRY publication, had an extensive article in MOTOR in March 2013.   That article, and some controversy (and comments by one well-known 'guru' in the BMW motorcycle repair business recently on an Internet mailing list) has prompted me to ADD more commentary.  I have NOT changed my own practice, intent, or recommendations; but I AM going to give you more information on what automobile manufacturer's are publishing/recommending!...and maybe some of WHY!
 

Some experienced mechanics believe that if antiseize is used it is easier to overtorque and pull threads. THAT IS TRUE!   BUT, it is true only if the person is NOT adjusting TORQUE lower, to compensate for the use of antiseize.

You should be using a torque wrench, AND lowering the official torque value (which is withOUT antiseize compound unless specifically so-stated).  Since popular antiseize compounds act like a lubricant, the applied torque (if a book value means NO antiseize) MUST be reduced...not quite 1/3 reduction is roughly correct.

Yes...SOME manufacturer's are concerned with over-torquing on spark plugs.  .SOME types of spark plugs, such as taper seat types, are NOT to be used with antiseize compounds, as they are degree- tightened, and so the manufacturer makes a blanket statement not to use antiseize compounds, rather than providing full information, which is more effort for them, and, frankly, the full information is a bit complicated.  Other manufacturers say OK, and give the needed lower torque values.  Some manufacturer's do not mention use of antiseize at all, and SOME SAY YOU MUST USE IT.
 

Some manufacturer's also SAY they are concerned with with HEAT conduction.  They probably think that the heat range of the spark plug is changed by the use of antiseize compound. Frankly, it IS possible, but it is such a tiny bit of a single heat range, and does not always happen, that I believe any such considerations SHOULD BE IGNORED.     I have NOT seen ANY problem in real life situations if the torque is set properly, even if close to properly. I have tested for results with and without antiseize, in virgin heads and used heads, for many factors.  I have NOT FOUND ANY PROBLEMS.

I have use some sort of antiseize compound on my own bike's spark plugs...and I have over 580,000 miles on just airheads, and I use it on my customer's bikes too.  When I do that, it was S.O.P. in my shop to always tell the customer that if anyone ever R/R plugs, that they must account for the use of antiseize, even if they don't apply any, as it works into the head metal somewhat.
 
I've NEVER 'pulled' spark plug threads out of my, nor any of my customer's cylinder heads.  I NEVER have problems removing my spark plugs. 

As you can see, I am IN FAVOR of using anti-seize compound on spark plug threads; and personally feel that if the official torque values are reduced (assuming originally specified withOUT antiseize compound), and you use the antiseize compound, you are LESS likely to have pulled thread problems, heat range will be more consistent, and there will be less problems with hard particles of carbon that deposit themselves into the cylinder head threads, which cause 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. Using oil or WD40 will tend to eventually result in hard carbon deposits in the threads, and thus tend to wear the threads, and tend to change the effective torque.  DO NOT USE OIL NOR WD40! 

Some manufacturer's recommend penetrating oil for tight plugs during removal.   Most fail to tell you to REMOVE that oil before installing a new plug.

NOTE:  ONCE antiseize is used, it tends to work its way into the aluminum head metal....from that point on, it is best to always use antiseize.  

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

READ THIS:
Some spark plugs seem to have a bit less diameter or other irregularities on the threads, and that can promote carbon and wear.   Thread and quality control problems have been reported on Champion and 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, although NGK makes a GOOD PRODUCT and I have NEVER seen a problem with NGK plugs manufacturing.   Bosch MAY plate the threads, which helps avoid use of antiseize, if you are so inclined.  NGK plates some spark plug bodies, and on some they put a coating.  The latest NGK literature, specifically mentions using antiseize:  it should be used on the unplated plugs.  I will get into this much more deeply right HERE:

If you were to access the factory repair manuals for a lot of bikes and cars, you would find that they are NOT consistent with use of antiseize.  Some say you must, some don't say anything, some say you must not.

2010 Honda Pilot:  Honda says to apply antiseize compound.
2007 Subaru Legacy:  gives a torque rating 'without oil on the spar 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 antiseize. 

NGK, the spark plug manufacturer, gives the strongest advice against using antiseize.    They have an entire technical bulletin on the subject.  Condensing it here for you, they do not want the compound used on plated plugs, and they say not to use it ON INITIAL INSTALLATION.   Then they stop talking about the plating, and confusingly say that all their plugs are coated with 'special trivalent zinc-chromate shell plating'
which is designed to prevent both corrosion and seizure, eliminating need for compounds or lubricants.
Apparently, NGK does not want anyone to remove a spark plug unless a NEW ONE is then installed.

Autolite says its plugs are nickel-plated, and says antiseize compounds 'can have a torque multiplying effect...'....but then says nothing else.   However, engineering books from Autolite say things that contradict its other book information, and says you MUST USE a thin-film coating of high-temperature nickel antiseize...and specifies, specifically, certain plugs and problems 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:  use antiseize, and use the 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.

***The exact amount of torque to use when installing spark plugs:
You CAN tighten spark plugs by hand, no torque wrench, once you have a good feel for it, but use of the torque wrench is HIGHLY recommended.    I've seen various forces needed to properly seat the crush rings, so I suspect crush rings vary; and some are not easily removable, so folks don't.  Contrary to many books, I am OK, with NOT replacing the crush rings, IF they are in good condition, and IF you use a torque wrench!!!  Many books say 1/2 turn on NEW crush rings after the spark plug is JUST seating to the head and crush ring; and they may also say 1/4 or 1/3 on used crush rings.  DO NOT depend on this!  I suggest you do NOT do that. I suggest, strongly, that you use a torque wrench.

Since we all know that many of you are cheapskates and will not use fresh crush rings for those plugs where they can be easily removed, the values I am giving here are generally safe values, with new or used crush washers/rings...used with a drop of antiseize spread onto the spark plug threads:

    3/4" reach, 14 mm, about 12-13 ftlbs.  This is the stock top spark plug size.  Avoid going over 14 ftlbs.

    1/2" reach, 14 mm, about 9-10 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. I'd prefer less on the 1/2 reach, say about 8, but this may well not be enough to ensure a
        positive seating. In fact, it may not be enough on an already used crush washer.  You can try.  Be
        careful.  JUST snug is right. You don't want the plug too loose either...if it loosens and rattles
         out, that pulls/wears threads.  I've been able to use up to 12 ft lbs sometimes.

Some have installed even smaller spark plugs for the lower plugs.  I've had good results with 8-9 ftlbs.

Some have installed a welded custom-made thick washer at the lower spark plug threaded hole, that allows the top and bottom spark plugs to both be 3/4" reach.  If you have this style of lower spark plug threaded area, then use the recommendation of 14 max, same as the top plug.

NOTE:  with no antiseize 'ever' used on your heads, you can torque to 16 footpounds on the TOP plugs; and if you have the 1/2" reach 14 mm bottom plugs, you can go towards 12-14.

One is supposed to, in most old literature, use a fresh new crush washer each time a plug is removed and replaced, but we all know that most of you won't, and many plugs have the washers fairly well captive, and finding new crush washers is often frustrating.  Be careful about the 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 and seat, and then go a certain number of degrees more.  This can still be done, if you have a decent feel for it.   I suggest you follow my above advice, and my below advice.

For those who are curious, some manuals have had recommendations on 10 mm plugs for 8-12 ftlbs in aluminum, and 12 mm plugs 10-18 ftlbs in aluminum.  Champion has used such values in their books...and I WELL remember that the 14 mm plugs used on the Mazda rotary engines had a special recommendation of 8-13 ftlbs.   New gaskets, every time, was recommended.   Recommendations for 16 ftlbs for clean and dry threads is my personal limit for 3/4" reach plugs, which are the standard plugs for Airheads.  Clean and Dry here means NO antiseize, or, nothing visible.

Airheads with dual-plug conversions will be more likely to have 1/2" reach lower spark plugs. It is especially important to NOT overtorque those lower plugs.

FYI, if you INSIST on NO antiseize:
14 mm, 1/2", 12-14 ftlbs, DRY, limits.
14 mm, 3/4", 16-18 ftlbs, DRY, limits (note that I use 16 as my personal limit)
12 mm, 1/2", 10-12 ftlbs, DRY, limits.

OFFICIAL BMW Service Information sheet on the late Airheads: 18.4 ftlbs (3/4 reach). You will not find ME using that overly high value, dry threads or not!!  NO WAY use that figure for threads with antiseize compound!!!!

*****The latest Bosch manuals, for 14 mm spark plugs in BOTH aluminum and 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 up to 16 is safe on the 3/4 reach plugs....NOT antiseize coated; but 7 is way 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!!!

Repairing spark plug threaded holes:

It is not uncommon to see damaged spark plug threaded holes in the cylinder heads.   This comes from excessive torque, dirty carboned 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 and an inspection with #1 eyeball, should tell you yes or no.     NOTE that if one EVER uses antiseize at a spark plug, that material can impregnate the threads, and thereafter it is best to continue with anti-seize, and to REDUCE maximum torque.  This is especially so with aluminum, and rather less with steel; and about in the middle with cast iron.

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 is better than commercial penetrating oil products!!   Apply the mixture several times;...allow to soak, even overnight if need be; and 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.

How to clean up and prepare well-damaged threads, and the head can be left in place on the engine:

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 (must be cleaned out later); using a vacuum cleaner in reverse to pressurize the cylinder from the end of the exhaust pipe (intake valve closed). ... Or, a combination of things.  If you intend to use the pressure method, before starting the process of tapping, be sure the exhaust valve on that cylinder is opened and intake closed.   If you are installing a Helicoil with the head still on the cylinder, you may want to use the grease-on-tap method, and the pressure method, both at the same time and wear eye protection.  NOTE that it is critical that the threads be 100% DEgreased before installing the Helicoil, and 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.....BEFORE installing the spark plug!!!  DO NOT FORGET TO CLEAN OFF THIS EXCESS UNCURED LOCTITE!  You do NOT want to Loctite your spark plugs into the Helicoil!  The heads get pretty hot, and that Loctite only gives a small amount of holding power when the head is hot.  In an emergency situation, or if you don't want to wait a few days, or?.....just clean the threads and install the Helicoil, and ride away.

It is VERY important to have a guide machined to allow exactly 90° drilling AND a guide machined for the the tap to start threads dead square to the surface.  Guides should be used for both drilling and tapping.
Tap handles often have a centering recess in one end, helpful with some jobs that can lay flat.

Here is a method to try if the threads are not too bad:
Be sure the piston is not anywhere's near fully outwards.  Take a small piece of cloth, and roll it up to fit moderately tightly in the spark plug hole.  Soak this in penetrant, then install in the hole, 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.  Keep the tap quite squarely to the surface. Back the tap out often. Use a bit of White Lead or good cutting compound meant for aluminum materials, on the tap during the thread reforming.  You can use antiseize 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 and dry.  THEN I put a dab of antiseize.....not much... on the first 4 threads (from the spark plug electrode tip end) before installing the new spark plug; and I then torque properly. Do NOT spread a lot of antiseize onto plug threads.  You want to totally avoid getting any on the ceramic tip area, where it WILL electrically short circuit the plug.    

Resistor caps:
From the Airheads LIST:  "If you have non-resistance caps, wouldn't it just be simpler to use a Bosch resistor plug for $0.99 from Napa so you still get your 5k Ohms? ......"

to which I replied:
There are reasons I do not recommend resistance plugs from ANY manufacturer.  'Resistor' plugs were originally made so as to reduce the current in the spark and leads, and thereby reduce Radio Frequency Interference (RFI).     It is possible that a reason might have been spark duration time too....as resistance will slightly increase the length of time the spark lasts, allowing better igniting of the fuel-air mixture...up to a point, and then the energy level during part of the striking arc falls off, and performance suffers.  There are also arguments against this theory of spark duration.

Some types of RFI interfere with other things, such as electronic ignitions, and electronic tachometers, sometimes in strange ways.   In our 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 and thereby getting to the Hall element in the canister on 1981+ models....it can destroy the part.    
You will find the same cautions for various reasons in such as the Boyer Microdigital ignition, etc. 

 If one uses resistor plugs and non-resistor caps, there is a danger that you might install non-resistor plugs later on, and injure your electronic ignition.   ALSO, if you install both resistor plugs and resistor caps, your spark can become too weak to reliably fire the mixture, particularly in leaner running bikes.   This has been seen with 5,000 ohm caps and resistor spark plugs.

The proper resistance in the circuit will ensure the proper ENERGY level (AND the LENGTH OF TIME the spark lasts; if you believe that theory).  The coil(s) and engine were designed with THAT in mind, amongst other things.   If the spark lasts too short a period of time, the ignition will not be as good, per some theories.  Same for too low energy, which can be reduced by excessive resistance.    The length of time the spark lasts is called Duration.

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 all these ...and only on points models, from 5000 back to 1000 ohms MIGHT give some extra ignition performance, in the highest rpm area...near redline+-.....and probably in starting in cold weather; and in marginal situations.   Thus, 1000 ohm caps are OK, in my opinion, for POINTS models.  In fact, I recommend the 1K caps for POINTS models.

 The 1981+ models have more energy coming from the coil(s), and are designed for and easily handle the 5000 ohm caps...and anything under 2500 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 and NO resistor plugs!

SOME 'resistor' spark plugs are not true resistor plugs...they have a small coil (possibly of nearly resistance-free wire), wound to act like an electronic 'choke'...or, inductance device.   The electronic ignition may NOT like it, and 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.

Original type Beru metal cap spark plug caps; wires, and other bits, assorted styles, are available from:  www.euromotoelectrics.com

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 dealers.   Bosch has told me that the nominal value for the resistors in their plugs is going to be 2000 to 6000 ohms.   At this time I can NOT recommend, and in fact I specifically caution against, using Bosch "resistor" plugs in 1981+ models, or any models you have converted to electronic ignition. I do NOT like to see ANY resistor plugs at all in Airheads.   You COULD use resistor plugs from Bosch if you removed the resistor spark plug caps and installed zero ohm caps (on POINTS models ONLY).  The worst thing about what Bosch is doing is that they have, purposely, or accidentally,....put these resistor plugs in the older small cardboard boxes.  If you purchase a Bosch spark plug (they DO make quality plugs! the German made ones anyway), check the box contents...read the number off the plug metal body itself, do NOT depend on the box numbers!  NOTE ESPECIALLY, that Bosch has been going to a 4 digit part number for plugs.  There is no good way for you to tell if the plug is a resistor type, or not, from that number, especially if it is the only number ON THE PLUG.    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, and with the old part numbers on them.
 

NON-projected nose platinum (or other precious metals) spark plugs MAY foul, misfire, and their gap arrangement is not the best for ANY airhead. Later style projected-nose platinum plugs with more than one surrounding ground electrode are a waste of money and may well NOT give reliable ignition...they can, at a minimum, occasionally misfire.  Single ground electrode, projected nose, platinum plugs are OK, but a WASTE of money, and there is some indication that the 'fine electrode' platinum plugs are NOT as good as the STOCK types.  Of course, this is contrary to manufacturer's advertising....nothing new about that sort of thing.   The main reason is the work that BMW did to ensure that the incoming charge is swirled and passes 'just so' at the spark plug.  Save your money, DO NOT buy specialty spark plugs, not even those from Bosch.... but DO purchase Bosch or NGK standard spark plugs; NON resistor types. Avoid Champion spark plugs, I don't like the threads.    ****The latest NGK Iridium spark plugs are constructed with a double shelf, or call it a tiny step in the insulator side near the end.  This offers a secondary spark area, that also cleans the center area.  PRELIMINARY tests CONFIRM NGK's claims.  I have yet to do dyno tests on any BMW bike with this type plug. 

Do NOT use resistor plugs....if the part number on the spark plug body has an R in it, it is likely a resistor plug. Some plug makers, including Bosch in its new numbering system (which I dislike, they offer no information by number as to head range nor size).... do not use that R, simply using an all numbers no letters part number, so BEWARE!  If you INSIST on using resistor plugs because you are too lazy to wait for an order for the non-resistor types, and use the stock resistor caps, your ignition MAY be a bit weak, but I don't see any problems beyond, MAYBE, hard starting and occasional misfiring...depending a whole bunch on engine and electricals condition.   You may not even notice any misfiring...but your fuel mileage WILL suffer.
 

Using spark plugs that have an extended nose/tip is important for proper combustion on an airhead and K bike engine.  

ELECTRODES:

This is 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; or, that have VERY powerful ignitions.  Years and years ago there were almost no such ignitions, and the usage of long lasting precious metal spark plugs was for industrial pumping engines, etc.   More recently, especially in the nineties and to the present, many cars have manufacturer's recommend plug changes as late as 80K...or even 100K miles.   This is primarily due to the use of UNleaded fuels, and somewhat to the cleaner burning of fuel injection.  Many modern cars are running somewhat higher rpm, more sparks per mile, and using very high powered ignition systems....that is harder on spark plug electrodes; they can erode away.  For these vehicles, I almost always recommend you use the manufacturer's stated make and model of spark plug.

In the old days, before the change to unleaded fuel in the eighties, spark plugs HAD to be replaced around 15,000 miles due to leaded fuel fouling.  Those fuels are now long gone; and, the present additives, which do deposit somewhat on the spark plugs, are vastly less inclined to electrically foul the spark plugs.  Engines are also 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 a bit cleaner...but eat spark plug metal a tad faster in some situations.  Because modern engines are fuel-injected and burn as lean as possible and very close to optimum at nearly all times; they need high powered ignition systems.  Most cars have 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.   It is true that nearly all cars have series resistances in the ignition leads...but that only cuts down the current some, and it also makes the spark and plug last longer....spark duration is the word.   Spark duration due to resistance is not totally accepted by all folks or engineering nerds.

High powered ignitions eat electrodes, particularly if the resistance in the circuit is 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 done.....the net result IS an improvement in lowering over-all electrode wear.   There are also problems with the cars that are using one coil per two cylinders...they need increased electrical energy.   So, modern cars CAN be hard on spark plugs....and CAN, often, use precious metal plugs to advantage:
1.  The precious metal lasts longer, so gaps are more constant, erode less.
2.  The precious metal plugs typically have smaller central electrode, leading to higher temperature of the center electrode being possible and somewhat easier spark start when the engine is cold or hot.

One method of reducing the rate of constantly increasing spark gap is to use precious metals, which, for various reasons, do not erode so quickly. Higher resistances, such as the use of 5K caps are helpful in reducing current (not voltage), which reduces metal erosion. 

  I see no reason to pay premium prices for an 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 over 40,000 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.  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.

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 models had X5D, then X5DC, and then XR5D) to a dual ground electrode type XR7LDC.  This XR7LDC was adopted by BMW dealerships for prior K models.   The two 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 in practice. The thin center and dual grounding electrode plug lasts longer, and 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 4163 inexpensive spark plug is an example of this substituting.

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, 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 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 MUCH WORSE than stock.   I have pulled multiple shrouded plugs (Bosch +4 Platinum's) out of airheads because they work lousily....and I can quote other examples.  The bottom line, for AIRHEAD owners, with stock or near stock engines (increased compression ratio or dual plugs is considered near stock by ME), is to use single ground electrode spark plugs, and use of the larger center electrode plugs works fine.
 

NOTE:  On dual-plugged heads it is common to use a lower spark plug that is one heat range step hotter, as for various reasons the lower spark plug may tend to carbon-up, otherwise.

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
             and 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, and the 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 and identified.   The above is the best information I have at the moment....but, see the CHART below.

Equivalents, Bosch and NGK     (Snowbum has charts crossing over almost every make of spark plug, even old English types, even back into the 1920's...just ask!)
Bosch Plugs with 'BC' are the 1/2" reach for BOTTOM plugs in those dual-plugged bikes that take 14 mm size; DC are 3/4" reach for top plugs.
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 and 7 series being covered by the NGK 6 series.
Warning!.....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"...no matter what Bosch or a salesman says!
WARNING!....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!!
 

Spark plug heat ranges are not 'exact' oftentimes when trying to cross one manufacturer's number with another.   Complicating this is that spark plugs were improved over the years, and a single part number may cover an actual slightly wider heat range.

Bosch has, on occasion, put their 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...open it and see what is printed on the plug base metal.  I'm anal enough to use an ohmmeter on the center electrode!

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.

Airheads spark plug gaps:
 

Spark Plugs for Classic K bikes (K1, K75, K100, K1100, all versions of these models):

(you can add K1200 here too, see text 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, so keep THAT in mind, you might have only one spark plug wrench size, and the larger size may not fit well with even your proper wrench anyway, considering the size of the casting 'hole' area around the spark plug.

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, and XR7LDC which was the last update, and actually was for the K1200, but is applicable to the earlier K models.  The XR7LDC has a dual grounding electrode.   "R" in a plug number means it is a resistor type.  BMW K bikes had 5KΩ spark plug caps.  The K75 bikes had a GAP in the spark plug lead, thus cannot be easily measured for the spark plug cap resistance.   Except for the XR7LDC, the plugs should be gapped (and 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 actually approximately or nearly the same heat range as a XR5DC, in actual use.  This situation was not at all common years ago. 

NGK makes a 12 mm, 3/4" reac, 5/8" hex, sparkplug.   Model DCPR7E, new number 3932, has removable terminals, tapered, shelf'd insulator, unsure of gap they come with.   I have not yet done testing on it. 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.

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

There is absolutely nothing wrong with using the original non-resistor, large electrode, inexpensive type of spark plugs in a Classic K bike!!!

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

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:    Here is a link to NGK's article on the caps, and wire assemblies.  It is much more detailed than the above:
http://www.ngksparkplugs.com/docs/Resistor_Covers.pdf

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 moulded, 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....I have almost never seen a failure of the resistor element inside them.

Revisions:
04/22/2003:  clarifications and emphasis here and there.
08/09/2003:  edit for 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 and wires, etc.
01/11/2007:  update with latest information, and add the NGK chart
03/25/2007:  add equivalents, Bosch and 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 automotive-tradition.de
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.
04/01/2012:  Clean up more of article.  Clarify that it 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.
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, and REAL industry information, that
                    is not, seemingly, read by those who have FAULTY reasoning on the subject.
                   
 

© Copyright, 2013, R. Fleischer

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