Spark plug threaded holes.
Tightening torques. Gaps.
ANTISEIZE... OR NOT?
Spark plug caps.
Resistor & non-resistor spark plugs.
© 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 LEFTMOST spark plug is appropriate for Airheads; it has a SINGLE GROUND ELECTRODE and a STANDARD SIZED CENTER ELECTRODE.
The middle spark plug, Bosch XR7LDC, has two ground electrodes, and can be used on 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 and right-most spark plugs have 2 or more ground electrodes and versions can have standard or a fine wire center electrode. BOTH of these have NO USE in Airheads, and NOT in Classic K bikes either 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.
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, and I am in total agreement.
After any spark plug has been in use, the metal 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 they have broken off and caused engine damage.
I agree, and my advice is: Do NOT re-gap plugs after they have been in use! IN USE here means that the engine has been run.
Note that as miles accumulate, the gap in spark plugs increases. This is almost always 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 spark plugs with precious metal tips are less likely to have this problem with advancing miles and the spark will find it easier on them to jump to the ground electrode(s), increasing the life a bit more. 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 and actual mixture burning conditions, type of ignition system, whether resistor spark plugs or not, ETC. Spark plugs GENERALLY last a very long time now, due to the use of UNleaded fuels. In the distant 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. They CAN last much longer now. For the Airheads, gaps even approaching .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 the gap, and MAYBE adjust it, on a BRAND-NEW, NEVER RUN PLUG.
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 combustion byproducts and carbon. Carbon from such as oil burning, or, very bad use of oil or grease on the threads, can get very hard, and very abrasive. It can cause the spark plugs to injure the cylinder head threads during removal. It is NOT a good idea to put such torque on a spark plug when attempting to remove it. Best to use a lubricant that seeps-into the threads. A mixture of 50% acetone in 50% Automatic Transmission oil, works nicely. In egregious instances, this can take a day or two.
Spark plugs screw into threads that are, especially at the inner-most end (of the head, and the plug), exposed to combustion heat and pressure and byproducts of gasoline and oil. Because of that, carbon tends to accumulate in the threads. As the threads wear from use, more carbon accumulates. Combustion is not the same in all Airheads, some have a fair amount of burning oil or carbonized oil, accumulating in the threads. As the spark plugs are installed/removed with carbon in the threads, the carbon can act like a pretty good abrasive. It also can act like a cement!
If reinstalling a used spark plug (BTW...never re-gap one) I like to clean the threads. If the threads in the head are carbon'd up, I may go so far as to rotate the engine to exhaust valve open, and 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. I've also used brass gun cleaning brushes. I've seen some spark plug threads that were, brand-new, not made all that nicely. Poor spark plug threads can damage the head threads.
Some BMW motorcycles PRIOR to the /5 series, I will just call them /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 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 foot-pound wrenches might have a rather subtle click when set much lower, say around 12 foot-pounds, cheaper ones possible even at higher settings. 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, especially YOURS, 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 Helicoils 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 a Helicoil type insert IS installed, do consider problems if not done correctly. 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 recommend against Time-serts, which can change the spark plug heat value required.
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 that....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. It is CRITICAL that the drilling and tapping be done squarely. ABSOLUTELY AND ALWAYS use a drill guide & tap guide!
MORE information on tapping
and installing Helicoils, head on, or head off the engine, is later in this
DO NOT USE GRAPHITE PRODUCTS FOR COATING SPARK PLUG THREADS. A corrosive type of reaction is possible on the aluminum head.
DO NOT USE GRAPHITE PRODUCTS FOR COATING SPARK PLUG THREADS. A corrosive type of reaction is possible on the aluminum head.
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 from those ham fisted about torquing things. 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. NOTE, however, that 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.
NOTE: Karle Seyfert, an expert who writes 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, 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 and pull 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.
You should be using a torque wrench, AND lowering the official torque value (which is withOUT antiseize compound unless specifically so-stated). Since popular anti-seize compounds act like a lubricant, the applied torque (if that book value means NO antiseize) MUST be reduced...about 20-30% reduction is 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 supposed to be used with regular anti-seize compounds, as they
are degree-tightened, and so the manufacturer typically makes a blanket
statement not to use anti-seize compounds, rather than providing full
information, which is more effort for them, and, frankly, the full information
is a bit complicated.
manufacturers say OK, and give the needed lower torque
values. Some manufacturer's do not mention use of anti-seize at all, and
SOME SAY YOU MUST USE IT.
Some manufacturer's also 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. Frankly, that IS possible, particularly
if the torque used is not correct, but with torque being reasonable, only a tiny
bit of a single heat range would be the change (at
worst case, otherwise none), and even that would very seldom 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 only close to
I have used some sort of anti-seize compound on my own various bike's spark plugs...and I have over 650,000 miles on BMW's, and I used it on my customer's bikes too, and on other brands. It was standard operating procedure (SOP) in my shop to always tell the customer that if anyone ever removes and replaces spark plugs, that they should account for the use of anti-seize, even if they don't apply any, as it works into the head metal somewhat. NO PROBLEMS...EVER. 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.
I'll expand all this a bit:
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 anti-seize compound), and you use the anti-seize 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, otherwise, can cause WEAR every time you R/R a spark plug. This is important, and if you have a two-stroke bike, it is even more important (but, important for BOTH).
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 (stuck/frozen) plugs during removal. Most fail to tell you to VERY thoroughly REMOVE that oil before installing a new spark plug.
NOTE: ONCE antiseize is ever used, it tends to work its way into the aluminum head metal....so, from that point on, it is best to always use anti-seize compound. Do not use graphite types. These 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, and that can promote carbon or 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. NGK makes a GOOD PRODUCT and 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, and on some they put a coating. I still use anti-seize on all of these.
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 right HERE:
If you were to access the factory repair manuals for many bikes and 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, and regarding taper seat plugs, etc., well above. 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
Chevy Cobalt says to NOT use antiseize....because if you DO, you will damage the cylinder head IF YOU
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.
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), and they say not to use anti-seize compound 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. 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, and 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...IF...you insist on re-installing an NGK spark plug, even if never actually having run the engine, you MUST use anti-seize compound. Once you unscrew the plug, the NGK super-thin 'plating' coating is, in essence, partially removed permanently, each time removes more, and the plug can then have galling and other problems.
NGK says nothing 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, and 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, and says you MUST USE a thin-film coating of high-temperature nickel anti-seize...and specifies, specifically, certain plugs and 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 antiseize compounds.
amount of torque to use when installing spark plugs:
1. 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.
2. I've seen various forces needed to properly seat the crush rings, much of this is because crush rings vary in how they are made; and some are not easily removable from the spark plugs, 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.
3. I suggest, 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.
Since we all know that many of
you are cheapskates and will not use fresh crush rings for those plugs where
they can be removed, 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 THINLY onto the clean spark plug threads (fingers work
well for this, you can push the stuff very thinly into the threads). You need NOT cover every thread....just a wee bit towards the electrodes end (combustion chamber ends), pushed into the threads with a finger tip...again, a WEE BIT....is enough, and as the plug is screwed-in, it is spread into the higher threads. I personally coat, VERY thinly, all-around. VERY VERY thinly!!! I use my fingers, or a tiny brush with fairly stiff bristles. The shortened-bristles end of a common 'acid brush' is perfect.
3/4" reach, 14 mm, about 13-15 ftlbs. This is the stock top spark plug size.
1/2" reach, 14 mm, about 10-12 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 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 my recommendation is about 13-15 ftlbs.
NOTE: with no antiseize 'ever' used on your heads, or exceptionally clean threads, 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....for non-welded-washer situations.
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 and seat, and then increase a certain number of degrees...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: wee bit of antiseize, torque wrench, proper torque.
Clean and Dry (NOT with anti-seize compound):
Recommendations for 16 ftlbs for clean and dry threads is my personal limit for 3/4" reach plugs, which are the standard top plugs for Airheads. Clean and Dry here, means NO antiseize, or, none visible.
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 & clean threads or not!! NO WAY should anyone use that figure for threads with anti-seize 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 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.... and 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 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 and an inspection with #1 eyeball, should tell you yes or no. NOTE that if one EVER uses anti-seize 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 inserts; and about in the middle with cast iron heads.
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; 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 reverse pressure method, before starting the process of
tapping, be sure the exhaust valve on that cylinder is opened and
intake closed. If you are pressurizing from the intake, then the exhaust valve must be open.
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. That usually works perfectly. 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 (treat the threads with a very 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, 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, and continue with the tool dead square. 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....you are unlikely to use that feature on the heads.
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, and 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 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 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 and 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; 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.
NOTE: 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 shaving cream
foam, or too much air movement to blow out the chips.
NOTE!....If using the pressure method, be SURE to wear eye protection...chips WILL likely be flying out the spark plug hole.
There are reasons I do not recommend resistor spark plugs from ANY manufacturer for BMW Airhead motorcycles. The main reason to use 'Resistor' plugs is to reduce the current in the spark and leads, and thereby reduce Radio Frequency Interference (RFI) levels. 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 (plasma)duration, and one gets into electrical spark characteristics in some esoteric ways, so I won't go further with that, here. There can be real problems with spark plugs that contain "resistors". I'll get into a bit later on.
Suffice it to say that a real resistor is incorporated into the stock or NGK aftermarket spark plug caps. Values of 1000 or 1200 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+).
Some types of RFI can affect 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. (4) Interference with the tachometer.
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. 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 at the same time. Yes, I am aware that BMW does it on some K bikes.
The proper resistance in the circuit will ensure the proper ENERGY level (AND the LENGTH OF TIME and heat component of the plasma spark; if you believe those theories). 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 a spark lasts is called Duration. Duration has nothing directly to do with DWELL....they do not describe the same thing.
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+-.....and probably help in starting in cold weather; and in other 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 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 and NOT to use resistor plugs!...although you may well get away with using both, without a degradation in performance.
>>>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, and may be so wound to incorporate a capacitance effect as well. 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.
You can measure the plug, if curious....with an ohmmeter.
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
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 (not sure for how long THERE). 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,....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...read the number off the plug metal body itself,
depend on the box numbers! NOTE ESPECIALLY, that Bosch has gone 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 just that number, unless you KNOW, and 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....but I am not going to try to keep up with that. Just look at the markings on the spark plug metal body. NO R allowed in the part number...if it is a conventional old Bosch number.
NON-projected nose spark plugs MAY foul
and/or misfire. Platinum-tipped or other precious metal tipped plugs ..very
especially 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 and even if you don't
feel it in the engine performance, they can waste fuel.
Single ground electrode, projected nose, platinum-tipped plugs are OK,
but a WASTE of money, and there is some indication that these '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 for these various things is the engineering work that BMW did to ensure that the
incoming charge is swirled and passes 'just so' at the spark
Save your money, DO NOT buy specialty spark plugs, not even those
from Bosch.... but DO purchase Bosch-German or NGK standard spark plugs;
NON resistor types. Avoid Champion spark plugs, I don't like
the threads I've seen on some of them.
****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 and wasting fuel...depending a whole bunch on engine and electricals condition. You may not even notice any misfiring...but your fuel mileage WILL or MIGHT suffer.
Using spark plugs that have an extended nose/tip is important for proper combustion on an airhead and K bike engine.
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; and/or, that have quite 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 the cleaner burning of fuel injected engines. Many modern cars are running somewhat higher rpm, more sparks per mile, and using very high powered ignition systems....that can be harder on spark plug electrodes; they can erode away faster. For these vehicles, I almost always recommend you use the manufacturer's stated make and model of spark plug.
Those of you that no longer have or use leaded fuels, AND use FULL synthetic oils (and in some instances part-synthetics), may well find that your spark plugs can last 60K miles in your bike. DO NOT bend the ground electrode, the plugs will work fine even up to even 0.040"+- gap.
Some of the new spark plugs with two grounding electrodes come pre-gapped, and the gap is wider than your manual says to use. Leave the gap alone. Example is the plug that BMW started specifying for the K1200 models: XLR7DC
In the old days, before the change to unleaded fuel in the eighties, spark plugs HAD to be replaced around 10,000 to 15,000 miles due to leaded fuel fouling. Mechanics would often sand-blast the plugs in a special blasting machine, and get more life. Leaded fuels are now long gone in the USA; and, the present additives, which do deposit somewhat 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. 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 that don't have a coil AT each spark plug, have series resistances in the ignition...that reduces the current , and it also makes the spark have a bit better duration and spark plugs last longer. Spark duration due to resistance is not totally accepted by all folks or engineering nerds.
High powered ignitions can 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-tipped 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 initiation.
One method of reducing the rate of constantly but slowly increasing spark gap, due to spark/plasma erosion, 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 60,000+ 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 K models had X5D, then X5DC, and then XR5D) to a dual ground electrode type XR7LDC. This XR7LDC has been adopted by many 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, by MAYBE half a
step, in practice. This 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
The XR7LDC comes pre-set for gap. It is wider than you are used to. Do not change the gap!!!
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....& 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 extended nose spark plugs, and use of the cheaper larger center electrode plugs works fine.
NOTE: As noted earlier....on dual-plugged heads... it is somewhat common to use a lower spark plug that is one heat range step hotter, as for various reasons the lower spark plug may otherwise tend to carbon-up faster.
VERY OLD Bosch numbers:
Bosch made these spark plugs, used on
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;
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.
Bosch no longer has the very old type numbers in their catalogs. Bosch has recently transitioned to 4 digit numbers, see the chart well 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.
Bosch and NGK
(Snowbum has charts crossing over almost every make of spark plug, even old English types, even back into the 1920's...)
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, early||Bosch||BC/DC Bosch||Bosch number series||NGK, older||NGK with removable nut||NGK withOUT removable nut||comments|
|W5D||W5DC||7591||BP7ES||1034||3995||W5 covers both W225T30 and the colder W230T30|
|Higher numbers are colder. That is, W230 is colder than a W175.||Higher numbers are hotter||Bosch plugs above ending in C cover a slightly wider heat range than the non-C and early Bosch to the left. Higher numbers are HOTTER||Be careful with these types of numbers, as you want to be SURE the ACTUAL spark plug, no matter what the carton has printed on it, is a NON-resistor. See note in paragraph after this chart.||Higher numbers are COLDER|
Bosch has, on occasion,
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
I'm anal enough to use an ohmmeter on the center
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:
Gapping brand-new plugs, NEVER EVER RUN, use 0.6
mm, 0.024" (preferred); ...to 0.7 mm, 0.028".
If they need gap setting, be very careful not to damage the central insulator. The place to put the bending pressure is close to the central electrode/insulator, but do NOT pressure the white insulator. Use the
proper tool, typically it is part of the spark plug wire gauge tool, which has hardened wires for measuring
the gap, and gives the best results. Plug gap will open up as the miles accumulate and the ground electrode will dish a bit. Dishing can be the other way, if the ignition coils are wrongly connected. Electrodes tend to erode very slightly faster with 1000 ohm caps as used on the old points bikes, but those tend to have better ignition, as opposed to 5000 ohm caps. I mean POINTS models ONLY, to be very clear here. Once worn, the gap MUST be measured with a ROUND WIRE gauge. My quite old recommendation was that if plug is 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 I now think that wear up to 0.040", and possibly a bit more, is OK. Use a wire gauge meant for checking spark plug gaps. Don't use a flat feeler gauge.
I am aware that some books say to use or set gaps to 0.7 mm (0.028") and some literature says that up to 0.9 mm is OK for late eighties to 1995. DO NOT gap-set previously run spark plugs.
Due to leaded gasoline having gone bye-bye,
spark plugs will last a lot longer than they used to. 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, and many have gone 60K, some more. 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 .040" generally, but perhaps slightly more); and the ground electrode is
NOT dangerously thinned.
****Once a spark plug has been in an engine and run ...EVER, ... DO NOT EVER adjust the gap. The reason is that ground electrodes in the spark plugs change their metal structure once heated, and bending them can weaken them, and there is a possibility, although this is quite rare, that the electrode will break off and damage the engine.
Spark Plugs for Classic K bikes (K1, K75, K100, K1100, all versions of these models):
(you can add K1200 here too, see text above, 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, so keep THAT in mind, you might have only one spark plug wrench size, and the larger size may not fit well with your tubular wrench, 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 as 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 nearly the same heat range as a
XR5DC, in actual use IN THE K BIKE.
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. 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. NO TESTING INFORMATION BY ME YET.
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!!! 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 Autolite 4163.
When installing 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.
Do not use the stuff on the electric contacts. This caution applies to the
rubber cover (boots) at the spark plug cap, and to the rubber boot at the coils.
If the coil electrical fitting end of the wire is tight-fitting into the coil
internal metal contact, it may not cause a problem, but do it correctly anyway.
The application will make removing the boots easier, and prevent electrical
contact corrosion from the elements, etc. You can coat the spark plug
white outer area a bit too.
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
Here is a link to NGK's article on the caps, and wire assemblies.
It is much more detailed than the above:
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.
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
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
undated, 2012: remove indexing section.
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.
sometime in 2013: Remove language button, as the 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, and REAL industry information, that
is not, seemingly, read by those who have FAULTY reasoning 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.
© Copyright, 2013, R. Fleischer
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