SPARK PLUGS;
Spark plug threaded holes;
torques; ANTISEIZE OR NOT;
spark plug caps; resistor & non-resistor spark plugs

sparkplugs.htm-32

©

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

The earlier BMW airheads,...let me just call them the /2 bikes, had some metal alloy and casting problems and very careful torquing, 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 considerably better.  There are always  problems, and these problems occur in several ways.  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 some 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, eventually the cylinder head threads start distorting and pulling out if they don't immediately rip out. While Helicoil installation, or welding, or other inserts, all do work OK, I am NOT a great fan of doing these things... and it is usually avoidable if one takes some extra care in the first place.  I don't like to see extra places for sharp bits of carbon combustion products to adhere to (they create hot spots in the cylinder heads) from using these 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 valves both open at the overlap period (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. ALWAYS use a drill guide and tap guide!

The use of antiseize is somewhat controversial, mostly I think this is due to SOME spark plug manufacturer's literature.   SOME have changed from recommending anti-seize, to not recommending it, or just ignoring the use, over the years.  I will explain what is going on:
 

Reasons to NOT use antiseize compounds:  

Some experienced mechanics feel that if antiseize is used it is easier to overtorque and pull threads.  My answer to that is 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 official applied torque MUST be reduced...not quite 1/3 reduction is roughly correct. SOME manufacturer's are
concerned with SOME types of spark plugs, such as taper seat types, and ....so they make a blanket statement not to use antiseize compounds. 
Other manufacturers say OK, and give the needed lower torque values.  Some manufacturer's 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. I have NOT seen this in real life situations if the
torque is set properly. I have always used some sort of antiseize compound on my own spark plugs...and I have close to 540,000 miles on airheads. 
I've NEVER 'pulled' spark plug threads out of my cylinder heads.  I NEVER have problems removing my spark plugs.
As you can probably see, I am IN FAVOR of using anti-seize compound; and personally feel that if the official torque values are reduced, 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. ONE thing you do NOT
want to do is use OIL!!!...and that includes WD40.....those will carbonize, and cause thread damage over a long period of time.  So, NOTE!...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.   

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.  

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.  Bosch tolerances seem to be better, and Bosch MAY plate the threads,
which helps avoid use of antiseize, if you are so inclined.  NGK spark plugs are known to be of good quality.

Spark plugs that, upon a wee bit of loosening, seem to then tighten up appreciably, should be oiled...a penetrating oil (or Kerosene!)...allowed to soak,
even overnight if need be; and  possibly carefully tightened a bit first, if that is not overtorquing, and then removed carefully to avoid damaging the
threads. I have had to clean up carbon'd threads MANY times...a good fresh sharp 14 mm tap works wonders, and I use a bit of White Lead on the
tap during the thread reforming.  In many an instance the tap will reform the threads, rather than cut them (it may do both) on the soft aluminum.   I
then thoroughly clean the threads, leaving the threads clean and dry.  THEN I put a dab of antiseize.....not much, on the spark plug threads, when installing the new spark plug; and torque properly. You do NOT have to spread a lot of antiseize onto a plug threads.  You want to totally avoid getting any on
the ceramic tip area, where it might electrically short circuit the plug. A wee dab, droplet, on a fingertip, spread into maybe the first 4 threads at the
tip end.     If you are going to use a tap to clean the threads in the cylinder head, I recommend you first insert some cloth (or??), rolled up to spark plug thread diameter, soaked in kerosene or other very slow evaporating petroleum solvent, and insert into the threads.  Let it sit overnight, before you use the lubricated tap.   This may well help soften the carbon and reduce wear on the threads.  I usually use cloth type shotgun cleaning tips for this.

It is VERY important to have a guide machined to allow the tap to start threads dead square to the surface...unless you are quite careful about it. 
If using a long tap, don't have the piston at top dead center!

***The exact amount of torque to use:
    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.  Since we all know that many of you are cheapskates and will not use fresh crush rings, the values I am giving here are generally safe values, with a drop of antiseize spread onto the spark plug threads:
    3/4" reach, 14 mm, about 12 ftlbs.  This is the stock top spark plug size.  Avoid going over 14 ftlbs.
    1/2" reach, 14 mm, about 9 ft lbs; this is the commonly used bottom spark plug size on dual-plug conversions.  This torque value is USUALLY just 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 a 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 10 to 12 ft lbs sometimes.
Some have installed smaller spark plugs for the lower plugs.  I've had good results with that same 8 ftlbs.
Some have installed a welded thick washer at the lower spark plug, 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 is 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.

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.   

Airheads with dual-plug conversions will likely 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.
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!!

MORE!

 I will use the Bosch manual information here as an example:  

    For removal.......you are supposed to loosen the spark plug a little bit, and if then becomes tight, use some thin oil or kerosene on the partially exposed thread, screw in, and then out after a few minutes. I say overnight if a problem.  Bosch's instructions for installation were:  "'use a torque wrench....or, if not available...hand tighten....until seated....then an additional 90°...avoid over or under tightening'".  Bosch does not point out, I will, that this is for a brand new plug and NEW, obviously never crushed before, gasket. ((I heartily recommend you watch out for overtightening, using that 90° method withOUT a NEW FRESH crush washer is VERY hazardous to your heads)).  I am hesitant to recommend you use Bosche's recommendation to use 90° on a NEW crush washer...it may well be too much.

Previous recommendations in SOME spark plug manufacturer's books has been as high as 16-18 footpounds for our 3/4" reach spark plug torque.  That is NOT a good idea, in MY opinion, for Airheads.

*****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 or antiseized!!! Unfortunately, no further information about the thread length, is given.    MY opinion is that 15 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 manufacturer's recommend penetrating oil for tight plugs during removal.   Most fail to tell you to REMOVE that oil before installing a new plug.

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!!!

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 unless a Helicoil is being installed, it is best to continue with anti-seize, and to REDUCE maximum torque.  

    The best repair for severely damaged spark plug holes is to remove the head, install a Helicoil.   Since many folks want a quicker way, here are some methods of repairing, that may work for you withOUT removing the head:
         Using a sharp tap, and very slowly and carefully, and quite squarely, rethread the hole, backing out the tap often.  This may re-form the threads into some sort of usable condition.   To prevent any chips from getting into the cylinder, there are various ways.  These INclude using a lot of grease on the tap; 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.  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 ever installing a spark plug!!!
In an emergency situation, just clean the threads and install the Helicoil, and ride away.

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 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.

 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.

1.  The proper resistance in the circuit will ensure the proper ENERGY level AND the LENGTH OF TIME the spark lasts.  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.  Same for too low energy, which can be reduced by excessive resistance.    The length of time the spark lasts is called Duration.

2.  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 went to 5000 ohms.  A change on 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.  Thus, 1000 ohm caps are OK, in my opinion, 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.  

3.  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.
 

WARNING!!
Bosch has been dropping the availability of NON-resistor spark plugs.  I expect, per what Bosch technical support folks have told me, that in the NEAR future you will not be able to purchase Bosch spark plugs withOUT resistors (MAYBE BMW dealers can supply them...MAYBE).   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.  This is especially so in 1981+ models, or any models you have converted to electronic ignition.  This does not include those points models with just a points booster amplifier; although I do NOT like to see 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); but I especially do NOT recommend this for 1981+ electronic ignition models or any with other types of electronic ignition...for the reasons stated.  The worst thing about what Bosch is doing is that they have, purposely, or accidentaly, or?....put these resistor plugs in the older small boxes.  If you purchase a Bosch spark plug (they DO make quality plugs!), 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, unless you see the box stating the older Wxxxx part number, which will have an R in it if a resistor type.  BUT, even this is NOT adequate, look at the spark plug metal body stamping itself!!!  There is more on this box problem later in this article!

Special spark plug electrode styles and recommendations:
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 electrodes 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.    I can't offer information on other brands at this time.

Depending on your particular model of year, engine, and modifications, you will typically have 14 mm 3/4" length spark plug threads, probably Bosch type W5DC to W8DC for the top and if dual-plugged, use the 1/2" length types W5BC to W8BC for the bottom (unless you have the welded washer modification, allowing the DC for bottom plugs).   If you have smaller than 14 mm lower plugs, use the proper equivalent head range types.   The heat range will depend on your bike CR, carburetion, and many other factors. Use stock gaps, about .024" to .028".  Be careful, some Bosch plugs have different gaps than needed for our Airheads...esp. the WR line.   

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.
 

EXPANDED :
This is an expansion on what is above.
This is edited from something I posted to the K-bmw list.

Where precious metal (iridium, platinum...) spark plugs shine 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 no such ignitions, and the usage was for industrial pumping engines and so on.   More recently, especially in the later nineties and to the present, many cars have manufacturer's recommend plug changes as late as 80K...or even 100K miles.    Many modern cars are running somewhat higher rpm, more sparks per mile, and using very high powered ignition systems....that is even harder on spark plug electrodes.  For these vehicles, I almost always recommend you use the manufacturer's stated make and model of spark plug.

In the old days, spark plugs HAD to be replaced around...roughly...15,000 miles due to lead fuel fouling.  Thos fuels are now long gone; and, the present additives do NOT electrically foul the spark plugs (or, are vastly less inclined to do so).  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 eat metal a tad faster.  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 many cars, if not most, have series resistances in the ignition leads...but that only cuts down the current somewhat; not, generally, the total energy, appreciably if at all;....but it also makes the spark last longer....spark duration is the word.   

High powered ignitions eat electrodes.  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 only a small improvement, so this has hardly been adopted universally, and won't be, IMHO.   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, strictly for TWO reasons:
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.

The bottom line is that spark plugs can have more center and outer electrode erosion than the old days.  One method of reducing the constantly increasing spark gap is to use precious metals, which, for various reasons, do not erode so quickly.  I see ZERO reason to pay premium prices for a K bike's spark plugs....at least not for Iridium or Platinum tips.   If, however, you are going to try for over 60,000 on your plugs, you might be interested in trying them.  I won't.   There ARE reasons to use certain types of spark plugs.   In my own K1100LT, BMW went from the recommended Bosch XR5DC to a dual ground electrode type XR7LDC.   The two have equivalent heat ranges in practice (in the K BIKE), ....never mind the 5 versus 7 here.   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 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.

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, will show 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.

OLD Bosch numbers:

Equivalents, Bosch and NGK:
NOTE!!   Plugs with 'BC' are the 1/2" reach for BOTTOM plugs in those dual-plugged bikes that take 14 mm size
Bosch     Bosch new series        NGK
W5DC        7591                        BP7ES
W5BC        7531                        BP7HS
W6DC        7594                        BP6ES
W6BC        7593                        BP6HS
W7DC        7500                        BP6ES
W7BC        7597                        BP6HS
W8DC        7505                        BP5ES
W8BC        7503                        BP5HS

Note:  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.

****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 WR plugs (not recommended for airheads) may have the wrong gaps for airheads!  check the gaps!

***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!!

MORE on the Bosch boxes:  It has been reported that Bosch has been putting their resistor plugs in the small cardboard boxes that are marked for the non-resistor plugs.   Also reported 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.

Using spark plugs that have extended nose/tip is important for proper combustion on an airhead and K bike engine.   Only true racing engines might not have, in some circumstances, such non-extended noses.   

  4.  There is no 'official' specification for spark plug manufacturer's on what the resistor in a resistor spark plug must be.  If that resistor is a coil of wire, or a resistor that happens to be under maybe 2500 ohms, you MIGHT have a FAILURE of the 1981+ electronic ignition system....a pricey failure.   I suggest you  use proper caps with real resistors, such as the Bosch or Beru or NGK.    Don't use resistor plugs; don't use resistance wires. Use of 1000 ohm caps is acceptable in some situations (points), as noted well above; and might offer a slight ignition improvement in cold weather. 

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.

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.

Return to Technical Articles Index

Return to HomePage