((This article revised by the author, 12/03/2007))

The resistance has more than one purpose:

It reduces spark plug tip erosion, and therefore greatly reduces any gap change over the spark plug life, and does this by reducing the electrical current flow. It does NOT reduce the applied voltage.

Reduces some types of radio interference.

Works in a complicated electrical way with the coil to lengthen the time the spark lasts after being started. That is a simplification, as there are some other beneficial and rather complicated effects as well. Note that if the resistance is too large a value, then the spark ENERGY (combined voltage, current, and with SOME time effect) will be lessened.

On the electronics ignition models, the plug caps, whether NGK or BMW or whatever, should be nominal 5000 ohms.

There is a bit of benefit to using the 1000 ohm caps on any of the points models. This can NOT be safely done on the electronics ignition models.

On the 1981 up, full electronic ignition models, the 5000 ohm resistance is used for all the previous reasons; AND, now is a NECESSITY to stop a type of harmful electric current/field, which can flow through the wiring as well as being radiated. Thus if 1000 ohm caps...or no caps...was used on these models, that could almost surely eventually damage the Hall elements in the canister, or even the electronic 'black box', called a module, located on these models under the gas tank. The damage could occur immediately....or, much later at a very inopportune time. It is NOT SAFE for the resistance on the 1981+ models to be less than 2500 ohms, and so 5000 was selected, a standard value for spark plug caps, and the different coils on the electronics models match nicely. The 1981+ models have much higher coil output than earlier models, so the 5000 ohm caps are hardly a problem and do NOT reduce performance, in fact, performance is increased on the 1981+ ignition.

 The spark energy that might be normally reduced by that 5000 ohm resistance is NOT, due to the different coils, the Module taking care of the increased current that points can not handle....and so the 5000 ohms greatly lowers the radiated electrical field which can get into the wiring on the bike making its way to the Hall devices in the electronic timing canister in the timing chest, ruining them, and this MAY not show up immediately!!, and THAT is the most common failure, usually occurring because the owner or uninformed mechanic removes a spark plug cap while the engine is running. This same problem may well occur with the Dyna III ignition unit, and definitely is a problem in the same way for the Boyer Brandsen Microdigital ignition.

The stock Hall canister, as well as the module under the tank, are all expensive devices from BMW, yet very reliable. Because of the sensitivity to damage it is NOT ACCEPTABLE, under ANY circumstances, to remove the spark plug wires when the engine is running, a common mode used by old BMW mechanics in the magneto era....and even into /5 and later era (wrongly on them). You CAN short circuit the spark plugs to the cylinder head fins on ANY model, if that is the way you want to balance your carburetors, but beware of open circuits here.

BMW does NOT use carbon resistance type of high voltage wire to the spark plugs, NOR, the often heavily advertised and touted spiral metallic type. These types should NOT be installed! Use either the stock type wires; or, you could use high quality silicon insulated solid copper wire (stranded conductors). Make sure that the core is solid stranded metal, usually copper or steel.

I HIGHLY recommend NOT using resistor spark plugs with the resistor plug caps. You COULD use non-resistor caps and resistor plugs, but it is VASTLY best to use the original type parts... with the following advice on using other than BMW wires/caps:

NGK makes plug caps in more than one resistance value. Use the 5000 ohm type LBO5F or LB01EP, or equivalent, or the 1000 ohm type, LBO1F or EP, for BMW POINTS models.

***SPECIAL NOTE! Bosch has been discontinuing the NON-resistor spark plugs we use in our airheads. Bosch makes very good spark plugs with nice threads and thread coatings (NGK makes a quality plug too, but Champion's threads are SUSPECT). I have spoken to Bosch Technical Support, and CAN NOT fully recommend the use of the Bosch Resistor plugs (such as WR7DC, etc.) in our 1981+ airheads, no matter if the spark plug caps are change to zero or 1000 ohm caps. BUT.....PROBABLY those Bosch resistor plugs ARE OK, with 1000 ohm caps on the 1981+ models (I think the spark likely too weak with the stock 5000 caps and those resistor plugs), and you certainly don't need resistance caps with the Bosch resistor plugs for the POINTS BMW models. If the spark gets too weak, the bike might seem to run OK, but in marginal conditions, such as very cold, or slightly marginal ignition components, even a dirty plug, etc....the bike might start hard, or misfire at higher rpm....and give poor fuel mileage...and cause excessive carbon deposits in the cylinder head and on the piston top. Under the worst conditions serious mechanical damage could occur.  Bottom line:  Use NGK spark plugs if in doubt.

 The author's website has an article, #32, with much greater in depth details on the spark plug caps and the spark plugs, with charts, etc.  I RECOMMEND you read that article.

The list of Technical articles is at:   http://bmwmotorcycletech.info

Snowbum (#1843)