Airhead Tech Pages

Hotter Coils - The Bosch Blue "K" Coils

John Petty

I needed some new coils for my new_to_me 1983 R100RT. I had heard of the Bosch "K" 6 volt "blue" coils and decided that if I had to pay for a pair of coils, why not get the hotter ones? I put a pair on my RT and they work just great. I immediately felt a slightly more robust engine under the seat without any other changes. I still have one of the boxes with some numbers. On the bottom of the coil it has the following:


An old fax from Bosch has the 6 volt part number listed as: 0 221 124 001 (with spaces)
On the boxtop it has the number: 00 016

The top number is probably a manufacture date/code of some kind and would probably be different for the ones you buy. The middle line on the coil bottom is the key - "K6V". The casings actually are blue in color unlike the standard black coils. I believe they were/are high performance coils for the Volkswagen Beetle - the original ones with air-cooled flat-four boxer motors. Almost required equipment if you have high compression pistons.

If you just want a cheap but exact replacement, then the black Bosch coils are what you want - 6 volt part number 0 221 100 025. The black coils may actually be a better choice for the older style ignition with points and condenser. A few people have mentioned that the points of pre-1982 bikes may not be able to handle the increased current as well as the electronic ignition of the 1982-1984 models. In 1985 BMW switched to a single coil with two outputs. Bosch also makes a pair of red coils that are for high performance 6-cylinder applications. DO NOT get these! They are 12 volt and overkill for Airhead twins - possibly even literally. Just check for the color of the coils (blue) and the voltage rating (6 volt) and you will be just fine.

No, you do not want the 12 volt coils even though the electrical system on all Beemers since and including the /5 have been 12 volt systems. I don't know the whole reason why, maybe because they are wired in series, but the 6 volt coils are what you need.

In my own humble opinion, NEVER, EVER balance your carbs by using the shorted-spark method!!! This may have been marginally OK on Airheads with points and lower current (much lower) but the higher current you will get from the blue "K" coils (and especially on an electronic ignition motor) will kill your coils. Yes, BTDT, bought new coils AGAIN! I guess it was the feedback from shorting the plugs or some other facet. I used the popular little spark plug extensions and always had a path to ground either through the spark plug or a metal grounding strap attached to the extension. It happened over the course of three adjustments but got a little worse each time. Yes, I know some people will tell you that I'm full of manure but I'm the one who had to go buy new coils because of it. Use some mercury tubes, a Twinmax, vacuum gauges, a good musical ear, or use a method I described in another article called "Parking Lot Balancing Act" that goes into much more detail.

John Petty ~ 1983 R100RT and 1996 R1100RT-P