Upgrade Your Early K-Bike Alternator
by Mick McKinnon
Upgrading the 33 amp alternator on the older K75/K100/K1100 to the newer 50 amp alternator, there by gaining a substantial increase in available wattage to run electrical accessories, is a pretty straight forward job for the average backyard mechanic. The included diagram is the 50 amp alternator found in the K1100 section of the BMW ETK.
All of the 50A alternators are the same part number (12 31 2 306 020). This alternator is used on all of the Oilheads and K-bikes EXCEPT the K1200RS/LT which use the 60A alternator (12 31 2 305 888). I do NOT know if this alternator can be used on the early K-Bikes.
The 33A and the 50A alternators are interchangeable with the exception of the clutch housing. You need to use the Clutch Housing (12 31 2 305 329) from the newer (50A) alternator. The earlier (33A) alternator uses a fan wheel and a woodruff key, but the newer (50A) alternator does not have the fan wheel or a slot for the key. On the Oilhead bikes, a pulley is installed instead of the clutch housing and driving dogs used on the K-Bikes. If you are considering this upgrade you should also get 3 new rubber 'Goobers' (Rubber Mounting) for the drive clutch (12 31 1 464 877).
Your wiring will need to be modified just slightly. The 50A alternators use a 8mm ring connector for D+ where the 33A alternator used a slip-on spade connector. Some folks have left the original wiring in place and simply added a new D+ wire from the alternator to the + terminal on the starter. This allows them to use either alternator.