Upgrade Your Early K-Bike Alternator

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by Mick McKinnon 

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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.

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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.

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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).

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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. 

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