You can rebuild your OE one with some of the kits they sell on ebay. A member names oldskewel rebuilt his with one of these kits. The seller for his is some guy in Ireland I think. I bought one for the alt on my n62 545i. Between the kit and the voltage regulator I am into it 65.00 or so. The seller provides videos on how to rebuild your alternator and it really does not look hard.

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2004 E60 545i Sport 6MT. Black/Black Build Date 6-2004 102k miles