‘ BMW used a friction-reliant design for the crank sprocket. There’ s no keyway, no mechanical device for locking that timing in place’
‘ BMW used a friction-reliant design for the crank sprocket. There’ s no keyway, no mechanical device for locking that timing in place’
pump and vacuum pump, and the other is the timing sprocket. Between each of those is a diamond-encrusted washer. The whole lot is clamped together with a bolt; there’ s no keyway or anything. There’ s no mechanical device for locking that timing in place.’
That sounds like a recipe for disaster on a high-torque turbo’ d engine, but Ben explains that this part of the design isn’ t the root cause of the timing slipping.‘ The diamond-encrusted washer design is used on quite a few engines, and it’ s not necessarily a problem in itself. The simulations we’ ve done for the diamondencrusted washer, the original setup, the torque capacity of that, if the bolt is tight, is actually adequate. The issue is, actually, that the bolt works its way loose.
The timing sprocket on the S55’ s crank hub is an entirely separate part; it’ s held in place by a diamond-encrusted washer and clamped with a bolt
Summer 2026 The Engine Rebuilder 53