The problem can be traced directly to the method used to connect the timing chain gears to the crank pulley, as standard BMW uses a three-piece design with friction washers between the clamping surfaces, meaning the assembly is held together purely on bolt tension with no physical keyways. This makes for easy assembly, installation, and adjustment of the timing chains with regards to setting the camshaft timing, but with no physical keyway the hub can ‘slip’ under large torque demand from the engine, meaning the camshafts will no longer be synchronized to the crankshaft correctly which at best will give poor running, but at worst will result in valve to piston contact and total engine failure.

The reason this affects tuned engines is to do with the aggressive torque delivery provided by the twin-turbo setup on the S55, which can see a significant torque ‘moment’ when the boost comes in, which in turn overwhelms the friction washer and crank bolts’ ability to keep the crank hub in place, even with the bolt remaining torqued to spec. This is why we strongly recommend having the issue addressed when tuning to any power level, to avoid a potentially much larger bill further down the road