What is the cause? It is somewhat difficult to pin down but look at this comparison of the S55 3.0 liter I6 crankshaft and the E9X M3 S65 V8 crankshaft:
The S65 crankshaft is on the right and you can see timing gear and oil pump gear are a part of the design whereas the S55 does not have the same design. The S55 quite frankly is simply an evolution of the N54/N55 geometry and design which is basically the same. That makes one wonder why the N54 and N55 do not see this problem to the same degree. Is it the higher redline of the S55 that is partially to blame?
Perhaps. You can say what you want about the BMW turbo era and it's easy power with bolt ons but the fact is if BMW raced the S55 design the same way they raced the S65 design no doubt this design flaw would have popped up in serious Motorsport competition and it would have been addressed before production.
It is not some fluke. Here is what BimmerBoost member @sA x sKy had to say about it and his own experience:
TPG Tuning supposedly has a solution but @JasonTPG has yet to tell us anything about it. Their idea was to cut a notch in the stock piece but it didn't work and they are trying a different approach now:
The TPG hub failed on member @sA x sKy three times:
Will the Gintani one piece crank hub solve this issue? Who knows. Maybe there just is no solution:
You can have your bolt replaced and tightened every so often which is probably the best way to go. Maybe someone will come up with someone else. Maybe not. Where we are at right now is BMW quite simply screwed up on the S55 design and doesn't care.
BimmerBoost reached out to BMW for comment on the issue but do not hold your breath for a response.