Originally Posted by Roelof

You would be amazed about the flex. Indeed some play etc is also hapening. I can tell you that an engine compleetly builded up with fresh bearings and almost no play on the rod with a 0.35mm head clearance at full rpm the piston of our engine is hitting the head. Making 40.000 or 45.000 rpm can make a difference of hitting the head and/or backplate or not.
With wrong hardened crankshafts we have seen that they will snap within an half our, hardened in the right way is keeping some flex damping the peak forces. We also had made conrods from a very strong piston material, because it had no flex it shattered in pieces in 15 minutes.

Flex is always needed, maybe you can not see it but it is there!