The head broke off at the second cooling rib from the bottom.
It looks like it was caused by the vibration of the engine.
Crazy nut probaly went a bit to low on wall thickness.
Maybe mine had just a crack from production.
Who knows, fact is that it went flying down the backstretch.
First I thought it was the complete head and that the screws came loose, but the engine was still running.
The head button and the lowest two ribs were still there.
Mike Webb, you are right, how did you know?