Jabba
How does a run away CHT relate to the bottom end being destroyed?
It could be related as follows. From time to time when using nearly 100% unleaded (especially during long hot slow climbs) the peak pressure moved back to TDC thus exerting a greater than normal load on the bottom end and high heat loads on the top end. The engine was strong enough to cop this a few times but couldn't do it forever.A bit like a mistimed engine--sometimes you get away with it but for how long?
Eventually with maybe a bit of help from detonation the engine throws in the towel.The first cylinder to overheat detonated and the bottom end weakened with the through bolt issue and earlier unwitting abuse, fails.
This is not necessarily what happened but it is a possibility. I wonder how many failures of this type of have occured in engines which never use mogas.
Cheers RA