Mains usually have an easy life as the crank just rotates. It's the rod bearings which get hammered. Imo it's an oiling/clearance issue as the other guys have suggested.

Mike.

 

Detonation leads to cap walk, and distortion of the brg shell.
This then leads to the bearing shells caving in at the parting line, wiping the crank of oil film, and the rest is history.
As already mentioned, insufficient clearance[s] will accelerate the inevitable..

BTDT, and have a t-shirt to prove it!