Its the main caps walking around, this chatter causes a crack at the main bolt holes, particularly number 1 2 and 3 main caps. These cracks run to the cam tunnel, then a cam bearing spins out because there is no press to hold it in the block, then the end result is a crack in the lifter valley. This is well known, there is no mystery really.
Then you have the blower guys who crank down on the tensioner and pull the crank snout sideways and this will break the snout and block.
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