I was thinking that it might be a fault in the crank/rods or cylinder walls. Possibly the cylinders were not honed/bored before the first rebuild and the pistons slapped around in the bore too much. The second time around it was just worse. Or maybe the crankshaft or rod bearing toleraces were too great and the piston slapped around some more. I guess the solution would be to bore the cylinders and main bearings, and maybe try out a new piston/rod setup.
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