Your Dad asked
this same question earlier on here, ...and it is being asked on HAMB too. Early on, I wrote that I suspected that it is detonation that is hammering out the babbitt. Based on all that has been written, I still stand behind my initial comment. Let me add additional thoughts...
Since the block has been replaced twice, the thoughts about multiple incorrect bore or cooling issues are unlikely between two different blocks. The same mindset applies to an oiling issue within two different blocks.
Now lets think this next thought through regarding crankshaft. Just "how far" would a crankshaft need to be out of round to cause this? Many engines out there have slightly out of round crankshaft journals and they survive more than any 500 miles. If it was severely out of round, the rod would be so stiff in one area of rotation that this after the 3rd or 4th time of assembly would throw up a 'flag' to the rebuilder. While we can be critical of some engine rebuilders, shouldn't one must assume that after the 2nd or 3rd time of doing it over, that ultra care during assembly would be taken where obvious issues would be corrected?
The only other possibility in my mind is the question that was posed in #14 of
the other post about whether the crankshaft has been drilled for oil pressure. If it has and the dipper has been closed off, this could be a logical explanation if there was an obstruction in the galley hole. Outside of that, the #4 rod area has the most amount of oil due to the inclination of the engine. If oil is prevelant in this area, then the chances of it being splashed up to keep a piston skirt from gaulding is very likely.
Therefore it is still coming back to a situation where detonation can easily be caused by too aggressive timing. Its effects appear in different ways but one needs to realize the extra forces that it can cause. It can cause ring land issues, it can cause a piston to rock in the bore, and it can cause rod bearing (babbitt) issues too. I wish I could see many of these parts as the engine is being disassembled to determine the true culprit.
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