I don't think the transfers are near wide enough to catch a ring. Plus they were beveled after widening.
I wonder if you might of had a little piston slap and with the widened porting it caught a port with the piston skirt and them wam broke it. I base this on how the pin boss broke like it did. Its either that or the pin boss is what failed.
Beveled yes, but the side is pretty square.
More importantly, it doesn't matter how wide it is if the ends of the ring make their way in there. Hard to piece it together from the pics, but how far was the widened edge of the transfer port from the ring ends?
Bingo.
A ring end definately did not drop into the transfer port. That wasn't even close. The only thing close was the lower ring end to the edge of the intake port. Square sides don't catch rings. It's flat tops and bottoms you have to watch out for, particularly on the exhaust roof.
Looks to me like the left side transfer snagged a ring on the way down and ripped the top of the piston off above the ring land shattering the brittle ring allowing the broken ring end to expand into the intake port causing the damage to skirt on the intake side. Skirt probably didn't happen all in one stroke, the ring end snag just started the failure and the skirt got beat to death with every stroke that came after.
I can tell you one thing for sure... it was not a circlip issue, both of those clips were in when the catsasstrophy took place.
I'd like to ask you something -- and I'm not pointing fingers, just gathering information, so don't take it wrong -- how wide were the ports as a % of bore diameter ? As you know, the general rule of thumb for a work saw is 65%. I noticed that one or two of your recent 660 build were going up to 70% ? Again, I'm not jumping to conclusions, just curious.
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