Well ... that sucks! So that leaves us "amateurs to guessing" as to what went wrong??? So ... excellent pictures and what I see in the photo of the failed skirt, is a vertical were pattern slightly to the right side of the failure point??? All the small pistons grooves are gone from that area and the marks are vertical and not horizontal?? To me that says that piston was not specked to the same standards as the other three pistons?? Excess side loading on that piston skirt??? Perhaps that piston was in backwards or the "thrust bushing was in backwards??? I don't know?? But something "bad happened" and it would best to not duplicate that again.