Not Ranked
To add on to what Gary said. This happened to me a long time ago when I built a Z code 390, bored 0.080 over with a borrowed 406 tri-power setup for the top. The engine was going into a 1957 Ford short tail. During the build, when I was getting ready to put the pistons together, some friends came by with some beer to help (what could go wrong). The pistons were assembled, put into the block and the rest of the assembly seemed to go well, It ran really strong for about a month and a half, until I developed what seemed to be a miss. During a compression test, coolant came shooting out of one of the spark plug holes. After pulling the head on the problem side, I found a big gash in the cylinder wall. On disassembly, the problem was the wrist pin was hitting and eventually broke through the cylinder wall. I couldn’t find the internal snap ring or pieces of it anywhere. So either it was left out during the build, put in backwards or not installed all the way into the groove. When I read and saw your picture, it brought back some memories.
Hopefully you caught this in time before there is any visible damage. But you may have to pull this engine apart to be sure where that piece came from.
Best,
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