damaged piston what? why?

[Y-Blockhead]

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It appears that it was seizing. Did the engine get hot? Was the piston dry when you pulled it? Did you ensure the pistons were re-installed in the same hole they came out of? Is there any metal transfer to the cylinder wall from the piston?

I can't see anything going down the spark plug hole and getting past the compression rings to do that on both sides of the skirt but who knows??