I can't get down there to look into the spark plug hole and I don't have borescope. However, I did compress the valve springs down to make sure they moved freely. The 10% leakage tells me that there isn't a broken piston ring, blown head gasket, or even a hole in the piston. I'm not sure where the piston is at relative to the stroke but I probably should make sure it is at bottom dead center and do the leak down test again. That way I can make sure that there isn't a crack in the cylinder wall.
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