360 LA getting oil in #3 cylinder.

Detonation?

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Here are the dents in the block I was referring to. Those spots on the piston are coolant and oil from a rag I used to wipe everything off.

That’s usually detonation. The marks in the block are good, but usually when a gasket fails from something like that it keeps digging the mark deeper.

Fire from combustion just eats away the block. You see that mostly on cylinders 3-5 and 4-6, right between them.