Might also have been the head gasket, the area between the valves and crankcase, kinda hard to see.
In the future simply take a old spark plug, knock pout the guts, add a air fitting and pressurize the engine, if you hear air from the intake valve or exhaust valve or it comes out the crankcase breather, you have found the problem. In this case if you had done that AND the head gasket was blown, you would have heard air coming out of the oil fill. You might have assumed it was the rings, ?