I would think it's rings then. I highly doubt you made the once-round pistons unround while still able to drive the car normally, but it could be possible. I highly doubt the piston are out of round though. The newest 7G is still 15 years old, it's not a stretch to think the cylinder walls or the rings have enough wear to lose compression. I've only torn down one engine in my lifetime though (well, more than that if you consider reciprocating piston railcar air compressors "engines" (they have blocks that are almost identical to engine blocks) but I've never seen a piston out of round), you might want to have a more experienced engine builder chime in.
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