Even if you've specified pre-gapped piston rings, always check ring end gaps. Top compression rings and center oil rings have specific ring end gaps based on location. The top ring will have a wider gap than the center ring because it's a lot hotter on top. Another misconception is ring terminology. Only the top ring is a compression ring. The second ring is an oil control ring. What are traditionally called the oil rings are oil wiper rings in the bottom groove. The oil control ring carries oil up the bore and the oil wiper rings clear cylinder walls.