I gathered some junk lawnmower parts to build a go-kart for my kids. The engine needed new rings, but was the cylinder worn in an egg-shaped pattern, or did it simply need slightly over-sized rings? I did not have a micrometer for measuring inside the cylinder walls. I improvised.
(I do not have an engine to show in this Instructable, and have decided to use a holesaw to represent an engine cylinder. The bottom of the holesaw represents the top of the piston.)