Piston rings

Piston rings are springy metal rings that sit in grooves around the piston. They seal the gap between piston and cylinder wall so compression and combustion gases don't escape into the crankcase, carry heat from the piston to the cylinder and, on a four-stroke, scrape excess oil off the wall.

A four-stroke piston usually has three rings: the top compression ring, the second (scraper) ring and a multi-piece oil control ring. A two-stroke piston normally has one or two compression rings and no oil ring, because lubrication comes with the fuel mix.

Signs of worn rings are low compression, hard starting, blue smoke, rising oil consumption and blow-by. When fitting new rings, check the end gap in the bore, fit them the right way up (mark facing up) and make sure the cylinder wall is in good shape – hone it if needed.