Piston rings seal the piston in the cylinder: they maintain compression, scrape off excess oil and carry heat away. The 39.50 mm size is an oversize (larger than the standard 39.00 mm). Brand: SEE.
They should be replaced if compression drops, cold starting is hard, there is smoke from the muffler, or oil is being consumed. The size is given in the name; if the cylinder has been bored, oversize rings are needed.
