PURPOSE: To eliminate test working before shipment and to enable high load working in the early stage of operation by forming a phosphate film on the sliding surface of a piston ring after lapping and sand-blast, coating the whole surface of the ring with a solid lubricant, heating the ring, and impregnating oil into the ring under heating.

CONSTITUTION: A piston ring is mechanically worked and the peripheral surface of the ring is lapped, degreased and sandblasted. A phosphate film is formed on the peripheral surface by a known method, and the ring is washed and dried by heating. The whole surface of the ring is coated with a solid lubricant prepd. by dissolving graphite powder and powder of a low m.p. metal in an org. solvent to about 7W50μm thickness by spraying, and the ring is put in a heating furnace and held in an atmosphere at about 10W250°C for about 20W60min to bake the resulting film. The ring is then immersed in an oil bath heated to about 80W170°C for about 5W30min to impregnate oil into the ring. The surface treated piston ring is combined with a piston and fitted into a cylinder.