Piston ring of internal combustion engine

An internal combustion engine having an oblong cylinder wall and an oblong piston mounted to reciprocate therein, a continuous piston ring groove formed in said piston, an oblong piston ring assembly positioned within said groove and comprising three discrete rings, an outer ring, an inner ring and an intermediate resilient expander ring, said outer ring provided with an outer periphery in surface contact with said cylinder wall, said inner ring being formed of a material having a high degree of stiffness and substantially continuous about the periphery of said piston ring groove, with only one pair of ends said ends being in abutting relationship, said resilient expander ring being formed of a corrugated plate spring and supported on said inner ring to exert a predetermined expanding force on said outer ring, said inner ring being normally spaced from the inner periphery of said piston ring groove so that lateral displacement of said piston ring assembly is limited by the amount of said spacing during any lateral movement of the piston, thereby equally distributing the load of the corrugated plate spring of said expander ring positioned between the inner ring and the outer ring wherein said outer ring has only one pair of ends defining a single gap therebetween, said gap being positioned opposite from the ends of said inner ring along a central long axis of said oblong piston.