A plastic ring for internal combustion engine pistons. In a preferred embodiment, the ring has an outer diameter lip for engaging the cylinder wall, an inner diameter portion bottomed in the ring groove of the piston, and an axial groove intermediate the outer diameter lip and the inner diameter portion whereby combustion gases will act in the groove to expand the inner diameter into sealing contact with the bottom of the ring groove and expand the outer diameter lip into sealing engagement with the cylinder wall.