Piston ring

A piston ring adapted to be received by an annular groove formed in a piston disposed in a cylinder assembly of an internal combustion engine, comprising:

• an annular ring base member having a cutout portion, the annular ring base member having an outer sliding surface, first and second surfaces continuous to the outer sliding surface through corner portions and an inner peripheral surface to be fitted in the piston groove, the outer sliding surface contacting an inner wall of the cylinder when the piston reciprocates therein;
• and
• a nitriding layer formed on all the surfaces of the annular ring base member by a nitriding treatment, said nitriding layer comprising a diffusion layer and a brittle white layer formed on the diffusion layer,
• wherein a first portion of the brittle white layer which is formed on the sliding surface of the ring base member is substantially completely removed, and a second portion of the brittle white layer which is formed on the first, second and inner peripheral surfaces and the corner portions is at least partially removed so as to reduce a thickness of the second portion to not more than 5 .mu.m.