Abstract: In a method of manufacturing a side rail of a compound steel oil ring assembly in which a linear, strip-like wire material is passed between a plurality of pins and bent into an annular shape, a side of the side rail that will become the inner circumferential side thereof is brought into pressured contact with at least one pin formed to have a corrugated surface, thereby forming corrugations in the side of the side rail that will become the inner circumferential side thereof.

Abstract: A steel oil ring assembly having an upper rail, a lower rail and an annular space-expander embraced by the rails and having radially extending or axially extending corrugations. The spacer is split at one circumferential location to form abutting end portions which come into pressured contact with each other at a force point located on a plane passing through the centers of the corrugations that extends in an axial direction for radially extending corrugations and in a radial direction for axially extending corrugations. The free ends of the abutting end portions of the spacer form an opening having an angle of 10.degree..+-.9.degree. extending outwardly from the force point.

Abstract: A steel oil ring assembly having an upper rail, a lower rail and an annular space-expander embraced by the rails and having radially extending or axially extending corrugations. The spacer is split at one circumferential location to form abutting end portions which come into pressured contact with each other at a force point located on a plane passing through the centers of the corrugations that extends in an axial direction for radially extending corrugations and in a radial direction for axially extending corrugations. The free ends of the abutting end portions of the spacer form an opening having an angle of 10.degree..+-.9.degree. extending outwardly from the force point.

Abstract: A compound steel oil ring assembly has nitrided side rails and a spacer expander. The opposing end portions of each side rail are treated to prevent formation of a nitride layer solely at these portions in order to reduce their hardness. This reduces the degree to which the corner portions of the opposing ends wear away the side surfaces of a piston oil ring groove.