Combined oil ring
A combined oil ring is comprised of an oil ring and an expander. The cross section of the oil ring is a generally I-shape, and the outer circumferential surfaces of outer rail portions form sliding surfaces to make contact with the cylinder wall surface. The cross section of an expander installation groove formed on the inner circumferential surface of the oil ring is formed in a rectangular shape, and an expander formed of axially corrugated ring-shaped plate material is mounted in the expander installation groove. The expander is formed by forming a rectangular cross sectional plate material in a corrugated shape in the direction of plate thickness and further forming in a ring shape. The expander installation groove of the oil ring is not limited to a rectangular shaped cross section. For example, the cross section of the expander installation groove may have a trapezoidal shape widening towards the inner circumference, and the cross section of the plate material forming the expander may have a generally diamond-shaped cross section.