Oil ring, manufacturing method thereof, and piston structure using the oil ring

摘要

An oil ring 1 comprises an annular oil ring body 10 having a pair of rail portions 12 at its axially upper and lower portions and butt ends, and a coil expander 20 assembled in the inner periphery of the oil ring body 10 . The cross-sectional profile taken in the radial direction of the outer peripheral sliding surface (f) of the pair of rail portions 12 has a nearly straight region (f 1 ) with a length of equal to or more than 0.05 mm in the center and curved regions (f 2 ) formed on both sides of the straight region (f 1 ).