OIL RING, METHOD OF PRODUCING THE SAME, AND PISTON STRUCTURE USING THE OIL RING

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 (f1) with a length of equal to or more than 0.05 mm in the center and curved regions (f2) formed on both sides of the straight region (f1).