Oil-ring including two rail rings and an expander-spacer ring which bites into the rail ring

A composite oil-ring for disposition in an oil ring groove of a piston reciprocally positioned in a cylinder comprising:

• a pair of axially spaced rail rings;
• an expander-spacer ring disposed between said rail rings to expand said rail rings, said expander-spacer ring being laterally extending U shaped in cross section providing rail supporting legs and an inwardly open channel between the legs of the U, integrally formed with a plurality of axially extending rail engaging portions circumferentially spaced at the inner periphery of each of said legs, and having a plurality of slots extending radially cut from the inner peripheries of said legs, said rail engaging portions having circumferential central portions with a curvature substantially the same as and in the same direction as the curvature of the inner periphery of said rail rings and circumferential end portions with a curvature, in a free state before contacting said rail rings, in the same direction as and smaller than the curvature of the inner periphery of said rail rings.