Piston ring assembly

In combination;

• a piston recirpocatingly positioned in a cylinder, said piston having an outer peripheral piston wall, a circumferential ring groove extending inwardly from said piston wall and having substantially parallel upper and lower annular groove surfaces terminating at upper and lower annular groove edges at said piston wall, a pair of positive twist split rings of substantially the same size and shape positioned in said groove, each said ring being generally circular and having a transverse gap, each said ring having a generally rectangular cross-sectional shape including substantially flat and parallel upper and lower surfaces, substantially flat inner and outer surfaces, and a chamfer between said upper and inner surfaces, said outer surface intersecting said lower surface at a knife edge, said rings being positioned in said groove with said transverse gaps circumferentially displaced from one another, said rings being radially compressed to reduce the size of said gaps and to dish said rings upwardly, said knife edges on each said ring being in engagement with the wall of said cylinder, said groove and rings being axially dimensioned so that said rings are axially stressed with said upper surface of one said ring adjacent said outer surface thereof engaging said upper groove surface adjacent said upper groove edge and with said lower surface of the other said ring adjacent said inner surface thereof engaging said lower groove surface, and said rings having their facing upper and lower surfaces engaging one another over substantially their entire area to prevent relative rotation between said rings.