Abstract: A piston ring has a gap defined by ring gap faces oriented toward one another and a nominal outer radius assumed by the piston ring in a closed position in which the piston ring is in an installed state in an engine cylinder having a radius equalling the nominal outer radius of the piston ring. The piston ring has first length portions extending from both ring gap faces and having a central angle of at least 15.degree. and less than 25.degree. and second length portions immediately adjoining each respective first length portion and having a central angle of approximately 10.degree.. In the installed state the first length portions have a radius of curvature equalling the nominal outer radius and the radius of curvature of the second length portions is smaller than the nominal outer radius.

Abstract: A piston-and-ring assembly includes a piston having a circumferential groove which has an upper side wall and an opposite lower side wall; and a piston ring received in the groove and having opposite radial faces extending obliquely with respect to the side walls. The piston ring engages, with an axial bias, the upper and lower side walls with an annular marginal surface portion forming part of each radial face of the piston ring and extending substantially parallel to the side wall with which it is in engagement.

Abstract: A piston ring whose contour deviates from the circular shape, and in particular an elliptical or oval piston ring, which, when seen in the circumferential direction, has regions that, in the installed state as well as in the operating mode, exert less radial contact pressure than the remaining surface regions in order to reduce wear of and leakage by the ring.