Abstract: A retaining ring is disposed in an annular groove to maintain a cap member positioned in a circular opening. The retaining ring is substantially toroidal in shape having a circular cross section. The groove in which the retaining ring is seated, has a centerline which is defined by the intersection of two opposed surfaces of revolution. Each surface of revolution has a cross-sectional configuration which is defined by a radius of curvature having its center axially offset from the groove centerline toward the opposed surface of revolution. The forces exerted on the retaining ring by the cap member are transferred to the annular groove. The loading of the ring is supported by both surfaces of the annular groove.