The metal lock ring (30) of the invention has an oblong exterior periphery which allows the ring to have a small "width" and a larger "height". The small "width" provides a strong locking mechanism on the poly liner (14) and the larger "height" allows the ring to remain secure within the groove (18) of the metal shell (12) prior to and during assembly of the polyethylene liner (14). The ring has a chamfer on its inside leading edges defining its width to facilitate assembly of the liner (14). In the preferred embodiment, the lock ring has a constant thickness such that the inner periphery and the outer periphery are oval.
In an alternative embodiment, the metal lock ring (30) has a round inner diameter and a round outer diameter that are not concentric, thereby creating a thickening of the ring at one end (50). The shape of the alternative lock ring provides for substantial contact between the lock ring (30) and the poly liner (14) about the liner (14). The thickened area (50) of the ring (30) retains the ring (30) within the metal cup (12) prior to and during assembly of the poly liner (14). The thickened portion of the ring (30) may have scallops or notches (52) formed therein to increase the flexibility of the ring (30).