Sierra's 18-3721-9 Stainless Steel Lock Nut is designed for use with MCM inner transom studs and prop nut for Mercury 39, 60 (1968 and newer, 8 spline). It is interchangeable with GLM 21170, Mallory 9-72004, 9-72004-10, Mercruiser Stern Drive 11-21298, 11-22178, 11-24648, 11-29241, 11-33962, 11-34933, 11-82671015, and Mercury Marine 11-134391, 11-22178, 11-34933, 11-46355, 11-82671015, 11-859116, 11-95702. Includes five lock nuts per package. Each Sierra product meets or exceeds the original equipment part it replaces.