Sierra 18-3721 Stainless Steel Locking NutSierra replacement stainless steel locking nut for Mercury and Mercruiser Stern Drives. Sierra replacement engine parts are manufactured to meet or exceed OEM requirements. Thread Size: 7/16" - 20; Material: Stainless steel w/ nylon insert; For MR/Alpha Drives; For MCM inner transom studs and prop nut for Mercury 39, 60 (1968 & Newer, 8 spline); 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; Mercury Marine; 11-134391; 11-22178; 11-34933; 11-46355; 11-82671015; 11-859116; 11-95702;