{"product_id":"stainless-steel-locknut-for-mercruiser-mr-alpha-one-drives-replaces-11-859116-11-13439-1-sierra-18-3721-9-7-16-in-pack-of-5","title":"Sierra 18-3721-9 Stainless Steel Locknut, 7\/16-20 Thread, Mercury Outboard\/Stern Drive, Pack of 5","description":"\u003cp\u003eThe Sierra 18-3721-9 is a stainless steel locknut built for Mercury and Mariner outboard and stern drive applications. It is designed for use with MCM inner transom studs and the prop nut on Mercury 39 and 60 outboards (1968 and newer, 8 spline). Each locknut is machined from stainless steel with a nylon insert, which resists loosening from vibration and keeps a secure, corrosion-resistant hold in a wet marine environment. The 7\/16 inch - 20 thread size matches the original equipment stud and prop nut fitment it is designed to replace.\u003c\/p\u003e\u003cp\u003eThis part is sold in a package of 5 locknuts, making it a convenient stock-up option for routine drive system maintenance, rebuilds, or keeping a spare on hand. Sierra manufactures this locknut to meet or exceed the specification of the original equipment part it replaces, so it installs and performs as a direct equivalent. The stainless steel construction with silver finish resists rust and salt water corrosion better than plain steel hardware, which matters for transom and lower unit hardware exposed to moisture.\u003c\/p\u003e\u003ch3\u003eSpecifications\u003c\/h3\u003e\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eBrand:\u003c\/strong\u003e Sierra\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eManufacturer Part Number:\u003c\/strong\u003e 18-3721-9\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eThread Size:\u003c\/strong\u003e 7\/16 inch - 20\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eMaterial:\u003c\/strong\u003e Stainless steel with nylon insert\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eFastener Type:\u003c\/strong\u003e Locknut\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003ePackage Quantity:\u003c\/strong\u003e 5 locknuts per package\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eWeight:\u003c\/strong\u003e 0.11 pounds\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eFinish:\u003c\/strong\u003e Stainless steel\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eColor:\u003c\/strong\u003e Silver\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eOEM Cross Reference:\u003c\/strong\u003e 11-34933, 11-95702, 11-82671015, 11-46355, 11-33962, 11-29241, 11-24648, 11-22178, 11-21298\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eApplication:\u003c\/strong\u003e Mercury\/Mariner outboard and stern drive system\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch3\u003eFitment\u003c\/h3\u003e\u003cul\u003e\n\u003cli\u003eMCM inner transom studs and prop nut\u003c\/li\u003e\n\u003cli\u003eMercury 39 and 60 outboards, 1968 and newer, 8 spline\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch3\u003eReplaces (OEM)\u003c\/h3\u003e\u003cul\u003e\n\u003cli\u003e11-34933\u003c\/li\u003e\n\u003cli\u003e11-95702\u003c\/li\u003e\n\u003cli\u003e11-82671015\u003c\/li\u003e\n\u003cli\u003e11-46355\u003c\/li\u003e\n\u003cli\u003e11-33962\u003c\/li\u003e\n\u003cli\u003e11-29241\u003c\/li\u003e\n\u003cli\u003e11-24648\u003c\/li\u003e\n\u003cli\u003e11-22178\u003c\/li\u003e\n\u003cli\u003e11-21298\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch3\u003eFrequently Asked Questions\u003c\/h3\u003e\u003cp\u003e\u003cstrong\u003eWhat thread size is the Sierra 18-3721-9 locknut?\u003c\/strong\u003e The Sierra 18-3721-9 locknut has a 7\/16 inch - 20 thread size, matching the original equipment transom stud and prop nut thread it is designed to replace.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eHow many locknuts come in a package?\u003c\/strong\u003e Each package of the 18-3721-9 contains 5 stainless steel locknuts, so buyers get a full set for a service job or spares for future use.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat is the 18-3721-9 locknut used for?\u003c\/strong\u003e It is used with MCM inner transom studs and the prop nut on Mercury 39 and 60 outboards, 1968 and newer with 8 spline configuration.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat material is the locknut made from?\u003c\/strong\u003e The 18-3721-9 is made from stainless steel with a nylon insert, which resists corrosion in marine environments and helps the nut resist loosening from vibration.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eDoes the 18-3721-9 replace Mercury OEM part numbers?\u003c\/strong\u003e Yes, this Sierra locknut replaces Mercury\/Mariner OEM numbers 11-34933, 11-95702, 11-82671015, 11-46355, 11-33962, 11-29241, 11-24648, 11-22178, and 11-21298.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eHow much does the 18-3721-9 locknut package weigh?\u003c\/strong\u003e The package of 5 Sierra 18-3721-9 locknuts weighs 0.11 pounds.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eIs the Sierra 18-3721-9 a genuine or aftermarket part?\u003c\/strong\u003e The 18-3721-9 is a Sierra-manufactured aftermarket replacement part, built to meet or exceed the specification of the original equipment locknut it replaces rather than being a Mercury-branded OEM part.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat finish does the locknut have?\u003c\/strong\u003e The 18-3721-9 has a stainless steel finish in silver, chosen for corrosion resistance in wet marine conditions.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWill the 18-3721-9 fit any outboard other than Mercury 39 and 60?\u003c\/strong\u003e The stated fitment for the 18-3721-9 is Mercury 39 and 60 outboards, 1968 and newer with 8 spline; it is not specified for other outboard models.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhy does this locknut use a nylon insert?\u003c\/strong\u003e The nylon insert in the Sierra 18-3721-9 helps the locknut resist loosening from engine and drive vibration, keeping the transom stud and prop nut connection secure.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat is included when I order the 18-3721-9?\u003c\/strong\u003e An order of the 18-3721-9 includes a package of 5 stainless steel locknuts with nylon inserts, sized for 7\/16 inch - 20 threads.\u003c\/p\u003e","brand":"Sierra","offers":[{"title":"Default Title","offer_id":48988162556123,"sku":"SIE-18-3721-9","price":16.89,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0839\/7586\/4539\/files\/sie-18-3721-9__67619.1722512328.500.750.jpg?v=1785188149","url":"https:\/\/www.goodvibesmarina.com\/products\/stainless-steel-locknut-for-mercruiser-mr-alpha-one-drives-replaces-11-859116-11-13439-1-sierra-18-3721-9-7-16-in-pack-of-5","provider":"Good Vibes Marina","version":"1.0","type":"link"}