Sierra 18-3721-9 Stainless Steel Locknut, 7/16-20 Thread, Mercury Outboard/Stern Drive, Pack of 5
Sierra 18-3721-9 Stainless Steel Locknut, 7/16-20 Thread, Mercury Outboard/Stern Drive, Pack of 5
The 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.
This 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.
Specifications
- Brand: Sierra
- Manufacturer Part Number: 18-3721-9
- Thread Size: 7/16 inch - 20
- Material: Stainless steel with nylon insert
- Fastener Type: Locknut
- Package Quantity: 5 locknuts per package
- Weight: 0.11 pounds
- Finish: Stainless steel
- Color: Silver
- OEM Cross Reference: 11-34933, 11-95702, 11-82671015, 11-46355, 11-33962, 11-29241, 11-24648, 11-22178, 11-21298
- Application: Mercury/Mariner outboard and stern drive system
Fitment
- MCM inner transom studs and prop nut
- Mercury 39 and 60 outboards, 1968 and newer, 8 spline
Replaces (OEM)
- 11-34933
- 11-95702
- 11-82671015
- 11-46355
- 11-33962
- 11-29241
- 11-24648
- 11-22178
- 11-21298
Frequently Asked Questions
What thread size is the Sierra 18-3721-9 locknut? 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.
How many locknuts come in a package? 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.
What is the 18-3721-9 locknut used for? 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.
What material is the locknut made from? 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.
Does the 18-3721-9 replace Mercury OEM part numbers? 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.
How much does the 18-3721-9 locknut package weigh? The package of 5 Sierra 18-3721-9 locknuts weighs 0.11 pounds.
Is the Sierra 18-3721-9 a genuine or aftermarket part? 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.
What finish does the locknut have? The 18-3721-9 has a stainless steel finish in silver, chosen for corrosion resistance in wet marine conditions.
Will the 18-3721-9 fit any outboard other than Mercury 39 and 60? 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.
Why does this locknut use a nylon insert? 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.
What is included when I order the 18-3721-9? 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.