Keeping your Mercury outboard running smoothly starts with the right components securing critical moving parts. The Sierra 18-3752 stands out as a precision-engineered crankshaft retaining nut designed specifically for the driveshaft assembly. Crafted from high-strength materials, it provides the torque-holding power needed to withstand the rigors of marine environments, from choppy saltwater runs to long freshwater hauls.
Unlike standard nuts, this high-strength version resists loosening under vibration and high RPMs, preventing catastrophic failures that could leave you stranded. Boat owners swear by its durability, often reporting years of trouble-free performance even in demanding conditions.
Perfect Fit for Popular Mercury Models
This retaining nut is a direct replacement for OEM parts in a wide range of Mercury and Mariner 2-stroke outboards. It’s commonly used in 45 to 125 HP three-cylinder engines, ensuring seamless integration without modifications. Key compatible models include:
Mercury 45-50 HP (various years)
Mariner 60-90 HP inline models
Force outboards in the 70-120 HP range
Chrysler marine engines sharing the same powerhead design
Always cross-reference your engine’s serial number for the best match, as slight variations exist across production runs. This part restores factory-spec tension, maintaining optimal alignment between the crankshaft and driveshaft.
Simple Installation for DIY Mechanics
Swapping out a worn retaining nut doesn’t require a full teardown. With basic tools like a torque wrench, impact driver, and thread locker, you can complete the job in under an hour:
Secure the engine on a stable stand and remove the flywheel or lower cowling as needed.
Apply penetrating oil to the old nut if seized, then carefully loosen with proper socket size.
Clean threads thoroughly to ensure a snug fit.
Thread on the new Sierra 18-3752 nut, applying blue Loctite for extra security.
Torque to manufacturer specs—typically 40-50 ft-lbs—then recheck after a test run.
Pro tip: Inspect the driveshaft key and washer during replacement to catch wear early, extending the life of your lower unit.
Why Upgrade to High-Strength Construction
In marine applications, vibration and corrosion are the enemies of fasteners. The Sierra 18-3752’s high-strength alloy fights back, offering superior tensile strength over generic hardware store alternatives. It locks down the driveshaft with unyielding grip, reducing play that leads to gear whine or premature bearing wear.
Owners of fishing boats and ski vessels report fewer mid-season breakdowns, attributing it to this nut’s ability to maintain precise crankshaft positioning. It’s not just a part—it’s peace of mind on the water.
Long-Term Maintenance for Peak Performance
To maximize lifespan, integrate checks into your annual service routine. Look for thread galling, looseness, or corrosion during oil changes. Pair it with fresh gear lube and impeller inspections for a bulletproof powerhead.
If you’re trailering frequently, double-check torque after each trip. This proactive approach keeps small parts from becoming big problems, saving you from costly shop visits.
