The Mercury  HD Prop Washer and Nut Kit (OEM Part Number 12‑840383A2)  is a precision-engineered OEM part by Mercury Marine (Quicksilver). Designed for HD 1.25 in prop shafts, it secures propellers in high-torque applications. Its components include a brass locking prop nut, flat washer, 1.5" Belleville (cupped spring) washer, and a 1⅞" spacer, each manufactured to OEM spec.
 
The Belleville washer maintains axial preload under thermal and load cycling, stabilizing clutch and hub alignment.
 
Fitment and Model Compatibility
 
This kit is for use on HD 1.25 in propshaft (Fat Shaft) configurations found in multiple Mercury drive systems: Bravo I XR and XZ sterndrives (including 2.5 and 3.0 Liter Sport Masters), Bravo XR One, Bravo XR Three, Torque Master I & II drives, Fleet Master drives, and Verado with 5.25" gearcases. It serves as a replacement for part 12‑840383A1 and is exclusively OEM.
 
Tightening Torque and Mechanical Best Practices
 
Use a torque wrench to tighten the locking nut to 55 ft‑lb (≈ 75 Nm) to achieve correct preload without over‑compressing the spring washer. Proper orientation of the Belleville washer (cupped face outwards) is crucial for spring performance. Splines should be cleaned and lightly greased with Mercury 2‑4‑C or Special Lube 101 if you plan to leave the prop on the shaft during storage.
 
Preload Mechanics and Durability
 
The Belleville washer actively compensates for wear and thermal expansion. Its spring force maintains consistent axial compression, resisting propeller looseness induced by vibration, transient torque spikes, or hydrodynamic loading. Replacement Spring Washers are available on this website in singles and 10-packs.
 
Service Intervals and Inspection
 
Adhere to annual inspection protocols. Remove the assembly at least once per season, inspect for thread wear, galling on the nut or washer, and check for any deformation in the spacer. Replace the kit if wear is evident. Saltwater exposure accelerates corrosion—cleaning and lubrication during inspections are key to preventing seizure or corrosion‑induced fastener failure.

The Mercury 12‑840383A2 Washer and Nut Kit is an OEM component tailored for Mercury heavy‑duty gearcases using a 1.25 in Fat Prop Shaft. It ensures propeller retention via precisely engineered preload from the Belleville washer, secured to spec at 55 ft‑lb torque.
 
Proven compatible with Bravo XR (One, Three), Bravo XR, Sportmaster, Bravo I (XR/XZ), Torque Master I/II, Fleet Master, and Verado 5.25" units. Every element is engineered to OEM tolerances for corrosion resistance and mechanical reliability—key to sustaining drivetrain integrity in demanding marine environments.
 
Contact Mike Hill at mike@buckshotracing77.com w/ technical questions