Upgrade the OEM design for consistent propeller performance on your Mercury Racing outboard or HD MerCruiser with the Fat Shaft Flo-Torq Hub Kit for 1.25" Prop Shaft, replacing Mercury OEM part numbers 840389K06 and 8M0183487,
 
Built for high-load, high-RPM use, this extreme-duty hub delivers smooth power transfer, noticeably less vibration, and longer life for both prop and shaft—whether you’re chasing top speed on race day or running hard every weekend.
 
This kit is purpose-built for Mercury platforms that use SportMaster or TorqueMaster lower units with 1.25" (1-1/4") fat shafts, including the Verado, Pro XS, Mercury Racing EFI, and Optimax families. You’ll commonly find the larger fat shaft on Mercury Racing models such as 260 HP, 280 ROS, 300 Drag, 300 Pro Max, 300X, 300XS, 250R, 300R, 400R, and 450R.
 
Beyond outboards, the kit also fits Fleetmaster and Verado 5.25" gearcases and integrates cleanly with MerCruiser Bravo One XR and XZ sterndrives. If your setup uses a Flo-Torq (square-hub) propeller and a 1.25" propshaft, this kit is designed to drop right in. (It will not fit standard 1.00" small-shaft applications.)
 
On the water, the cushioned hub core maximizes the transfer of engine power to the prop while damping harshness that can lead to chatter, slip, and premature wear. Precision-engineered washers and a brass Nyloc prop nut lock everything down so the prop stays put in rough water, hard turns, and sustained wide-open throttle.
 
The result is a confident, repeatable feel at launch and at speed, plus the durability serious boaters demand. Installation is straightforward: clean the propshaft, assemble the included washers in the order shown in your kit photo, and torque the brass Nyloc prop nut to the OEM specification for your engine or drive per the Mercury service manual.
 
After your first run, re-check nut torque and you’re ready for a full season of hard use. Each kit arrives complete with one extreme-duty cushioned hub, one thick case-hardened thrust washer, one spring washer to secure the prop, one flat finishing washer, and one brass Nyloc prop nut.
 
For easy cross-reference and parts-counter search, this Buckshot Racing #77 hub kit replaces Mercury/Quicksilver OEM part numbers 840389A1, 840389A2, 840389A5, 840389K5, 840389K06, 840389Q06, 12-840383A2, and 8M0183487.
 
If you’re running a Mercury Racing outboard or Bravo XR/XZ and want a tougher, smoother, fat-shaft solution that keeps your Flo-Torq propeller planted, this is the hub kit to trust.
 
Contact Mike Hill at +1-714-697-1716 or email mike@buckshotracing77.com for technical support.