Mercury Racing introduced a new Prop Double Nut Kit for all Mercury Racing outboards with a 1.25-inch diameter propeller shaft. The Double Nut Kit is designed to better secure the propeller to the prop shaft. Mercury Racing recommends checking the torque on the fasteners regularly.
The new kit includes a brass half hex nut and a brass locking nut. When installing the propeller, the hub and prop assembly should be placed on the prop shaft, followed by the supplied washer and then by the brass half hex nut, which should be torqued to 85 ft. lb. The brass locking nut is then installed behind the half hex nut, and should also be torqued to 85 ft. lb.
Note that the previous kit uses a single prop nut that could be torqued to 55 ft. lb. The new system accepts more torque, and the double nuts better maintain that torque. However, if the nuts loosen, in extreme instances the propeller can begin rocking on the shaft. This can cause damage to the propeller shaft splines, and even lead to the propeller shaft breaking.
For this reason, it’s good practice to break out the torque wrench after a high-velocity session. To re-torque the hub, the rear locking nut must be removed. Torque the hex nut to spec, and then reinstall the locking nut and torque it to 85 ft. lb.
The new Prop Double Nut Kit (P/N 8M0179579) can be ordered from a Mercury Racing Authorized Dealer, and will replace the single-nut hub for all Mercury Racing outboard propellers.