How to Protect your Engines Vibration Mount While Torquing the Prop Shaft Nut
By: Ralph Shultz

If you have ever replaced your redrive belt and had to torque that large nut on the prop shaft you already have first hand knowledge as to how difficult it is to get that nut torqued to the required 150 ft/lbs. You probably also noticed how much strain this put on the rubber vibration mounts for the engine. That strain is easily controlled using the following procedure.

Start by placing a couple of pieces of ¾” thick hardwood or any suitable material (one on each side) between the Aluminum engine mounting angle and the engine vibration mount top plate as shown below. Then, using suitable sized “C” clamps, clamp these spacers between the mount pieces so as to lock up and render the rubber vibration disks immovable (solid). This makes it impossible for the torque applied to the prop shaft nut from being resolved out through the rubber vibration mounts, and effectively transfers the torque load to the root tube.

After the engine mounting has been made "solid", the prop shaft nut is tightened using two suitable wrenches. One wrench on the large nut and one on the prop shaft flats on the end of the shaft. This will allow you to torque the nut to about a third of the required torque without much effort. At that torque the prop shaft should be tight enough to keep it from rotating in the tower plate and allow a large socket to be placed over the nut and with a cheater tube used over a breaker bar, the rest of the required 150 lbs of torque can be applied very easily.


In torquing the prop shaft nut it is wise to keep an eye on the tension on the drive belt, as in some assemblies, particularly the Hagger 2.2:1 low redrives, some changes occur during this torquing that tend to tighten the belt tension. This usually happens, if it is going to happen, in applying the first 50 lbs of torque.

As can be seen in the picture above, I have the low 2.2: 1 redrive on my plane. I have used this method of torquing “that nasty” nut with excellent results many times in the past and I'm sure it will work just as well for you. Now “Go do the right thing”. :-)