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Gearbox friction torque test.

Roger Lee

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Hi all,


I thought I would talk about the gearbox friction torque and when it needs to be done and how.

It is a mandatory check every 100 hrs or the Annual and it only takes a few minutes, but it is very important. It has a place on the Rotax inspection check list to record your test info and should be logged in your logbook for the inspection. (The owners that come to me, don't bother to look in the book, it's there.B))


You will need a set of metric allen wrenches and a fish scale. Something that reads between 0-50 lbs. Don't use something that read really high as it won't be accurate down at the bottom numbers where we want to measure. I use a digital fish scale. You will need a piece of string too.


As far as the how to perform this check, well let's take a look.

While you have the prop spinner off for the inspection is a good time to do this. First remove the M8x20 plug screw for the locking pin . It is located about 3.5" above the magnetic oil plug. It is gold colored and has an upward angle on the head. It is a number 5 or 6 allen screw. Remove it. Get the black 1 3/4" crankshaft locking pin out of the Rotax orange tool kit. We will need to rotate the prop which can be done with one plug out of each cylinder or they can be left in. You will need a flash light to find the crankshaft locking pin grove. You are going to shine the light down the hole where you just took out the screw where the locking pin will be inserted. Slowly rotate the prop counterclockwise while looking in the hole. You may only see blackness, keep rotating until you see a shiny smooth oily surface. Continue to rotate slowly and you will see what looks like a notch or a "V". If you continue it will be shiny smooth again. Stop and go back to that "V" or notch. Move the prop back and forth slightly to get it centered. Now take the locking pin and screw this in. It should stop with 3/4" still sticking out. Hold the prop and slightly wiggle it while you snug up the locking pin into the center of the notch. Now your crankshaft should be locked. Absolutely do not screw the locking pin in further if it just keeps going. That means you missed. Take it out and look for the smooth oily surface and notch again. This is very easy so don't worry.

Now with the crankshaft locked the prop can still move approximately 30 degrees. Push the prop back all the way counterclockwise until it stops. Take a tape measure and measure out from the center of the prop hub outward along the blade between 24-30 inches. It doesn't matter what number you pick. If you pick 26", 28" or 30" it doesn't matter. Put a little pencil or a mark at your measurement.

You need your string now around the prop at your mark. Now take your scale and hook it on the string. In a nice straight and even pressure pull, pull the scale and prop. Watch how much the scale reads. I usually do this 2-3 times to make sure my reading is accurate. So let's say your reading was 17 lbs. To find the torque measurement we take that number 17 lbs. and multiply it times the number of inches you measured out from the prop center. Let's say you used 28" and had a 17 lb pull. 28 X 17 = 476 in/lbs

That is a good number and is the number to record on the Rotax inspection sheet. We look for a bottom number of 300 and the high end of 530 in/lbs. In this case the higher the better. If you are at 490 in/lb verses 340 in/lb you are much better off. No it isn't worth the effort to pull the gearbox to go from 450 to 510 and getting the friction right on is not an exact science. It is accomplished by pulling the gearbox and using shims to add more pre-load to the bellville washers that act as our spring tension.

Your done, take out the locking pin and don't loose it. replace the plug screw at

133 in/lbs torque.


If you are a member of the R.O.A.N. website you can look at this video that demonstrates this very well.





The method to locate the notch with the flashlight is the easiest.

Locking pin location pic.pdf

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