Hello guys

I had this 3W 75i for the past year or so on my plane.
The engine is great so far. Powerful and easy to start.
Engine is in very good condition. Never crashed, although I had a couple of rough landings.

Recently, while I was warming up the engine for a flight, the engine stalled suddenly on the ground.
Upon closer inspection, I realised that the prop bolts have sheared (all 6 of them!).

Luckily, I managed to get my local machinist to unscrew the 6 bolts that were stuck.
Then I installed another prop Menz 24 x 12, drilled with a 3W Drill Guide.
It worked fine for 10mins or so .... then the engine stalled again.
The prop bolts have sheared again (5 out of 6 this time).

I don't really know what I have done incorrectly (screwed up somewhere).
I do notice that the holes that I have drilled on the props are slight bigger than the screw diameter..... to make it easier for me to install the screws .........

Does anyone else have the same problems?
Is the bolt shearing thing commonly seen?

Thanks!

Those bolts don't actually shear, but break by bending to and fro due to engine torque variations. The threaded bolt part that protrudes from the prop drive washer is very prone to this failure mode.

First, use high tensile steel bolts. Any old bolt just won't work.
Second, drill them with as little play as possible; drill from the prop back side
Third, tighten the bolts very carefully, untul all is seated well, without crushing the wood.
Fourth, check and tighten at regular intervals.

fifth - make certain they did not bottom out -
common mistake
I find you need two sets of bolts (different lengths)- to handle the variations in hub plus spinner thicknesses.
Actually I over drill my ZDZ 80 props to 5mm holes -to allow swapping props with DA 100's for testing purposes
never a problem
I am certain some believe these bolts are drive pins -
Which, they are definately NOT.

sixth-read the above again, it contains all your answers. Those bolts do not stop the prop from turning due to shear strength, it's from the clamping/friction being applied.

Well here's another one to check for. This is ESPECIALLY true when you run carbon props. The center stud that your prop slips onto needs to be checked for runout. They don't bend but they can get out of whack. Using a dial indicator measure and if it's out by more than .015 go ahead and true it up. I actually true mine to within a couple of thousandths because it's so easy to do. The back side of that stud is bowl shaped which allows it to move a bit. All you need to do is find the high side and rap it with a plastic mallet till runout is correct.

This applies to 3W's and DA's alike. My new DA150 was .040 out from the factory. Quick rap with the hammer and she was within .002.

Rich Sornborger

forty thou? that is a surprise.
I can measure that with a yardstick.

I agree with Dick. I went through 3 sets of prop bolts and finally figured out they were bottoming out. I added another prop washer and have not had a problem since.

I suppose that is what is meant by "pilot error"