Harold  I've found that if you can just hold the can and tighten the prop. if you are using a pusher prop. it will stay on. the prop does not have to be torq down as much as a IC.

I've tried holding the can (motor), but there is not much room given the
mounting.  It just doesn't seem tight enough.  Perhaps a spanner wrench to hold the back of the motor (is one available?).

I have also run into this problem, especially when using a prop adapter that has a smooth face where it touches the back of the prop....or when using a spinner with a smooth aluminum backplate. What I have done is cut some small washers out of 220 grit sandpaper to place between the slick surfaces. That usually gives me enough friction that I am able to tighten the prop nut sufficiently.

Bob

Hi All,
Same here, but I make mine out of two pieces of sandpaper glued back-to-back if none of the surfaces is knurled.

take care,
  Dean

I tried filing flats at the very front of the prop adapter, ahead of where the prop nut rests.  Put a small wrench on that to hold the adapter while tightening the nut. 

if you are using a pusher prop. it will stay on. the prop does not have to be torque down as much as a IC.

Don't agree with that one - if it's a pusher then the direction of rotation is trying to loosen the nut off.  If it does get loose it will spin off in about 5 milliseconds.  (Don't ask me how I found out.)  I also found that the soft aluminum collet has a hard time gripping the polished chrome shaft unless it's good and tight.  I'll try the sandpaper washer - it sounds like that'll do the trick.

Paul

On an IC engine I resorted to J-B Weld to glue the prop extension to the thrust washer on the engine.   Might try J-B Weld to hold the prop adapter.