Robin
I removed the threaded prop adaptor from my Leopard motor, turned the motor around and used a standard plain flexible coupling on the other end of the shaft.

Robin – Further to the earlier post by Dave Milbourn. If you wish to undertake a comparison with a M3.5 thread, it is the standard thread used for domestic electrical fittings e.g. the screws which attach the socket outlets to the steel box embedded in the wall.

The other line you could pursue would be to try another fitment with a known thread size e.g. the normal thread size used for model boat props is M4 X 0.7

Dave Marks

If you look at the motor you'll see that the prop adaptor has a cruciform plate which is screwed to the red motor case. If you remove the four Allen screws then this plate can be taken off. You then mount the motor with the other end of the case as shown in the photo of my installation. The shaft is 3.17mm diameter (aka 3.2mm nominal) which is a standard coupling size. **LINK**

As I've been known to say once or twice, it works for me!

DM