Most Helicopter engines have a 1/4" threaded shaft. The area on the crank where the tapered collet goes on is usually 8mm (very close to 5/16"). I had run into your same problem a few times back in the day, I just took one of my sockets and modifies it with a belt sander. The second issue you may run into is that it looks like the shaft and nut are sticking up above the the hub a little. This will keep the clutch from sitting where it needs to be.