ORIGINAL: flyX
...I think the shaft might not be long enough to mount a prop perperly...
...As memtioned above..threads are on the out side of the shaft for airplane engine
The threads are on the inside of the shaft for a heli...
What heli engines are you talking about??
The engines I've had are identical to the plane engines, except for the head and the carb (larger head for heat dissipation and a better carb). I've never seen a shaft with threads on the inside! The OS 50SX-H even came with a drive washer. The OS 37 SZ-H doesn't come with a drive washer, but you can just use the OS 32 SX drive washer for it. In fact, I had to buy the 32SX drive washer for my 37 to install it in my Venture 30 heli, since it required it.
As long as you have, or can get a drive washer and an airplane muffler for your heli engine, you should be able to use it in a plane.