The cranks on them, for the most part are to short to mount a prop. They are setup to mount a clutch.

I had an old O.S. SE .21 car motor with a full sized crank on a plane. I had to take most of the heat sink off. I ran it with a 8.5X6 prop and it ran right at 21,000 RPM with 30% fuel and tuned pipe. It was a good combo with the Aritronics Jetfire 20.

Running a 6 inch prop would not be a good idea. It would go way beyond the limits of the prop. You could easily shatter the prop and stuff lying every where.

Dru.