We break the engine in at full throttle with a 12 x 8 propeller. We start out very, very rich and gradually work the mixture to peak RPM.

When the engine's had about an hour's run time on it, we'll have leaned it to peak RPM and see mid- to high- 12,000's.

Fly with an 11 x 9, 12 x 7, 12 x 8 props. 12 x 6 or 7 aren't too bad, either. It all depends upon what your plane needs to fly the way you want it to. If the engine turns in the mid- to high 12,000's, you'll be OK.