I broke in an OS 55 AX this last weekend (for the GP Cherokee review that I'm doing) and had no problems at all. I used the prop that I planned to fly with (according to OS's instructions), which was a 12x7. I also had a Pitts muffler on the engine as well. I didn't have any issues at all with the break-in. The instructions call for alternating between rich (4-stroke) and lean (2-stroke) operation through the first tank. I actually ran 2 tanks through the engine doing this. The best way to sum it up was that it runs like an OS.

Ken