Hey Spit,
I have a well used 4*40 that had an OS55 on for engine break in. I used an APC 11x5. At 4 3/4 pound it was fast and had unlimited verticle. It seems many are running this engine with huge props, but I don't get it. I tried a 12x5 and verticle and speed were both down from the 11x5 so I quit playing with props.

When I had the ST51 on, it flew best with the 11x5 also. The 12.25x3.75 was slow, but the torque was incredible.

Have fun, MikeB