These are all too small for the .55-AX. OS recommends a 12x7-8 or a 13x6-7. I feel that the 13" prop is probably a bit of a stretch but likely doable. The 12x7 is near ideal. If you use an 11 inch prop on this engine it is probably best to keep the pitch fairly high (i.e. 11x8 or more). Despite the advertised "practical RPM", this engine leans toward low end "grunt" rather than a high RPM screamer. It has the torque for the 12 inch props and thrives in that RPM range.