There is nothing magic about the OS AX series engines that allows them to run much greater prop loads than other engines with the same displacement. At most one pitch number greater...maybe. Follow the prop advice the guys have given you here if you want to keep it cool and want power. About your 55AX, run 12X5APC for3D or 11X7APC for sport MAX on stock muffler 15% &OS#8. From what I gather, the recommended props in the manual are rather extreme to keep tip speed down, hence the noise. APC props are about the quietest you will find. I will give OS one thing, with so many guys following the manual to the letter, they seem to be handling the heat well.