I have an OS 55 AX w/ a pitts muffler. I am currently using a 12.25x3.75 prop (recommended by local hobby store) all this is on a Model tech twister. What would be a good replacement prop for this plane? I am using this plane to learn 3d on.

Thanks in advance

I have been using an APC 13x4 and a Zinger 13x4 ....
Both have worked well on my 55ax on a CAP profile...

Wat...

SORRY ONCE AGAIN BAX !!!!! Should follow my own TAG LINE !!!!!

I run an APC 12x4 and like the quick response!

Whatever propeller will allow the engine to turn in the mid-to-high 12,000's with good throttle response is a good propeller. The one you're using should be OK, as long as it gives you the proper pull.