Agree with Aussiesteve and Tianci. The two props which will work best with an AX on your CAP are either an APC 10 x 7 (not for your ops) or Bolly 11.5 x 5 depending upon what you want to do with it and how you want to fly it. For prop hanging as best it can, the 11.5 x 5 Bolly or alternatively an 11 x 5 APC is the way to go. On the AX which is an absolute boar of motor in class once run in if tuned right, personally I'd go the 11.5" blade.

The APC 12¼ x 3¾ is a great prop, but really belongs on a dedicated fun fly or 3D ship. The = or + 11 inch diameter x 5" pitch ones mentioned above are "no brainers" which will work better all round on your CAP.

Question for Aussiesteve. What sort of a run have you had with your Magnum 4 strokes? Reliable? Any problems at all? Carburation etc?