That mount won't work on a OS 25 FX due to rear needle valve clearance. Some guys have ground them down to fit. Others use this OS METAL mount:
http://www3.towerhobbies.com/cgi-bin...?&I=LXCB61&P=7
Or this Hayes plastic mount:
http://www3.towerhobbies.com/cgi-bin...?&I=LXK805&P=7
If they can't get the custom mount that was made by Mike Langlois and sold by Don Stegall.