I replaced my tower hobbies .75 for a OS.91fs and after ordering a new mount for my top flite mustang found that the engine drops right into my old mount. The mounting holes fit perfect and the spinner backplate is just where its suppose to be. Anyways just thought I would mention it in case somebody else runs into the same situation.

Good information. I'll try to remember that

Nice factoid Jeff - thanks for sharing.

just replaced my TH.75 with a OS fs91 surpassII as well and found the same to be true, but also found that the Big Stik that the 91surpass went on also has vertical capability that i never saw with the TH..........just thought i add that tidbit of info too.