I have a Nexstar that came with the .46 FXi. The plane now has a 46 FX. Exact fit. I did try to mount my OS 50 SX in it once and it would not fit. The 50 SX I believe has the same case as the 46 AX and LA. Suddle difference but enough to goof the Nexstar motor choices up without modifying the mount. Oh and before someone asks, no there is no noticeable diference in performance between the 46 FX and FXi engines.