the maxx conversion mounts wont work on a nitro rustler. a .21 is just too big, you would need a longer chassis to make room for it. even .18's are very hard to fit, i know the fantom/peak/epic .18's idle screws are unaccessable because they are blocked in the rustler's engine compartment. plus with the torque of a .21 or .18 you would be melting trans gears and going through diffs like crazy. nobody makes upgrade gears for the nitro rustler trans either. even if you loosen your slipper clutch you will end up melting through friction pegs.