I tried using the losi 22 one, I think it was the 83 tooth one, what i had to do was drill out the central hole where the spur goes onto the main shaft. Not sure what the diameter for the hole is but once you do that then it worked for me. To be honest it is probably easier to stick with the standard spur gear and try changing the pinion instead. But if you want to change the spur then just do what I did and that should work fine