TQ Racing has a Robinson Racing 60T 48P spur gear that I use that fits nicely. PN# 1860. The 48P gears are much easier to set the mesh with as the spur and pinion aren't the easiest things to see from the rear of the car if your using the Exotek conversion chassis. I use 36 thru 40 pinions to pretty much cover the full range of gearing on a 17.5T motor. Adjust pinion choice for 21.5T accordingly.