Here are some charts for Slot.it gears. Good sftermarket gears should be ready to run, stock gears often need to be run in with a polishing compound. With sidewinder spur gears make note that two different diameters are used. Inline pinion gears are smaller in diameter than the ones used with anglewinder and sidewinder spur gears. With inline gears it is common practice to use the end of the motor shaft to position the rear axle and crown gear. If you race your cars it is good practice to use spacers to position the rear axle assembly. You can then shorten the motor shaft and adjust the crown gear for a perfect mesh.