Bushings are the round parts that the axles of the gears fit into. Replacing them would require that you re-shim the gearbox.

Bushings can be made from plastic, brass, or steel. Some bushings are solid, while others contain ball bearings. Ball bearings improve performance by allowing the gears to turn more freely. The disadvantage in ball bearing bushings is that they are more expensive and can sometimes break if used with a powerful piston spring.

Make sure the bushings are fully seated in the gearbox before you begin shimming.

To install bushings in the gearbox, first try pushing them in with your fingers. If you need more force to get them in, try to use something soft like the back of a rubber screwdriver grip.