Tamiya 70121 Pulley Unit Set is similar in function to a gear box and contains parts compatible with other Tamiya products. The pulleys can be mounted using the base and pulley mounts included with the set and driven with the included motor.
The set includes seven pulleys of various sizes along with an assortment of single and double bushings that make the pulleys compatible with round and hexagonal shafts of both 3 mm and 2 mm diameters. Depending on the bushing, the pulleys can either idle on a shaft or rotate with it. The double bushings allow for pulleys to be connected and sit next to each other on the same shaft or screw. Other parts in the set include a motor, a belt guide for use with the motor's shaft, power bands, rivets, and a screw and nut set.
Base Length: 4-3/4" (12.3cm)
Base Width: from 1/2" to 1" (1.5cm to 2.5cm)
Base Height: 1/4" (7mm)
$8.89 
Large Pulley Set - Tamiya 70141
Small Pulley Set - Tamiya 70140