These small brushed DC gearmotors can deliver a lot of power for their size. This version has a 6V brushed DC motor combined with a 156.3:1 metal spur gearbox. The gearmotor is cylindrical with a diameter of 20 mm, and the D-shaped output shaft is 4 mm in diameter and extends 18 mm from the face plate of the gearbox.
Metal Gear Motor 172:1 23DX56L
This gearmotor consists of a low-power, 12V brushed DC motor combined with a 171.79:1 metal spur gearbox. The gearmotor is cylindrical, with a diameter just under 25 mm..