154:1 Metal Gearmotor 20Dx44L mmPololu
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 154:1 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 10.7 mm from the face plate of the gearbox.
Key specs at 6V: 90 RPM and 250 mA free-run, 120 oz-in (8.6 kg-cm) and 3.2 A stall.