Pololu 313:1 Metal Gearmotor 20Dx46L mm 6V CB with Extended Motor Shaft (Item 3721)

These small brushed DC gearmotors can deliver a lot of power for their size. This version has a 6V brushed DC motor with long-life carbon brushes combined with a 312.5:1 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. This version also has a 6 × 2 mm extended motor shaft.