Pololu 25:1 Metal Gearmotor 20Dx41L mm 6V with Extended Motor Shaft

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 25:1 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.

Key specs at 6 V: 590 RPM and 170 mA with no-load, 22 oz-in (1.6 kg-cm) and 2.9 A at stall.