These micro gearmotors are incredibly tough and feature full metal gears. They have a gear ratio of 250:1 and operate up to 12 volts and have a stall torque of 60 oz-in. and a max speed of 110 RPM. Each micro gearmotor sports a 3mm D-shaft.
Note: The micro gearmotor enclosures are sold separately, you can find them here.