These Micro Metal Gearmotors from Pimoroni are ideal for small robot platforms or other light mechanical loads. They have a 3mm diameter D-shaped output shaft which is 9mm long and a 1mm diameter back shaft which is 4.5mm long. The back shaft is a great spot to install an encoder to track the motor's speed and position. All the motors in the range are the same size but with different gear ratios, allowing you to choose the one that's right for your next project.