This gearmotor consists of a low-power, 12 V brushed DC motor combined with a 171.79:1 metal spur gearbox, and it has an integrated 48 CPR quadrature encoder on the motor shaft, which provides 8245.92 counts per revolution of the gearbox’s output shaft. The gearmotor is cylindrical, with a diameter just under 25 mm, and the D-shaped output shaft is 4 mm in diameter and extends 12.5 mm from the face plate of the gearbox.
These cylindrical brushed DC gearmotors are available in a wide range of gear ratios and with five different motors (two power levels of 6 V motors and three power levels of 12 V motors). The gearmotors all have the same 25 mm diameter case and 4 mm diameter gearbox output shaft, so it is generally easy to swap one version for another if your design requirements change (though the length of the gearbox tends to increase with the gear ratio). All versions are also available with an integrated 48 CPR quadrature encoder on the motor shaft. This dynamically-sortable table can help you find the gearmotor that offers the best blend of speed, torque, and current-draw for your particular application.
Detailed information at Pololu.