This two-part aluminum assembly mounts directly to 4 mm diameter motor output shafts like those on our 25D mm gearmotors and allows you to drive most scooter, skateboard, inline skate, and other common wheels that are compatible with popular 608 ball bearings. Two M4 set screws are included for securing the threaded adapter piece to a motor shaft, and three 14 mm M3 screws are included for clamping the wheel between the threaded adapter piece and the washer.