Fits a mendel X-axis motor, and can be screwed into place with 3 M3 screws.I drilled the holes up to 2.9mm and put 3 M3 screws in there.That really made the pulley stick, without destroying the motor shaft.Turned out my problem was with my power supply, not the pulley begin loose.Drill the screwholes to 2.9mm, drill the shaft hole to 4.9mm.Press the pulley onto the shaft, and fasten it securely with the 3 screws.You may have to mount it while the motor is mounted on the reprap mendel - I did. Also, the thick part faces down, not up as it intuitively would.