OctagonStar Flexible Couplings 5mm to 8mm NEMA 17 Shaft for RepRap 3D Printer or CNC Machine2PCS Flexibility of this coupler removes stress between motor and driving component. Compatible with NEMA 17 stepper motors (for 5mm to 8mm shaft diameter). Useful in RepRap 3D Printers and small CNC machines. Dimensions: 19mm x 25mm. NOTE:OctagonStar Quality Assurance.Item package or product surface have OctagonStar brand identity. If you feel to buy isn't OctagonStar item, you can return or exchange.