This stepper motor coupler is machined  with special tolerance to be used with NEMA 23 stepper motor shaft with standard diameter of 6.35mm. 

These couplers are made of machined aluminum and have a spiral cut that makes them slightly flexible so they can be fit to two shafts even if they are not perfectly co-linear and will help reduce bending effects. 

• 6.35 mm inner diameter -------> 5mm inner diameter
• Outer diameter:  20mm
• Length:  25 mm