CNBTR Aluminum Shaft Coupling Rigid Coupling Coupler Motor Connector 3mm to 5mm Red Pack of 2 3mm to 5mm: Hole at one end is 3mm/0.2inch, the other end 2mm/0.08inch Simple structure, red Coupling Coupler with two Screw Spanners. Aluminum Material Coupling Coupler, good use for the shaft and motor. Applies to: Connect optical axis of two different diameter or connect a motor with optical axis.