This coupling consists of two parts. Plug the two parts into the bore of the wheel from each side respectively, then fasten them with an M4*18 screw. Two M3*5 set screws are also needed to fix the coupling onto the motor shaft. Specification: Material: Aluminum Alloy Inner Diameter: 4mm Outer Diameter: 11.9mm Length: 6mm / 23mm Part List 1×Shaft Coupling (2 Parts) 2×Screw, Spring Lock Washer and Plain Washer assemblies M4×18 (1 Backup) 3×Set Screw M3*5 (1 Backup)