These hubs are intended for use in connecting wheels and motors. CNC lathes are used to manufacture rigid aluminum products with precise tolerances. The tiny setscrew provides mechanical support for the motor shaft. An Allen wrench is included with the setscrew for tightening. It is sufficient to puncture the four screw holes in order to connect any round parts to the engine. Please verify the dimensions that have been uploaded to the product image gallery. Hubs work with any motor that has shafts that are 3 mm in diameter. Some of them are displayed below. Included in the kit: Two Hubs Eight M4 machine screws Two setscrews One Allen wrench