Our coupler is specifically selected to work with our 1/2 HP gear motor, and our keyed ½” shaft grain mills including all of the Pro Series mills. It is an L075 series coupler from TB Woods with one half having a 22mm bore with 6mm keyway, and the other half with a 1/2 " bore and 1/8” keyway, along with a blue urethane spider jaw coupling insert for superior torque handling and plenty of cushion. We ALSO offer this coupler set for use with our standard 1/2 and 3/8 shafts that have three flats for a setscrew. The setscrew only attachment works well when one of the flats on your drive shaft is widened slightly with a file, and you use loctite on the setscrew for no loosening.