ARE Coupling Shoulder Nut
Replacement ARE Shaft System Coupling Nut, this is the special wide shoulder nut that fits inside the coupling attaching the shaft to the coupling. The wide shoulder is for the set-screw to lock down into the nut-shoulder.
Thread size: 3/4-12 (odd propitiatory ARE Thread, you will NOT find this tap or die)
Wrench size: 1 1/16"
Torque to: 60-70 Ft. lbs.