Spring coupling 5/8"x5/8". Fits Bell & Gossett PD-38, 39, 40 booster pumps.
The breaking of the pump coupling (coupler) indicates the motor mounts have lost their resiliency - The motor shaft and pump shaft are now out of alignment. Always replace the motor mounts once your pump coupling breaks - And keep an extra pump coupler in the boiler room. They always seem to break on the coldest day in January!