Spring coupling 1/2"x1/2". Fits Bell & Gossett 100 1"-2" and HV 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!