A progressing cavity pump/motor includes a stator tube (12), a rotor (20) within the stator tube, and a coupling assembly (30) interconnecting the upper and lower rotor sections. The coupling includes a drive adapter (32) and a socket adapter (36) for transmitting torque between the upper and lower rotor sections, and at least one alignment surface (60) in cooperation with a mating surface to rotationally position the drive adapter within the socket adapter at a selected circumferential position. The pump/motor may be reliably assembled in the field while maintaining precise axial and rotational positioning of the rotor sections.