A progressing cavity pump/motor includes an upper stator tube (14), a lower stator tube (16), a rotor (20) sitting between the upper and lower stator tubes, and a coupling assembly (30) interconnecting the stator tubes comprising an outer sleeve (32), an inner sleeve (36) and a nut (60) for threaded engagement with at least one of the sleeves to bring a stop surface on the inner sleeve into mating engagement with the stop surface on the outer sleeve. The pump/motor is usually assembled in the field while maintaining precise axial and rotational positioning of the stator tubes.