Abstract: A hydraulic powered stud tensioner as employed to tension stud-like members in forming flanged joints in pipes or like vessels. A housing containing a plurality of segment shaped jaws having thread portions for engaging the stud threads. An actuating sleeve rotatably supported in the housing and having inner camming surfaces. The camming surfaces are arranged so that rotation of the sleeve to one extreme moves the thread portions radially inward into engagement with the stud threads and to the other extreme moves the thread portions radially outward and free of the stud threads. An improved hydraulic actuator having a piston and an improved fluid seal on one side of the piston. An air chamber formed on the opposite side of the piston for receiving compressed air and applying force to the piston in a reverse direction to facilitate disengagement of the jaws from the stud threads.