Abstract: Apparatus for screwing, unscrewing, and handling large bolts such as stud bolts for the cover of the core tank of a nuclear reactor. A circular guide rail is positioned above the periphery of the core tank. A main chassis is suspended from and movable along the guide rail. The main chassis includes two vertical beams and a plurality of horizontal guide beams which extend over the core tank periphery toward the interior thereof. A secondary chassis includes a number of bars which cooperate with the guide beams to permit horizontal movement of the secondary chassis in a direction substantially perpendicular to the direction of movement of the main chassis. A screwing mechanism is vertically movable on the secondary chassis and includes a gripping device for gripping the screw and a jack for compensating for the weight of the screw. A second gripping device can be included for gripping a washer.