Apparatus employs a device for automatically threading and unthreading studs into tapped bores of a pressure vessel containing nuclear fuel to detach or fit a cover thereto. The apparatus also employs a device with a carrier ring supporting piston and cylinder units for tensioning the studs while nuts are fitted thereon. The stud threading device is supported on a carriage displaceable along a circular track to access each stud in turn. The stud threading device has a drive member, such as a tube or rod, which is driven by a rotary drive as well as a reciprocating drive. At the lower end the drive member has a coupler which fits onto a coupler at the upper end of each stud. The couplers on the studs take the form of bayonette-type fittings with U-shaped grooves for receiving locking projections on the coupler of the drive member to establish a connection resistant to tension and relative rotation.