Abstract: An annular piston has a bore threaded to a bolt end and is disposed between a base and a nut in axially spaced relation to the nut, there being an annular cylinder slidably receiving and surrounding the piston and defining an annular pressure chamber therewith, the cylinder being disposed between the base and the nut so that the nut can be tightened to act axially through the cylinder against the base in the absence of any pressure in the chamber. With pressure present in the chamber of a magnitude which tensions the bolt, after pre-tension has been overcome, the nut is free to be rotated to a desired setting.