Abstract: A reaction nut for location on a threaded bolt, particularly a bolt to be nsioned by a hydraulic tensioning device, includes a plurality of internally-threaded segments (22) movable between an outer-most inoperative position radially spaced from the bolt (2) and an operative position making threaded engagement with the bolt (2). The segments (22) have tapering surfaces thereon. A primary ring member (16) is movable axially relative to the segments (22) and has a corresponding tapering surface (18) thereon. A first resilient device (20) biases the tapering surface (18) of the primary ring member (16) into engagement with the tapering surfaces on the segments (22). A cam then (26) moves the primary ring member (16) from a rest position to an axially displaced position against the bias of the first resilient device (20) whereby the segments (22) are moved by an associated second resilient device (24) between their operative and inoperative positions.