A hydraulic nut (20) for tensioning a bolt (232) is disclosed having a stepped annular piston (22) slidable within a housing (21) for reducing the complexity of manufacture. The locking collar (27) is threaded to the housing (21) on a buttress thread (28) to minimise distortion due to thread loads and locks against the piston (22) instead of the clamped article for minimising tensile relaxation during transfer of load from the hydraulic tensioner to the locking collar (27).
