A hydraulic nut for tensioning an assembly comprises a mechanical threaded locking collar to minimize load loss during transfer of hydraulic load to the locking collar, an annular piston, an annular pressure area to generate an axial load under hydraulic pressure, a hydraulic pressure port and preferably metallic seals to maintain the hydraulic pressure within the annular pressure area that operates in an elastic region and continues to actuate hydraulic pressure after repeated operating temperature cycles. The seals of the hydraulic nut have a ‘U’ or ‘C’ shaped cross-section.