An interface device for tensioning or relaxing a bolt in a nut-and-bolt assembly wherein the bolt extends in an axial direction. The interface device comprises a frame having a first end portion and a second end portion. The first end portion is provided with a rotatably-mounted nut ring for receiving and rotating a conventional nut on the bolt. The second end portion comprises a mechanical tensioner nut configured for reacting on the frame and for receiving a part of the bolt that extends beyond the conventional nut. The mechanical tensioner nut is configured for being driven by a low-pressure torque tension tool to tension or relax the bolt in operational use of the interface device. The device provides a very compact torqueing solution for use with low-pressure torqueing tools.