A straight threaded rod has three reference elements spaced apart from each other. A shoulder and a knob having a fourth reference element are screwed onto the rod. A spring is mounted on the rod between the knob and the shoulder. The rod is inserted through two axially aligned holes in two abutting pieces, with the shoulder bearing on a first surface of one of the two pieces. By adjusting the position of the shoulder and the knob along the rod it is possible to apply a calibrated clamping force to a pair of abutting pieces, regardless of the thickness of the pieces.