A shear head screw 10 (Fig. 3) comprises a first (screw) head 14A and a second head 14B coupled by a shear neck 16 so that the screw can be tightened by the first head 14A until the screw shears at the neck 16 leaving the second head 14B and a threaded stem 12 tightened to a predetermined torque determined by the shear force. The screw thus applied can subsequently be loosened by the second head 14B. A shear head adaptor 108 (Fig. 19) comprises a socket 112 drivingly engageable with the screw head 106, and a drive head 114 coupled by a shear neck 116 to the socket 112.