Abstract: Method and apparatus for tightening a nut and bolt assembly to ensure that the bolt shank is tensioned correctly using a wrench having two relatively movable detector parts carried by the body of the wrench, one part being a cylindrical outer casing which engages the abutment surface non-rotatably, the other part being a stem extending axially from the body and having its outer end engaging the body of the nut and its inner end connected to a holder for a microswitch arranged to cooperate with an actuating member fixed to said outer casing, said microswitch being in an electrical circuit which controls power to the wrench and/or a visible or audible signal.

Abstract: A screw having a head and a threaded shank and, in the head, a cruciform shaped recess comprising a central cavity and four grooves radiating from the central cavity at 90.degree. angular spacings, the bases of the grooves lying on a conical (or pyramidal) surface and wherein the included angle at the vertex of such cone (or pyramid) has a value within the range of 40.degree. - 45.degree., the preferred value being 40.degree..