in fig3 and 4 in which the end face 38 of the bolt fastener 14 is flush with the top 38 of the nut 24 referring now to fig6 there is illustrated a typical application using a frangible wrenching collar in the illustrated fastener the head 49 of the bolt 50 is not provided with wrenching flats instead the end 51 of the bolt fastener 50 has a hexagonal socket 52 to receive a key member not shown for immobilizing the bolt fastener during the application of the nut 54 in non interference fit applications this is necessary as the prior art fastener systems with mechanical locks preclude free spinning nuts and the key member is necessary to restrain the bolt against the frictional resistance of the mechanical lock one of the disadvantages of this prior art system in addition to those previously mentioned is that the frictional resistance is variable and results in a variable tension force in the fastener the preload of the fastener system which is the limiting torque which can be applied through the frangible collar includes both the frictional resistance of the fastener and the tension force of the fastener since the frictional resistance is an uncontrolled variable it results in an imprecisely controlled tension force on the fastener affecting the fatigue life of the assembled.