A lockbolt for installation into apertured workpiece members, comprising a pin having a head and a tail end provided with locking grooves and a single pull groove, wherein an installation tool having a collet with a corresponding shape to the pull groove is used to apply an increasing pulling force to the pin tail, thereby to push the collet towards the workpiece, and as the force applied by the hydraulic piston further increases, causing the collar to be swaged into the lock grooves, and halting the force applied by the tool either at a predetermined maximum value or when the pin tail breaks at a breaker groove formed by the single pull groove.