A fastener, comprising: - a pin member having an elongated pin shank adapted to be located in aligned openings in a plurality of workpieces and terminating at a first end in a head which engages a surface on one side of the workpieces and at a second end in a grooved portion which extends past an opposite surface on an opposite side of the workpieces, the grooved portion including a locking portion having a plurality of lock grooves defined by circumferentially extending pin grooves and associated pin shoulders terminating in pin crests, the locking of the pin member having an outer diameter (Da'); - a substantially straight pull portion extending from the second end of the pin member and including a plurality of pull grooves, the pull portion having an outer diameter (Da') which is smaller than the outer diameter (D B) of the locking portion of the pin member; and - a collar including a generally straight collar shank adapted to be swaged into the lock grooves of the pin member in response to a relative axial pulling force applied to the pull portion of the pin member, thereby providing a desired magnitude of clamp load on the workpieces, the collar, when swaged, having collar grooves and collar shoulders that interlock with the pin grooves and the pin shoulders, the pin member and the collar being made from different materials having ultimate shear strengths of different magnitudes selected such that yielding of the pin member is substantially avoided upon swaging the collar thereto, wherein the pull portion of the pin member has a length that is less than the length of the locking portion of the pin member in order that a protrusion length of the pull portion from the second end of the pin member is relatively short, and wherein the pull portion is adapted to remain on the second end of the pin member after the installation of the fastener; characterised in that the collar is an as-headed collar which is produced and strain hardened directly from cold working without any thermal processing, and the collar shank of the as-headed collar has an outer diameter and an inner diameter, and wherein the ratio of the outer diameter to the inner diameter of the collar shank is between 1.47 to 1.55, the collar shank having a thin wall thickness designed to resist overpacking of the lock grooves of the pin member thereby reducing the swage load required to swage the as-headed collar. - a pin member having an elongated pin shank adapted to be located in aligned openings in a plurality of workpieces and terminating at a first end in a head which engages a surface on one side of the workpieces and at a second end in a grooved portion which extends past an opposite surface on an opposite side of the workpieces, the grooved portion including a locking portion having a plurality of lock grooves defined by circumferentially extending pin grooves and associated pin shoulders terminating in pin crests, the locking of the pin member having an outer diameter (Da'); - a substantially straight pull portion extending from the second end of the pin member and including a plurality of pull grooves, the pull portion having an outer diameter (Da') which is smaller than the outer diameter (D B) of the locking portion of the pin member; and - a collar including a generally straight collar shank adapted to be swaged into the lock grooves of the pin member in response to a relative axial pulling force applied to the pull portion of the pin member, thereby providing a desired magnitude of clamp load on the workpieces, the collar, when swaged, having collar grooves and collar shoulders that interlock with the pin grooves and the pin shoulders, the pin member and the collar being made from different materials having ultimate shear strengths of different magnitudes selected such that yielding of the pin member is substantially avoided upon swaging the collar thereto, wherein the pull portion of the pin member has a length that is less than the length of the locking portion of the pin member in order that a protrusion length of the pull portion from the second end of the pin member is relatively short, and wherein the pull portion is adapted to remain on the second end of the pin member after the installation of the fastener; characterised in that the collar is an as-headed collar which is produced and strain hardened directly from cold working without any thermal processing, and the collar shank of the as-headed collar has an outer diameter and an inner diameter, and wherein the ratio of the outer diameter to the inner diameter of the collar shank is between 1.47 to 1.55, the collar shank having a thin wall thickness designed to resist overpacking of the lock grooves of the pin member thereby reducing the swage load required to swage the as-headed collar.