The invention provides a fastener (2) for securing a workpiece comprising a plurality of workpiece members, wherein the fastener comprises a shank (4) and a radially enlarged head (6), the fastener further comprises axial voids (40) and splines (18) provided on the internal wall of the bore of the fastener, such that when the fastener is installed into a workpiece by a mandrel, breakstem or pin, once the shank of the fastener has contacted with the walls of the workpiece apertures, the crests of the splines are deformed by the mandrel, breakstem or pin, and material of the crests is displaced into the voids.