the bore is cold worked in response to forcible insertion of the fastener section 20 into the bore typically the bore initial diameter is nominally 0 246 inches and the diameter of section 20 is nominally 0 248 to 0 249 inches in this regard the work panels may typically consist of a metal or metals of less hardness than the fastener metal for example the fastener may consist essentially of titanium and the work of aluminum the first section 19 is conically tapered forwardly from the head and toward section 20 further section 19 is sufficiently oversized in relation to the workbore that interference forces develop in response to forward displacement of the fastener into the work thereby to effect displacement of work material between first and second locations defined as follows the first location 30 is proximate the initial junction 22a between bore 12 and countersink surface 15 and the second location is at the work surface adjacent and bounding the countersink surface as seen in fig2 the second location is defined by a rearward bulge 31 extending annularly at the second location and is produced in response to radially outward deformation of the work material at the first location by section 19 in addition no bulging is produced at the work countersink surface 15 provided that the relationships as referred to are maintained further the work material appears to be cold worked at and between the locations 30 and 31 sufficiently that fatigue strength is very substantially increased over what is produced when all dimensions are maintained except that section 19 is not tapered but has the outer diameter of section 20 and no bulge appears at 31 for example fatigue.