the shoulder portion 168 can have a diameter that is slightly less than the diameter of the opening 37 of the bung 34 which acts to substantially center the guard 152 within the opening 37 the inner and outer radii 196 198 of the skirt portion 188 are dimensioned so that the distal end 194 of the skirt portion 188 does not contact the exterior threads 40 of the bung 34 while contacting the flange portion 44 of the outer ring 22 the inner radius 196 of the skirt portion 188 can be set radially outward from the exterior threads 40 at a distance that enables the flange portion 44 to flex as the flange portion 44 courses over the exterior threads 40 a method for building up the bung subassembly 110 is as follows insert the guard 152 into the opening 37 of the bung 34 so that the flange portion 182 of the guard 152 covers the opening 37 fig8 place the outer key ring 22 so that the flange portion 44 rests on the proximal end of the threads 40 of the bung 34 fig9 place the assembly tool 154 so that the distal end 194 rests on the flange portion 44 of the outer key ring 22 and so that the assembly tool 154 is substantially centered about a central axis 202 of the outer key ring 22 fig1 exert an axial force f that is parallel the central axis 202 so that the flange portion 44 of the outer key ring 22 courses over the exterior threads 40 of the bung 34 until the flange portion 44 is below the distal end of the threads 40 and surrounds the neck portion 112 of the bung 34 fig1.