portion 31 of uniform maximum diameter is greater than the internal diameter of the thinner wall portion 19 of the shell and in this example is equal to the uniform external diameter of the shell shank 17 see fig4 the head 29 has an underhead portion 32 the external surface of which is in elevational section a smooth arcuate curve this curve blends smoothly with the surface of the adjacent part of the stem which is the adjacent end of the retaining portion 33 of the stem the end of the head has a slightly domed surface 30 the assembled rivet is shown in fig4 the stem having been inserted through the bore of the shell until the inner periphery of the tail end face 25 of the shell shank meets the stem head 29 part of the way up the underhead curved face 32 the free end part of the pulling portion 26 of the stem projects well beyond the head 18 of the shell the leading part of the retaining portion 33 of the stem has been drawn into about half of the length of the part 20 of the shell bore producing an interference fit this interference holds the stem firmly assembled in the shell the exterior of the corresponding part of the shell shank may have been slightly expanded by.