210 inch with the radius 74 being 0 200 inch and the axial extent of the two curves being 0 1941 inch the interior diameter of surface 76 may be 2 5080 inches the axial extent of the convex portions also increases with successive steps referring to the third stage of forming as shown in fig8 die 90 has a surface 92 for establishing the convex transitional portion a surface 94 for establishing the concave portion and a cylindrical surface 96 in this embodiment the axial extent of the two curved portions indicated by the letter e has been increased in this embodiment the radius of portion 92 may be 0 260 inch for example the radius of portion 94 may be 0 200 inch and the interior diameter of surface 96 may be 2 4670 inches it will be appreciated that the axial extent e of the combined curves 92 94 and the radius of surface 92 have been increased and the interior diameter of the die at 96 is reduced in successive stages similar changes occur in the subsequent dies in the fourth stage shown in fig9 die 110 has a surface 112 to create the annular convex surface on the necked in portion and surface 114 to create the concave portion and the cylindrical portion 116 the axial extent f of the two curved portions 112 and 114 exceeds axial extent e of die 90 of.