49 CFR §179.400-7

1. (a)Tank heads of the inner tank and outer jacket must be flanged and dished, or ellipsoidal.
2. (b)Flanged and dished heads must have—
   1. (1)A main inside dish radius not greater than the outside diameter of the straight flange;
   2. (2)An inside knuckle radius of not less than 6 percent of the outside diameter of the straight flange; and
   3. (3)An inside knuckle radius of at least three times the head thickness.