Section 179.400-7 - Tank heads(a) Tank heads of the inner tank and outer jacket must be flanged and dished, or ellipsoidal.(b) Flanged and dished heads must have-(1) A main inside dish radius not greater than the outside diameter of the straight flange;(2) An inside knuckle radius of not less than 6 percent of the outside diameter of the straight flange; and(3) An inside knuckle radius of at least three times the head thickness.