CFR /  Title 49  /  Part 179  /  Sec. 179.202-4 Thickness of plates.

The wall thickness after the forming of the tank shell and heads must be, at a minimum, 9/16 of an inch AAR TC-128 Grade B, normalized steel, in accordance with Sec. 179.200-7(b). [80 FR 26749, May 8, 2015]