49 C.F.R. § 178.346-2
The type and thickness of material for DOT 406 specification cargo tanks must conform to §178.345-2, but in no case may the thickness be less than that determined by the minimum thickness requirements in §178.320(a). The following Tables I and II identify the specified minimum thickness values to be employed in that determination.
TABLE I-SPECIFIED MINIMUM THICKNESS OF HEADS (OR BULKHEADS AND BAFFLES WHEN USED AS TANK REINFORCEMENT) USING MILD STEEL (MS), HIGH STRENGTH LOW ALLOY STEEL (HSLA), AUSTENITIC STAINLESS STEEL (SS), OR ALUMINUM (AL)-EXPRESSED IN DECIMALS OF AN INCH AFTER FORMING
| Material | Volume capacity in gallons per inch of length | ||||||||
| 14 or less | Over 14 to 23 | Over 23 | |||||||
| MS | HSLA SS | AL | MS | HSLA SS | AL | MS | HSLA SS | AL | |
| Thickness | .100 | .100 | .160 | .115 | .115 | .173 | .129 | .129 | .187 |
TABLE II-SPECIFIED MINIMUM THICKNESS OF SHELL USING MILD STEEL (MS), HIGH STRENGTH LOW ALLOY STEEL (HSLA), AUSTENITIC STAINLESS STEEL (SS), OR ALUMINUM (AL)-EXPRESSED IN DECIMALS OF AN INCH AFTER FORMING 1
| Cargo tank motor vehicle rated capacity (gallons) | MS | SS/HSLA | AL |
| More than 0 to at least 4,500 | 0.100 | 0.100 | 0.151 |
| More than 4,500 to at least 8,000 | 0.115 | 0.100 | 0.160 |
| More than 8,000 to at least 14,000 | 0.129 | 0.129 | 0.173 |
| More than 14,000 | 0.143 | 0.143 | 0.187 |
1 Maximum distance between bulkheads, baffles, or ring stiffeners shall not exceed 60 inches.
49 C.F.R. §178.346-2