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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Interpretation Response #25-0050

Below is the interpretation response detail and a list of regulations sections applicable to this response.

Interpretation Response Details

Response Publish Date:

Company Name: Fleener Consulting LLC

Individual Name: Art Fleener

Location State: IA Country: US

View the Interpretation Document

Response text:

August 7, 2025

Art Fleener
Fleener Consulting LLC
3741 Mathews Road
Ames, IA  50014

Reference No. 25-0050

Dear Mr. Fleener:

This letter is in response to your April 23, 2025 email requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to the minimum thickness for Department of Transportation (DOT) specification Motor Carrier (MC) 331 cargo tanks. You explain that Pipeline and Hazardous Materials Administration (PHMSA) previously issued a memo to the Federal Motor Carrier Safety Administration's (FMCSA) Hazardous Materials Division, providing clarification on the minimum thickness requirements for cargo tanks under the HMR, including those for MC 331 cargo tanks. You ask questions regarding the minimum shell thickness requirements.

We have paraphrased and answered your questions as follows:

Q1. Is the memo from PHMSA to FMCSA, as referenced in your incoming email, regarding the minimum shell thickness of an MC 331 specification cargo tank still valid?

A1. Yes, this memo remains valid for the specific scenario that it addressed at the time it was drafted. Please keep in mind that an interagency memo, like the one referenced in your incoming email, is intended for "internal use only." This memo is not meant to be shared outside of the agency, as it serves as internal guidance for specific situations.

Q2. For an MC 331 specification cargo tank, what thickness is considered below the minimum requirement?

A2. In accordance with § 178.337-3(e), the minimum thickness of the tank shell and head shall be determined using structural design requirements in Section VIII of the American Society of Mechanical Engineers (ASME) Code or 25% of the tensile strength of the material used. Additionally, the minimum required thickness for an MC 331 cargo tank constructed before October 1, 2003, is determined by the thickness indicated on the U-1A form minus any corrosion allowance, as required in § 180.407(i)(9). If no corrosion allowance is indicated on the U-1A form, then the thickness of the tank shall be the thickness of the material of construction indicated on the U-1A form with no corrosion allowance. For an MC 331 cargo tank constructed after October 1, 2003, the required minimum thickness is the value that has been marked on the cargo tank, as required by §§ 180.407(i)(9) and 178.337-17(b).

Incidentally, § 180.407(i)(9) states that the minimum thickness for an MC 331 cargo tank constructed after October 1, 2003, can be found on the specification plate, but this is incorrect. However, the minimum thickness markings can be found on the name plate in accordance with § 178.337-17(b) and this will be corrected in a future rulemaking.

I hope this information is helpful. Please contact us if we can be of further assistance.

Sincerely,

Dirk DerKinderen
Chief, Standards Development Branch
Standards and Rulemaking Division

§ 178.337-3(e), § 178.337-17(b), § 180.407(i)(9)

Regulation Sections