PHMSA letter of interpretation

Requirements for Cylinder Repair and Marking

Pipeline and Hazardous Materials Safety Administration · May 27, 1999 · Reference No. 99-0128

PHMSA says to use caution with this letter. On August 8, 2002, RSPA finalized Docket HM-220D, which consolidated the requirements for qualification, use, and maintenance of cylinders in Part 180, Subpart C. As a result, the references provided in this guidance are no longer accurate. Please see Part 180, Subpart C for the current requirements for the qualification, use, and maintenance of cylinders. Although not all references to requirements previously codified in § 173.34 have parallel requirements in Part 180, Subpart C, you may wish to review § 180.211 for additional information.

Reggie’s quick take

Someone asked whether a company that is neither a cylinder manufacturer nor an authorized repair facility may braze, weld, or use similar heat methods to attach fittings to a DOT 4L cylinder's piping system, or to the foot ring, neck ring, top ring, or top ring supports of a DOT-4 series cylinder to attach an identifying or marking plate. The answer is no for both. Under § 173.34(i), welding or brazing repairs on DOT-4 series cylinders may be done only by the cylinder's manufacturer or by a repair facility approved by PHMSA's Associate Administrator. The letter also addresses stamping depth, but it gives no fixed limit.

Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.

This responds to your letter, dated May 10, 1999, concerning the requirements for cylinder repair and marking in the Hazardous Materials Regulations(HMR; 49 CFR Parts 171.180). Your specific questions are paraphrased and answered below.

Q1: Is a company that is neither a cylinder manufacturer nor an authorized repair facility allowed to braze, weld, or perform similar applications involving het to attach gas inlet, outlet, and/or vent fittings to the piping system of a DOT 4L cylinder?

A1: No. Section 173.34(i) of the HMR authorizes repair of a DOT-4 series cylinder by welding or brazing to be performed only by performed only by a manufacturer of this type of DOT specification cylinder or by a repair facility approved by the Associate Administrator for Hazardous Materials Safety.

Q2: Is a company that is neither a cylinder manufacturer nor an authorized repair facility allowed to braze, weld, or perform similar applications involving heat to any portion of the foot ring, neck ring, top ring, or top ring supports of a DOT-4 series cylinder for the purpose of attaching an identifying plate or marking plate pursuant to §173.34(c)?

A2: No. See response to Question 1 above.

Q3: What is the maximum allowable stamping depth for DOT specification cylinders?

A3: Generally, markings on cylinders must be of a size and depth that will not create harmful stress concentrations. The marking application must ensure that the wall thickness from the root of the stamping to the interior surface is greater than or equal to the minimum wall thickness as prescribed in the manufacturing specification. The person applying the marking may need to consult with the cylinder manufacturer to determine allowable stamping depths.

A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.