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This letter is in response to your May 25, 2017, letter requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to the repair of seamless DOT 3-series specification cylinders. You describe a scenario in which a DOT 3AAX specification cylinder was not equipped with external neck threads at the time of manufacture. You ask if the cylinder may be threaded with external threads and be considered a “re-threaded” repair under the provisions of § 180.212(b)(2), and therefore not require prior approval.
The answer is no. The provisions of § 180.212(b)(2) apply only to cylinders already machined with threads. If a person wishes to machine external threads on a cylinder originally manufactured without external threads, the work must be done in accordance with § 180.212(a), which requires an approval under Part 107, Subpart H.
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