49 CFR 179.100-17 - Closures for openings.
(a) Closures shall be of approved design and made of metal not subject to rapid deterioration by the lading. Plugs, if used, shall be solid, with NPT threads, and shall be of a length which will screw at least six threads inside the face of fitting or tank.
Title 49 published on 2014-10-01
no entries appear in the Federal Register after this date.