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§179.200-21 Closures for openings.
(a) All plugs shall be solid, with NPT threads, and shall be of a length which will screw at least 6 threads inside the face of fitting or tank. Plugs, when inserted from the outside of tank heads, shall have the letter “S” at least 3⁄8 inch in size stamped with steel stamp or cast on the outside surface to indicate the plug is solid.