Mich. Comp. Laws § 125.559

Sec. 9. Standard full weight wrought iron, galvanized iron or steel or copper pipe, shall be used throughout. Underground piping shall be galvanized or copper or brass. The supply pipe or pipes shall be not less than 1/4 inch in diameter. Unions shall be ground type with conical seating with faces of metal. Flanged or packed joints shall not be used. Valves shall be constructed so that the stem cannot be withdrawn by continual operation of hand wheel. The packing gland shall be provided with a separate shouldered unthreaded follower having a beveled contact space for the compression of the packing. All threaded joints shall be made with glycerine and litharge, or shellac, or other approved compound. All pipes shall be rigidly supported and protected against mechanical injury. Gas supply pipes must be provided with shut-off valves.