How to Cut, Thread and Assemble Black Iron Gas Pipe

Why this matters

A threaded gas joint fails in one of three ways: a thread cut with dull dies that tears instead of cutting, dope smeared into the bore where it ends up in an appliance's gas valve, or a fitting tightened against an upstream joint that quietly backs off two threads. None of the three shows on the day if you skip the test, and all three show as a smell in a closet a month later. This card is the sequence from a stick of pipe to a made-up, tested run, with the reason for each step.

This is gas work. Do it only where your license covers fuel gas piping in that jurisdiction; in many places a plumbing license does, and in some a separate gas fitter's license or a permit pulled by one is required.

Before the first cut: isolate the gas and set up the shop

If you smell gas on arrival, nobody touches a switch, a phone or the meter inside the house. Everyone goes outside, and the call to the gas utility is made from there. The rest of this card assumes a house with no odor.

Isolate the section you are opening. Close the shutoff upstream of the tie-in point; if the tie-in is on the main run, close the meter service valve (a quarter turn so the tang crosses the pipe). Tag the valve with your name and the time so nobody reopens it, and lock it where you have a meter lock and the utility allows you to use one. Prove the isolation: light a burner on an appliance downstream and watch it go out, then turn that burner back off. Note every standing pilot in the house now, because each one will need relighting when gas is restored.

Set up threading away from the living space. Cut and thread in the garage or outside, not in the furnace room. Threading oil is combustible, it smokes when a power threader runs hot, and a sulfurized oil mist is an irritant: keep the work area ventilated and never thread near an appliance burner or a pilot. A power threader's chuck will wind up a loose sleeve, a glove or a rag in under a second: no gloves on the hand near the chuck, sleeves rolled or tight, foot switch only, and the machine stopped before you reach toward the die head. Chips off a threader are razor sharp and oily; brush them, do not wipe them.

Material check. Gas pipe on this shelf is standard weight (Schedule 40) black steel with malleable iron fittings. The model codes allow galvanized steel, but some utilities and local amendments do not accept it for natural gas; check locally. Read the stencil on the stick before you cut it.

Cut square, then ream the bore

1. Cut with a wheel cutter, square. Lock the pipe in a pipe vise with the cut line about 6 in from the jaws so the pipe does not whip. Tighten the cutter a quarter turn per revolution. Feeding harder than that flares the wall and rolls a heavier burr inside. A square cut matters because the die starts square to the end it is fed onto; a cut 1/16 in out of square starts the thread crooked and the fitting makes up cocked.
2. Ream the inside burr until the bore is full diameter. A wheel cutter pushes a lip into the bore. Left in, it restricts flow on small pipe and, more to the point, it sheds a curl of steel that travels to the first appliance control downstream. Ream until you can run a fingertip around the inside edge without catching anything.
3. Knock off the outside flare. A light pass with a file or the reamer's outer edge lets the die start without tearing the first thread.

Thread with sharp dies and plenty of oil

1. Check the dies before the first thread of the day. Look at the teeth. A chipped or rounded tooth leaves a torn, shiny, ragged thread where a sharp die leaves clean, dull-gray crests. Torn threads are the leak path; dope does not reliably fill them, and the adopted IFGC or NFPA 54 does not permit pipe with stripped, chipped or otherwise damaged threads to be used.
2. Oil the die and the pipe continuously. Use a thread cutting oil, not motor oil or a penetrant. Oil cools the cut and carries the chip out. A dry die overheats, the thread smears, and the die dulls in a few joints.
3. Run the thread to length, not by feel. On most hand and power dies the thread is complete when the end of the pipe is flush with the small end of the die. Check the die maker's instructions, because that reference varies by die head. NPT is tapered (1 in 16 on diameter), so a thread run too long gives a fitting nothing to wedge against and it bottoms on a thin, loose thread; a thread run short makes up with too few threads engaged.
4. Back the die off, then clean the thread and the bore. Wipe oil and chips off the threads with a rag and a wire brush, and swab or blow out the bore. Cutting oil left on the threads keeps dope from bonding, and chips left in the bore travel the same path as a reaming burr.

Gauge the first thread on a new stick or a freshly set die head: a fitting should run on by hand about three to four turns on 1/2 and 3/4 in pipe before it gets snug. One turn by hand means the thread is cut oversize (too short or the die is set wrong); a fitting that runs on nearly to the end by hand means the thread is too small and will not seal.

Seal the male threads only

1. Use a sealant rated for the gas you are carrying. The adopted IFGC or NFPA 54 requires thread joint compound resistant to the action of the gas, which means a pipe dope listed for gas service or a PTFE tape made for gas (commonly a heavier, yellow-marked tape). White plumbing tape on a gas joint is a red flag to an inspector for good reason. See related: Pipe Threads + Sealant Selection Reference.
2. Apply to male threads only, and leave the first thread bare. Dope on the female thread is pushed into the bore by the advancing pipe. Starting a thread back from the end keeps the sealant out of the gas stream. Work dope into the roots of the threads with the brush; a heavy blob on top of the crests does nothing but squeeze out. Tape goes on clockwise looking at the pipe end, so the fitting tightens it rather than unwinding it, with the first thread left bare.

Do not use both dope and tape on one joint unless the sealant maker's instructions call for it. The combination lubricates the joint enough that people over-tighten it into a split fitting.

Make up and back hold

1. Start by hand, then wrench. Run the fitting on by hand until snug, then turn it with a wrench. On 1/2 and 3/4 in pipe, two to three wrench turns past hand-tight is typical, leaving a couple of threads showing. If a fitting goes past that with no rising resistance, back it off and look at the thread before you go further.
2. Back hold every time. Put a second wrench on the fitting or pipe you are turning against, so the torque goes into the joint you are making and not into the one behind it. Without a back hold, turning a new elbow onto a tee rotates the tee on its upstream thread. A quarter turn of loosening on an existing joint is enough to leak, and that joint was not in your test plan because you did not touch it.
3. Never back a fitting off to align it. Tighten to the next position that points the right way. A threaded joint backed off to line up is a joint that has given up its seal; if overshooting is a problem, the run needs a union where unions are allowed (exposed, accessible locations only, never concealed in a wall).

Worked example: a range branch off the basement main

A customer is adding a gas range in a kitchen above a finished basement with an open joist bay in the utility room. The main is 3/4 in black iron at 7 in water column. Branch size is set by 'Gas Line Sizing for an Added Appliance' and came out 1/2 in for this range and length; we do not redo it here.

Isolation. The tie-in point is a capped tee left on the main years ago, so only the meter valve isolates it. Meter valve closed and tagged at 9:05. Water heater pilot and furnace pilot both noted, and both went out within a minute of lighting a burner test.

Layout. The capped tee's branch outlet is 3/4 in. A 3/4 x 1/2 bushing is not used; we use a 3/4 x 1/2 reducing elbow, so the reduction is in a fitting rather than in a bushing inside a fitting. From there the run goes 14 ft along the joist bay to a point under the range, then up through the floor.

Cutting a length. For each piece, cut length = center to center, minus each fitting's center-to-face, plus the thread engagement at each end. Measure center-to-face on the actual fitting in hand rather than from memory. For one 1/2 in horizontal piece between two elbows 36 in center to center, with each elbow measuring 1 in center to face and taking about 1/2 in of thread (illustrative values: measure your own fittings): 36 minus 1 minus 1 is 34 in, plus 1/2 plus 1/2 is 35 in to cut. Recheck from the other direction: a 35 in pipe with 1/2 in disappearing into each elbow leaves 34 in exposed, and adding the two 1 in center-to-face distances gives 36 in center to center.

At the appliance. The riser comes up through the floor behind the range to an accessible shutoff valve, within the distance the adopted code sets from the appliance (commonly 6 ft, same room), then a sediment trap (a tee with gas entering the top of its run, a capped nipple below and the outlet off the side, as the code figure draws it), then the listed appliance connector. Most ranges are exempt from the sediment trap requirement in the adopted codes, but this shop puts the trap on every new branch where it fits, because it costs one tee and one capped nipple.

            shutoff valve (accessible)
     elbow +--------[V]--------+ elbow
           |                   |
           |                   | gas down into
           |   to range <------+ tee: in the top,
           |   connector       |      out the side
           |                   | nipple, 3 in min
           |                 [cap]  sediment trap
 ==========|============================ kitchen floor
           | riser
           |
           +-- 1/2 in run, 14 ft in bay --+ 3/4 x 1/2
                                          | reducing ell
 -----------------------------------------+-------
   main 3/4 in (7 in WC)        old capped tee

Make-up. Every joint was doped on the male thread, first thread bare, and made up with a back hold on the fitting behind it. The reducing elbow on the old tee got a back hold on the tee itself, because that tee is threaded onto a main nobody had opened in twenty years.

Test. Appliance connector not yet attached and the outlet of the new shutoff capped, so the range's regulator is not exposed to test pressure. The branch was tested per the pressure, gauge and hold time in 'Gas Line Pressure Test + Leak Detection SOP' and held with no drop. The old main and the existing tee were bubble-tested with leak solution at operating pressure after the meter valve was reopened, because they were outside the tested section.

How to verify you got it right

The pressure test is the finish test, not the bubble test. Leak solution finds a leak you already suspect; a gauge holding for the full test time proves the whole section. Use the test pressure, gauge and duration in 'Gas Line Pressure Test + Leak Detection SOP' and the adopted code, with appliance regulators and valves isolated from the test pressure.

Then restore gas, and that has its own checks.

• Open the meter valve slowly with your eyes on the meter dial. With every appliance valve closed, the test dial should not move. A dial creeping with everything off is a leak, and you close the valve and find it before going further.
• Leak check every joint you made, and every existing joint you held against, with leak solution or a combustible gas detector at operating pressure. Never with a flame.
• Connect the range and leak check the connector ends. Then purge the new branch of air through a range burner or to outdoors, as the adopted code and the appliance instructions allow, with no other ignition source near the purge point and the room ventilated, and light every burner until the flame is steady and blue.
• Relight every standing pilot you noted at isolation, and watch each main burner light and run. A pilot you forget is a no-heat call tonight.

If a joint bubbles at operating pressure, close the valve upstream, back the joint apart, inspect the thread, recut it if torn, and remake it. Do not just tighten a leaking joint further: a joint that leaks after proper make-up has a thread problem that more torque will turn into a cracked fitting.

References

• IFGC or NFPA 54, as adopted: material, thread joint compound, damaged-thread, shutoff valve, sediment trap, purging and testing requirements.
• See related: Gas Line Pressure Test + Leak Detection SOP.
• See related: Gas Line Sizing for an Added Appliance.
• See related: Pipe Threads + Sealant Selection Reference.
• Die and threader manufacturer instructions for thread length reference and machine guarding.