How to Rethread Plumbing Pipes

Shane Grey

Plumbers and pipefitters rethread steel plumbing types in two ways: manual rethreading with a pipe die and automatic rethreading with a pipe threading machine. Both methods produce precise results and both are within the capabilities of a do-it-yourself plumber.

A threaded pipe and fitting construct a fire sprinkler supply line.

Learn how to rethread plumbing pipes and create custom lengths of supply pipe for your plumbing project.

  1. Place the pipe into the saddle of a pipe vise or bench vise. Tighten the vise’s clamp to secure the pipe. Stretch a tape measure across the pipe’s length and use a pencil to mark the position of a cut, if applicable.

  2. Turn a pipe cutter’s adjustment knob to open the cutter’s jaw. Place the open jaw over the pipe, position the jaw over the mark, and tighten the adjustment knob to secure the cutter’s blade against the mark. Squirt two or three drops of cutting oil onto the pipe beneath the cutter’s blade. Turn the cutter around the pipe twice. Tighten the adjustment knob and apply a couple more drops of cutting oil. Turn the cutter around the pipe until it slices and severs the pipe.

  3. Insert a reamer into the cut end of the pipe. Turn the reamer to clean and remove burrs from the pipe’s interior.

  4. Attach an NPT die to a die stock. Squirt two or three drops of cutting oil onto the cut end of the pipe. Thread the die onto the cut end of the pipe. Turn the die stock’s handle to rotate the die around the pipe’s circumference. Turn the die stock 180 degrees at a time. After a 180-degree turn, slightly back off to cast out metal shavings. Continue to cut and back off until the pipe nearly runs through the entire die. Remove the die and die stock from the pipe.

  5. Loosen the vise’s clamp. Lift the pipe from the vise’s saddle. Clean the newly threaded end of the pipe with a rag.

  6. Tip

    Buy cut lengths of pipe to reduce work time.