Pipe Threading

Before any cutting starts, the system must be depressurised. For heating, that means draining at least the affected section. For water, it means closing the supply and opening the lowest tap. The tradesperson then works roughly like this:

1. Cut the pipe square, ideally with a pipe cutter, so the end is straight.
2. Remove burrs inside and out and clean off rust and old paint.
3. Fit the correct size die head and coat the pipe with cutting oil.
4. Cut the thread with an even motion. With a hand tool, turn back briefly every so often to break the swarf.
5. Clean the thread, check its length and surface, and test fit the fitting.
6. Seal the thread with hemp and jointing paste, sealing cord or PTFE tape, then tighten the joint.

Why does the oil matter so much, and why the fuss about starting straight? If the die goes on at an angle, the thread ends up crooked. The fitting then sits askew and the joint tends to weep. Without oil, the thread tears and leaves a rough surface that is hard to seal.

Water, heating and gas pipework in Europe uses pipe threads in inch sizes, often simply called BSP. In practice, these markings matter most:

• R: an external tapered thread cut directly onto the pipe. It narrows as you tighten it and seals in the thread together with the sealing material. This is the standard thread on steel pipe for all three uses
• Rp: an internal parallel thread in fittings, into which the tapered R thread is screwed. Some fittings have a tapered internal thread instead, marked Rc
• G: a parallel thread that does not seal on its own. It is used where a flat washer or O-ring does the sealing, for example on unions, radiator valve tails and pumps

Internal threads almost always come ready-made in fittings and valves. Cutting an internal thread on site is rare. What matters is that the thread type and size on the pipe match the fitting. American NPT threads look similar but are not compatible with BSP, so watch out for imported parts. If in doubt, the valve manufacturer's data sheet will tell you.

Black steel pipe is the classic material in older heating systems. It threads well by hand or by machine and gives a strong joint that can be taken apart again.

Galvanised pipe is common in water supply pipework in flats and older houses. Cutting a thread naturally removes the zinc coating at that point, so a good seal is important to keep moisture away from the bare steel. Galvanised pipe should not be welded or heated, which makes threading the natural choice.

Stainless steel pipe is harder and tougher on tools. It needs sharp dies suitable for stainless and is usually threaded by machine.

Plastic and multilayer pipe, copper pipe and thin-walled steel press systems are not threaded. They are joined with press, compression or soldered fittings, or with threaded adaptors. Badly corroded pipe is also unsuitable, because the wall left after cutting the thread would be too thin.

A ratchet hand threader is enough for smaller diameters and a few threads, for example a repair in a bathroom or a radiator connection. Its advantage is that it fits into tight spaces where a machine simply will not go.

A powered threader or threading machine is used for larger diameters, harder materials and larger numbers of threads. The threads come out more even, the work goes faster, and there is less strain on the pipe and on the wall it is fixed to. If a section can be removed, the tradesperson will sometimes cut it out, thread it on the machine and refit the finished piece.

How much work is involved, and so what the job costs, depends mainly on:

• The pipe diameter and the number of threads needed
• The material, since stainless and old hard pipe are slower to cut
• Access, for example a pipe tight against a wall, in a corner or behind the boiler
• The condition of the pipe, because a corroded or furred end has to be cut back to sound metal
• Whether the heating has to be drained and refilled, or the water shut off for the whole house
• Whether it is water, heating or gas, as gas comes with stricter testing requirements

In practice, the thread itself is often the quick part. Most of the time goes on freeing a rusted old joint and preparing the spot.

On drinking water pipework, only sealing materials approved for potable water should be used, meaning pastes and cords marked for that purpose. The thread should be clean, with no leftover oil or swarf that could get into the system.

On heating, the joint has to cope with heat and repeated warming and cooling. The sealant must be suitable for heating systems, and the joint must not be overtightened, or the fitting can crack. If the pipe is going to be chased into a wall or floor, every hidden joint must pass a pressure test before it is plastered over.

On gas, threads may only be cut and joints made by a qualified, registered gas engineer. The work is followed by a pressure test and a safety check, with written records. Altering gas pipework can also involve the gas network operator. Whether building regulations require a notification or approval for your job is something to check with the local building authority. If you smell gas, do not use electrical switches or appliances, open windows, leave the building and call the gas emergency number or 112.

In a block of flats, risers and shared pipework must not be drained or altered without agreement from the building management or owners' association.

When you accept the work, look at these points:

• The thread is smooth, with no torn teeth or burrs
• The fitting or valve sits straight, not at an angle
• Not too much thread or sealing material sticks out of the fitting
• On water, the system was put under pressure and the joint stayed dry
• On heating, the system was refilled and bled, and the pressure gauge did not drop during the test
• On gas, there is a written record of the pressure test and the safety check
• The area has been cleaned of oil and swarf