Internal thread on valves – how to properly seal and connect the pipe

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Internal thread on fittings – what it actually is and why it matters

If you have ever bought a ball valve, filter, check valve, or any other fitting for a home plumbing or water supply system, you have probably come across the term internal thread – briefly denoted by the letter F (from the English female, meaning "female"). It is one of the most basic terms in plumbing practice, yet also one of those where amateurs – and sometimes even more experienced plumbers – make mistakes that later cost time, money, and nerves.

This article will explain what an internal thread is, how it works, its dimensions and tolerances, and most importantly – how to properly seal and connect it with piping so that the joint does not leak and lasts many years without problems. We will start from the basics, go through practical procedures, and end with specific tips from installation practice.

What is an internal thread – technical definition and standards

An internal thread (Female thread, abbreviation F) is a thread cut into the inside of a hole – for example, in the body of a valve, elbow, reducer, or coupling. An external thread (Male thread, abbreviation M) is screwed into it, which is in turn cut on the outside of a pipe or fitting.

In European plumbing practice, pipe threads according to ISO 7/1 and ISO 228-1 standards are predominantly used, commonly referred to as G-thread (cylindrical, parallel) or Rp-thread (conical, internal only). In practice, we also encounter the American NPT thread, but the G/Rp system is dominant in European plumbing fittings.

Dimensions are traditionally given in inches, although this is a nominal (apparent) dimension, not the actual diameter of the hole. So:

1/2" – nominal diameter, actual internal thread diameter approx. 18–20 mm
3/4" – actual diameter approx. 24–26 mm
1" – actual diameter approx. 30–33 mm
5/4" (1 1/4") – actual diameter approx. 38–41 mm
6/4" (1 1/2") – actual diameter approx. 44–47 mm

These dimensions directly correspond to fittings in the range – for example, ball valve 1/2" FF,P for water has an internal thread G 1/2" on both sides and is intended for pipes with this nominal dimension. Similarly, ball valve 3/4" FF,P corresponds to a 3/4" pipe – which is a very common dimension in home plumbing for main supplies.

The designation FF (Female-Female) on ball valves means that both sides of the fitting have an internal thread – that is, a pipe or fitting with an external thread is screwed onto both sides. This is by far the most common type in home plumbing, as it allows for flexible connection in various directions.

Types of sealing principles for internal threads

Many people think that the thread itself seals. This is a misconception that causes many faults. A metal thread by itself does not seal – a capillary gap remains between the thread turns, through which water will pass under pressure. Sealing is always achieved in another way, depending on the type of thread and the pressure system.

Cylindrical thread G (BSP Parallel)

A cylindrical thread has parallel walls – the diameter of the thread is the same from the beginning to the end. For a cylindrical internal thread G, sealing must be ensured mechanically – either by thread sealing material (Teflon, hemp with paste, liquid sealant) wound on the external thread, or by flat sealing (rubber O-rings or flat gaskets) if a seating surface is designed.

Cylindrical thread G is the most common on internal threads of fittings – practically all ball valves for water use it.

Conical internal thread Rp

A conical internal thread (Rp) tapers inward – the external conical thread R locks in place when tightened and, with sufficient tightening, creates a metal-to-metal self-sealing. In practice, however, sealing compound is also used here, as pure metal self-sealing requires very precise manufacturing and is prone to wear with repeated disassembly.

Sealing materials for internal threads – overview and comparison

The choice of the right sealing material is crucial. Not every seal is suitable for every medium and every pressure. In the practice of domestic plumbing and water distribution, we encounter the following options:

PTFE tape (Teflon)

The most popular and readily available sealing material. A white or blue thin-walled tape made of polytetrafluoroethylene is wound around the external thread before being screwed into the internal thread of the fitting. Advantages: inexpensive, easily available, does not require special tools, functional from -60 °C to +260 °C, chemically inert to drinking water, utility water, and most chemicals.

Disadvantage: if applied incorrectly (too few layers, wrong direction), it will not seal. The correct procedure for winding PTFE tape:

The thread must be dry and clean – grease, dirt, or moisture will reduce adhesion
Begin winding the tape from the base of the thread (from the body of the fitting toward the free end)
The winding direction is always clockwise – the same direction as the thread will be tightened
Standard 3–5 layers for plumbing, 6–8 layers for higher pressures
Gently stretch the tape while winding to ensure it fits into the thread grooves
Press the end of the tape with your finger

Common mistake with PTFE: winding in the wrong direction causes the tape to unwind and "slip" out of the thread instead of being pressed inward during tightening.

Hemp fiber with sealing compound

A traditional plumbing method that remains valid despite its age. Hemp fiber (shavings) is wound around the external thread in the direction of the thread and impregnated with sealing compound (e.g., Fermit, Loctite 577, Hempasit, and others). The compound prevents the hemp from drying out and creates a lasting seal. Advantage over PTFE: hemp better fills irregularities in the thread, and the joint can withstand greater vibration and shaking (e.g., from pumps).

Disadvantage: when used incorrectly (too much hemp), it can clog the pipe – it is very important not to push the fibers into the fitting. Hemp is also not suitable for hot water above 80–90 °C (it dries out and shrinks).

Liquid sealants (anaerobic adhesives)

Products such as Loctite 542, Loctite 577, or Henkel Dichtol are anaerobic – they harden in the absence of air, directly in the thread. They are applied to both the external and internal threads, the fitting is tightened, and the sealant hardens within 30 minutes to 24 hours (depending on the product and temperature). Advantage: excellent sealing, resistance to vibrations, suitable for metal threads. Disadvantage: disassembly requires heating or special tools. For standard fittings in domestic plumbing, where disassembly may be necessary, anaerobic sealants are not recommended – PTFE or hemp are more flexible.

Comparison table of sealing materials

Material	Temperature	Pressure	Disassembly	Suitability
PTFE tape	up to 260 °C	up to 10 bar	easy	cold/warm water, air
Hemp + compound	up to 90 °C	up to 10 bar	medium	cold water, vibrations
Anaerobic sealant	up to 150 °C	up to 40 bar	difficult	industrial, rigid joints

Installation procedure – how to properly connect a fitting with an internal thread

Theory is necessary, but practice decides. Here is a step-by-step procedure that works for installing ball valves and similar fittings with internal threads in domestic plumbing and water distribution:

Step 1: Pipe preparation

The pipe (copper, steel, galvanized iron, plastic with a metal thread) must be cut perpendicularly – a slanted cut causes misalignment of the thread and the risk of leakage. The external thread on the pipe must be complete, without damaged or flattened threads. Check that there are no dirt, metal chips, or rust (limescale) in the thread. Clean everything with a cloth or sponge.

Step 2: Applying the seal

Begin winding the PTFE tape from the base of the thread outward, slightly at an angle, in a clockwise direction (when looking at the end of the thread you are tightening). Each layer should cover 50% of the previous one. For water supply applications at pressures up to 6 bar (typical domestic water system), 4–5 layers are sufficient. For higher pressures (up to 10 bar), use 6–8 layers. Important: the tape must not extend beyond the first thread toward the fitting – it could detach and contaminate the system.

Step 3: Manual Threading

Mount the fitting onto the thread and manually screw it in as far as possible – at least 3–4 threads. If the fitting is difficult to screw in from the start, do not start pushing with a tool – the thread is likely damaged or you are cross-threading (crossed thread). Remove it, inspect, and start again. A crossed thread is one of the most common causes of later leaks.

Step 4: Tightening with a Wrench

Use an adjustable wrench or a spanner of the appropriate size to tighten. Hold the fitting with a second wrench – you must never allow the torque to be transferred through the entire pipe, as this may cause joints higher in the system to crack. Tighten firmly, but without excessive force – with brass fittings and plastic seals (PTFE), over-tightening may cause the body of the fitting to crack. As a rough guide: after manual threading, add another 1–2 turns with the wrench.

After installation, check the position of the ball valve lever – it should be in the position you need (open/close), and the lever should move freely without touching the pipe or the wall.

Fitting Dimensions – Which Valve for Which Pipe

Correct sizing is important not only for sealing, but also for the hydraulic performance of the entire system. With an undersized fitting, flow is restricted; with an oversized one, unnecessary costs and more complicated installation result.

As a practical guide, the following applies:

1/2" (DN 15) – typical for household appliances, sinks, toilets, garden valves. Ball valve 1/2" FF,P is the standard choice here.
3/4" (DN 20) – main household lines, most household water systems with up to 3 kW output. Ball valve 3/4" FF,P brass is among the most frequently ordered sizes.
1" (DN 25) – more powerful household water systems, pumps with higher flow, storage systems. Ball valve 1" FF,P for water systems and pumps is specifically designed for these applications.
5/4" (DN 32) – industrial applications, powerful pumps, larger garden systems. Ball valve 5/4" FF,P brass is used, for example, in irrigation systems with higher pressure drops.
6/4" (DN 40) – large distribution lines, filling stations, process water systems. Ball valve 6/4" FF,P with internal thread for these applications.

For more information on selecting the correct size, read the article What size of ball valve do I need – from 1/2" to 6/4" in this Knowledge Center.

Common Mistakes in Sealing Internal Threads and How to Avoid Them

Over the years of practice, the same mistakes keep repeating. The following overview will help you avoid unnecessary rework:

Mistake No. 1: Winding PTFE Tape in the Wrong Direction

If you wind the tape in the opposite direction to the threading direction, the tape will gradually unwind and collapse during installation. Result: the joint looks tightened, but it does not seal. Always check the direction – when looking at the end of the thread, wind the tape clockwise (to the right).

Mistake No. 2: Cross-threading

If you screw in the fitting at an angle (crooked), the threads are damaged and a "false joint" is created – the fitting holds, but there is a gap between the threads through which water can leak. Cross-threading is recognized when the fitting initially screws in easily, then suddenly meets extreme resistance or twists unstably. Solution: always manually check the alignment before using a wrench.

Mistake No. 3: Too Much Sealant

Excessive hemp fibers or PTFE tape that extends into the fitting can break off, enter the pipe, and clog filters, pump membranes, or check valve seats. Rule: always apply sealant only to the thread, not beyond it.

Mistake No. 4: Overheating During Soldering Near the Fitting

If you solder copper pipe near a fitting (e.g., attaching a soldered fitting close to it), the high temperature can damage the PTFE seal inside the ball valve or melt the plastic ball. Rule: always temporarily remove or cool the fitting with a damp cloth during soldering.

Mistake No. 5: Neglecting to Re-seal After Initial Pressure

PTFE and hemp settle slightly after the first pressurization, and the joint may begin to leak slightly. This is normal – after commissioning the system, always visually inspect all joints and slightly tighten those showing moisture. Later (after 24–48 hours of operation), the joint will stabilize.

Special Cases – Transition Joints and Reducers

In real water system installations, it rarely happens that all dimensions match directly. In most cases, some kind of transition joint must be addressed – for example, between a 1" pump flange and a 3/4" distribution pipe, or between a 1/2" fitting and a 3/4" pipe. In these cases, threaded reducers and transition pieces come into play.

Important rule: when reducing pipe size, seal each connection separately. A reducing fitting has an internal thread on one side and an external thread on the other – both connections must be properly sealed with PTFE or hemp. It is not uncommon for a customer to seal one connection and forget the other, then wonder where the water is coming from.

For home water supply systems with pumps, we always recommend checking what type of thread the pump's discharge and suction ports have directly – some pumps have external threads, others internal, and the appropriate fitting must be selected accordingly. We elaborate on this in the article How to choose a ball valve for a home water supply or pump.

Materials of fittings and their influence on sealing

The internal thread of a fitting can be made from various materials, which also affects the sealing:

Brass is by far the most common material in home plumbing. Brass threads are hard, resistant to corrosion (when in contact with drinking water, they meet the required standards), machine well, and the threads are precise. PTFE tape adheres well to brass threads. Disadvantage: when disassembled repeatedly (e.g., during service), the thread wears slightly. After about 10–15 disassemblies, we recommend checking the condition of the thread.

Stainless steel – excellent durability, but more pressure is needed for sealing – steel threads tend to "seize" without lubrication. It is recommended to apply a thin layer of vaseline to the thread before applying PTFE.

Plastic (PP, PVC) – plastic fittings have less precise threads and are more sensitive to overheating and mechanical damage. PTFE tape works here as well, but be careful with the tightening torque – a plastic thread can easily twist or crack. Never tighten plastic fittings with the same force as brass ones.

Learn more about material selection in the article Brass or plastic ball valve – comparison for plumbing.

Pressure testing and verifying the seal after installation

After completing the installation, it is necessary to verify the seal before covering the system (e.g., plastering, embedding in a shaft). Procedure:

Slowly fill the system with water – sudden pressurization can push PTFE out of an unseated joint
Let the system remain under operating pressure (typically 2–4 bar for home water supplies) for at least 15 minutes
Visually inspect each threaded connection – dampness or drops indicate that tightening is needed
Place a dry paper towel on the joint – the towel will catch even the smallest amount of moisture that the eye might miss
If leakage is suspected, use soapy water (foam under pressure) or a special detection spray

If a joint still leaks after tightening, it is usually necessary to completely disassemble it, remove the old sealing material, dry it, and apply new sealing material. Never add sealing material "on top of an existing joint" – sealing material must be applied to a clean, dry thread.

Maintenance of threaded connections – long-term prevention

Properly sealed threaded connections do not require regular maintenance – they can function for years without intervention. Nevertheless, it is advisable to visually inspect all fittings once a year, especially in areas with fluctuating temperatures (technical room, basement), where thermal expansion of the piping can gradually loosen the joints over time. For more detailed care of fittings, read the article Maintenance and service of ball valves in home water supplies.

Special attention should be given to fittings on pumps and water supplies, where motor vibrations continuously "work" on the entire piping system. For pumps, we recommend using flexible supply lines (hoses) or anti-vibration pads, which eliminate the transfer of vibrations to threaded connections.

Frequently asked questions (FAQ)

How many layers of PTFE tape are enough for a home water supply at 4–6 bar pressure?

For standard operating pressures in home water supplies in the range of 2–6 bar, 4–5 layers of PTFE tape wound in the correct direction are sufficient. If you have any doubts about the condition of the thread (old, slightly worn), add an extra layer – 6–7 layers will not harm, on the contrary, they ensure better filling of possible irregularities. For higher pressures (over 6 bar), we recommend at least 7–8 layers or a combination of PTFE and sealing paste.

Can I seal a plastic thread with PTFE tape the same way as a brass one?

Yes, PTFE tape is suitable for plastic threads – it does not chemically react with them and fills the gaps well. The difference is in mechanical tightening: a plastic thread (PP, PVC) is significantly more sensitive to over-tightening and can easily crack. Tighten only with manual force plus about 1 turn with a wrench, no more. If you are unsure, find the technical sheet of the fitting with the recommended tightening torque.

What to do if a threaded connection still leaks after tightening?

The first step is to try to slightly tighten the connection with a wrench (by 1/8 to 1/4 turn). If that doesn't help, you must disassemble the connection: unscrew the fitting, completely remove the old sealing material (PTFE or hemp residue from both threads), dry it, check the condition of the thread, and apply new sealing material. Never add sealing material to an existing, wet connection – the new sealing material will not seat properly and the leak will not stop. If the thread shows signs of mechanical damage (deformed threads, grooves), replace the entire fitting or the valve.

What is the difference between FF and FM fittings with internal threads?

FF (Female-Female) means both sides of the fitting have internal threads – pipe with external threads is connected on both sides. FM (Female-Male) has one side with internal and the other with external thread – this configuration is suitable for direct installation into radiators, tanks, or as a transition piece. For most applications in home water supplies (connection to the distribution system), FF fittings are used. The difference is discussed in more detail in the article Ball valve FF vs. FM: what is the difference and when to use which.

Can I reuse a fitting that I have already disassembled – or do I need to replace the sealing material?

The fitting itself is reusable – the brass body, ball, and seat are not damaged during standard disassembly. However, the sealing material (PTFE, hemp) must be completely removed and new material applied with each disassembly. Never reuse old PTFE – after one use, it is partially deformed and will not seal reliably in a new installation. Hemp fiber is also a one-time use – after disassembly, it becomes brittle and loses elasticity.

Is it better to seal with hemp or PTFE – what do experienced plumbers recommend?

Opinions vary in practice, but for home water supplies and cold or slightly warm water distribution, PTFE tape is more convenient and safer for a layperson – it is easier to apply, does not require paste, and has almost no risk of clogging the system. Experienced plumbers prefer hemp with paste for vibrating applications (directly on pumps) and for larger sizes (5/4", 6/4"), where hemp better fills the larger gaps in the thread. A combination of both (a thin layer of hemp under PTFE tape) is also a legitimate technique, but for home DIYers, it is unnecessarily complicated.

Conclusion – reliable sealing is not a coincidence, but a process

An internal thread is a fundamental building block of every threaded connection in home plumbing. Whether you are installing a simple 1/2" ball valve for a garden system or a more powerful 1" valve for a home water supply, the sealing principle remains the same: the right material, the correct direction, the right number of layers, and gradual tightening without excessive force.

The most common mistakes – cross-threading, PTFE tape wound in the wrong direction, and insufficient sealing material – can be easily eliminated if you take your time before installation, inspect the threads, and follow a proven procedure. A properly sealed joint will take care of itself for years without any intervention.

If you are looking for a specific valve by size, check out the entire category of fittings on atria.sk – from 1/2" to 6/4", brass with lever, all in stock and ready for shipment. And if you are unsure about the choice, other articles in this Knowledge Center will help you with every aspect – from selecting the size through material comparison to installation and service.

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