Internal vs. External Thread: What Type of Adapter Do I Need
Internal vs. External Thread: Which Type of Transition Do You Really Need
One of the most common sources of confusion when purchasing heating fittings is the question of threading. The customer stands before the choice and does not know whether their elbow, valve, or device has an internal or external thread – and thus what the fitting must have on the other end. A wrong choice here is not just a delay; in the worst case, it means dismantling the piping, returning to the store, and unnecessary costs. This article will give you a solid foundation to correctly identify the thread, choose the right type of transition, and avoid common mistakes we see in practice every week.
Basic Terminology: What Is a Thread and Why the Type Matters
A thread is a spiral-shaped profile cut or rolled onto the surface of a pipe, fitting, or threaded part of a device. Its primary function is to mechanically connect two components and, in combination with sealing, hermetically seal the joint so that the medium (water, glycol mixture, etc.) does not leak and pressure does not escape.
When selecting a coupling or transition, it is not enough to know only the pipe diameter. Three other parameters are equally important:
- Thread type – internal (IG, internal, female) or external (AG, external, male)
- Thread standard – BSP (British pipe thread G, most common in European heating systems) vs. NPT (American, rare here) vs. metric (M)
- Thread size – given in inches (1/2", 3/4", 1") and corresponds to the nominal diameter, not the actual measured diameter of the thread
In the HEPWORTH heating systems we work with in this category, it is almost exclusively BSP (G) threads. If you see the marking G 1/2 or R 1/2 on a device, it is precisely this type. Overview of thread types: R denotes external conical, Rc internal conical, G external cylindrical, Rp internal cylindrical – in heating practice you mainly encounter G (cylindrical) on a range of couplings and transitions.
How to reliably distinguish internal and external threads on site
Identifying a thread is not a science, but if you are doing it for the first time, it can be a bit confusing. Here is a practical procedure I recommend to everyone who is about to buy a coupling or transition:
Visual inspection
You look at the end of the pipe, valve, or device. If you see the thread (spiral) on the outer edge of the pipe or nipple – you have an external thread (AG, male, external). If the thread "disappears into" the hole and you have to literally look into the hole to see it – it is an internal thread (IG, female, internal).
Tactile test
Place your finger on the end of the fitting. An external thread is "sharp" – you can feel the spiral profile on the surface. An internal thread is "smooth" on the outside, as it is hidden inside. When you put your finger inside, you can feel it there.
Mutual rule
An external thread ALWAYS screws into an internal thread. Never an external into an external or internal into an internal – that simply does not fit. This principle is the same as with any screw and nut: the screw has an external thread, the nut an internal one.
Measurement as a check
If you are still unsure, use a caliper. For an external thread, you measure the outer diameter of the threaded part (e.g. approx. 20.9 mm for 1/2" BSP). For an internal thread, you measure the diameter of the hole (also approx. 20.9 mm for 1/2" BSP). Both would correspond to the same nominal size of 1/2" – which confirms that they fit together. More detailed tables can be found in the article Dimensions of couplings with sealing: how to correctly measure the pipe diameter.
Compression fitting vs. straight transition: when to use which
In this category, you will encounter two basic types of fittings. To understand which thread you need, it is first necessary to understand what each of these components does. A detailed analysis can also be found in the article Difference between a compression fitting and a straight transition. Here, we will focus on the aspect of threading.
Compression fitting
A compression fitting is used to connect copper (or other compatible) pipes to fittings that have a thread. The fitting works on the principle of a compression ring – you insert the pipe into one end, tighten the nut, and the ring creates a hermetic connection without soldering. The other end of the fitting has a thread – and this is where you ask: internal or external?
A compression fitting 1/2" x 15 (for 15 mm copper pipe) has an internal BSP G 1/2" thread on the threaded side. This means that the device you are connecting it to (a radiator valve, pump, expansion tank) must have an external G 1/2" thread. If you are unsure, look at the Compression fitting 1/2" x 15 – the type of thread is clearly stated in the product parameters.
The same applies to the Compression fitting 3/4" x 15 – the dependency is the same: the pipe side is compression (15 mm), and the fitting side has an internal BSP G 3/4" thread.
Straight transition with thread
A straight transition is a fitting that uses a compression connection for the pipe on one side and has a thread on the other – but here you also encounter a variant where the thread is internal "from the side of the pipe". In other words, the transition allows you to connect a pipe to a device with an opposite orientation of the thread, which a standard compression fitting cannot handle.
Straight transition 1/2" x 15, internal thread is a typical example: one side has a compression connection for a 15 mm pipe, the other has an internal G 1/2" thread. This fitting is used, for example, when your outlet from the wall or device has an external G 1/2" thread – the transition accepts it and at the same time allows you to connect a copper pipe without soldering.
For larger dimensions, Straight transition 3/4" x 22, internal thread serves the same purpose – the same principle, but for a 22 mm pipe and a G 3/4" thread.
Typical situations in practice: when you need internal and when external thread
In practice, we repeatedly encounter the same configurations. The following scenarios will help you quickly orient yourself without having to go into theory.
Scenario 1: Connecting a radiator valve
You have a panel radiator to which you want to install a thermostatic valve. The thermostatic valve has an external G 1/2" thread on the inlet (standard for this type). Your pipe is copper 15 mm. You therefore need a fitting that compresses the 15 mm pipe on one side and has an internal G 1/2" thread on the other, so you can screw it onto the external thread of the valve. The correct choice: Compression fitting 1/2" x 15 or Straight transition 1/2" x 15, internal thread.
Scenario 2: Connecting a circulation pump
A circulation pump (e.g. Grundfos UPS or similar) has an internal G 1 1/2" or G 1" thread on the inlet and outlet. You are connecting a 22 mm copper pipe. Here you need a reduction with an external thread on the side of the pump and a compression connection on the side of the pipe. Note – in this case, you are looking for a fitting with an external thread on the side of the device. In the range, you encounter this with larger dimensions; for smaller circuits, the Compression fitting 3/4" x 22 is suitable.
Scenario 3: Wall outlet with external thread
A nipple with an external G 1/2" thread protrudes from the wall (a typical case during reconstruction, where the original plumber installed a steel nipple). You want to connect a 15 mm copper pipe to it. You need a fitting with an internal G 1/2" thread on one side. The correct choice: Straight transition 1/2" x 15, internal thread.
Scenario 4: Connecting an expansion tank
Membrane expansion tanks have an external G 3/4" thread on the outlet/inlet (nipple). Connecting copper pipes of 22 mm is solved by the Compression fitting 3/4" x 22 – with an internal G 3/4" thread on the side of the tank.
Scenario 5: Replacing an original fitting with a different pipe diameter
The older system had a 22 mm copper pipe, the new part of the reconstruction is 15 mm. You need to reduce the diameter and at the same time maintain the threaded connection. A combination of fittings and possibly reducing bushings comes into play here – or the Compression fitting 3/4" x 15, where the 3/4" thread fits the larger device and the compression connection secures the 15 mm pipe.
Reducers, nipples and other transition elements: when the basic pair is not enough
In practice, basic combinations are sometimes not sufficient. The system may give you two fittings with the same type of thread (both external or both internal), or you may need to overcome a difference in dimensions. Additional elements come into play here:
- Reducer nipple (reduction AG/AG): Has external threads on both sides, but of different dimensions. Allows you to connect two fittings with internal threads of different sizes.
- Extension nipple (nipple AG/AG): External threads on both sides of the same dimension. Extends the external thread if the fitting does not have enough thread or is embedded.
- Reducer coupling (IG/IG): Internal threads on both sides, different dimensions – for connecting two outputs with external threads of different sizes.
- Adapter AG/IG: External thread on one side, internal thread on the other – typically used to change the direction of the thread gender in the system.
This is used, for example, when connecting a larger 1" loop to a boiler with a 3/4" output, or vice versa when connecting a secondary loop via a manifold. Compatibility of different elements in the Hepworth system is addressed in a separate article Compatibility of Hepworth couplings with other systems and pipe materials.
Sealing of threads: what influences the choice
Choosing the right thread is not only about the type (internal/external) and size, but also about knowing how the joint is sealed. The cylindrical BSP thread (G) is not sealing by itself – it requires additional sealing: PTFE tape or hemp fiber with paste. The conical thread (R/Rc) can be sealing by itself under certain conditions, but in heating practice you will almost exclusively encounter G.
Incorrect sealing or the complete absence of sealing on a cylindrical thread almost always leads to a leak. More on this topic can be found in the article How to seal threaded joints in heating systems: PTFE vs. hemp fiber. For compression fittings (olive), you do not add sealing – the olive ensures sealing mechanically.
Most common mistakes when choosing thread type and how to avoid them
From practice, I know that most problems can be easily avoided – if the customer knew what to look for. Here are the mistakes I see repeatedly:
- Mixing up terms male/female, AG/IG: The customer confuses "male" and "female" (in English documentation) or AG and IG. Result: orders the opposite type. Solution: always verify directly on the fitting – what protrudes or what is inside the hole.
- Assuming the same thread on all devices: Not all boilers, valves and pumps have the same type. Some pumps have internal threads (which is less common), some valves have external. Always check the technical specification of the device.
- Ignoring thread size: The customer knows they need an internal thread, but does not know whether it is 1/2" or 3/4". They measure the outer diameter of the pipe, not the thread. These two numbers differ – for a 1/2" BSP thread, you measure an outer diameter of approximately 20.96 mm, not 12.7 mm (which would be the actual half inch).
- Mixing standards: Using a BSP fitting on an NPT thread or vice versa. Threads seem to twist, but do not seal and can damage the thread profile. For devices imported from the USA or some Asian countries, verify the standard.
- Trying to directly connect two of the same thread types: Mistake described in the diagram above. A connecting element is always needed.
Product overview: the right choice for common situations
For most common heating installations in family homes, the following selection covers the situations you will encounter. A summary in the form of a practical table:
| Situation in practice | Pipe | Thread on fitting | Correct fitting |
|---|---|---|---|
| Thermostatic radiator valve | 15 mm Cu | AG G 1/2" | Coupling with compression 1/2" x 15 |
| Wall nipple (AG), Cu pipe | 15 mm Cu | AG G 1/2" | Straight adapter 1/2" x 15, internal thread |
| Expansion tank, boiler output | 22 mm Cu | AG G 3/4" | Coupling with compression 3/4" x 22 |
| Boiler nipple (AG 3/4"), Cu 22 mm | 22 mm Cu | AG G 3/4" | Straight adapter 3/4" x 22, internal thread |
| 3/4" valve on 15 mm Cu distribution | 15 mm Cu | AG G 3/4" | Coupling with compression 3/4" x 15 |
Special Cases and Exceptions
Devices with internal thread on both sides
Some fittings (e.g., pressure-reducing valves, filters, certain control elements) have internal threads on both sides. In such cases, you need fittings with external threads on the side of the fitting and compression ferrule on the pipe side for both inputs/outputs. If you cannot find such fittings in the basic range, the solution is a combination: a standard coupling + an extension with an external thread.
Devices with external thread on both sides
Less common, but they do exist – for example, some wall-through fittings, sleeve couplings. In this case, you need a fitting with an internal thread on both ends. A transition with an internal thread is the correct choice in this case.
Boilers with European vs. British connections
The majority of boilers sold on the Slovak market have outputs with an external thread of G 3/4" or G 1" (British BSP). Some older Czech and German-made boilers may have metric threads of M30x1.5 or M32x1.5. Identification is more difficult in these cases and you will not get by without a caliper and comparison of the pitch.
Stainless steel and plastic fittings
The principle of internal and external threads applies equally to stainless steel and plastic fittings. However, these materials may have different tolerances and when combined with brass Hepworth fittings, I recommend always checking the tightening torque to avoid damaging the plastic thread.
Installation and common mistakes when tightening
Correct identification of the thread is only the first step. Even if you have the correct fitting, incorrect installation leads to leakage. A few practical rules:
- Never start tightening without sealing on a cylindrical thread (G). PTFE tape is wound clockwise, i.e., in the tightening direction, in 2–3 layers.
- Never twist the thread – after initial resistance, stop and finish tightening by max. 1–2 turns further. A twisted brass thread is damaged.
- When tightening a compression ferrule (olive), use a wrench to hold the body of the fitting – not just the nut. Without holding, the body of the fitting may rotate and damage the olive or the pipe connection.
- When retightening after a pressure test (service inspection after winter or after reconstruction), do not over-tighten – more in the article Inspection and retightening of compression fittings during annual heating service.
A complete installation procedure can be found in the article Step-by-step installation of a coupling with compression fitting on copper pipe.
Frequently Asked Questions (FAQ)
How can I tell if the fitting has a BSP or metric thread?
The most reliable method is to measure the thread pitch using a thread gauge (set of templates) or a caliper and compare it with a table. BSP 1/2" has a pitch of 1.814 mm (14 threads per inch), BSP 3/4" also 1.814 mm. Metric thread M22 has a pitch of 1.5 mm. If the pitch does not match the BSP values, it is likely a metric or NPT thread. In case of doubt, consult before ordering.
Can I use a direct transition with an internal thread instead of a coupling with a compression fitting?
It depends on the configuration. Both fittings have a compression ferrule for the pipe and a threaded part for the fitting. The difference is that the transition is mainly intended for connecting the pipe to a fitting with an external thread, while the coupling with a compression fitting can have various combinations. For the same dimensions and the same type of thread, they are often interchangeable – check the specific product parameters.
Why is the connection leaking even though I used the correct fitting?
The most common reasons: insufficient or incorrectly applied sealing (PTFE tape wound in the wrong direction or missing), undersized olive in the compression connection, damaged thread, or insufficient tightening. A detailed troubleshooting procedure can be found in the article Why the compression coupling is leaking and how to fix it.
Is there a difference between a G 1/2" and R 1/2" thread?
Yes. G denotes a cylindrical BSP thread – this does not seal by itself and requires sealing (PTFE, hemp). R denotes a conical BSP thread (external) and Rc denotes an internal conical thread – these can seal under certain conditions without added material, but in heating practice, sealing is added here as well. For HEPWORTH systems, the standard is G (cylindrical). When connecting a device with an R thread to a G nut, be careful – it may happen that the tightening goes smoothly, but the connection will be leaky or mechanically unstable.
Can I use Hepworth couplings on steel or plastic piping?
Couplings with compression fittings in the HEPWORTH system are primarily designed for copper and some other metal pipes. For plastic (PEX, MLCP), specific fittings with different ferrules and tightening torque ranges are intended. Mixing without verifying compatibility can lead to leakage or mechanical damage. More in the article Compatibility of Hepworth couplings with other systems and pipe materials.
What if I cannot determine the thread type even after a visual inspection?
Take a photo of the fitting from several angles including an axial view (from above/below into the opening), attach a ruler for scale, and consult with the seller. Alternatively, disassemble the fitting (if it is safe and the system is shut down) and bring it to the store in person. An experienced technician can reliably identify the thread without measuring – simply by sight and touch.
Conclusion: thread is not a detail
Selecting the correct type of thread – internal or external – is one of those steps where a small mistake causes an unnecessarily big problem: wrong fitting, disassembled piping, repeated purchase. The good news is that if you understand the basic principle (external thread into internal thread, never the other way around), you will handle the vast majority of situations that heating installation brings.
Always verify before purchasing: the type of thread on the fitting (external or internal), the nominal size in inches (1/2", 3/4", 1"...), and the pipe diameter (15 mm, 22 mm...). With these three pieces of information, you will find the correct fitting in the couplings with compression fitting category without problems. If you want to understand the selection in depth, I recommend reading the article How to choose the correct compression coupling for heating, where all parameters are analyzed in detail.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.