External thread vs. internal thread on elbow – what difference does it make

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External thread vs. internal thread on elbow – what difference does it make and why it matters

When solving heating systems or plumbing installations in practice, most people pay attention to the pipe diameter and the elbow angle. The question of the thread – specifically whether the elbow is fitted with an external (AG) or internal (IG) thread – often remains in the background. And it is precisely this that is the source of many unnecessary problems: incorrectly purchased parts, leaky connections, improvised workarounds, double trips to the store. If you have ever spent half an hour disassembling an older installation just to find out that you have no way to match an elbow with an external thread, you know exactly what I am talking about.

This article is about this seemingly minor, but practically crucial topic: what is the technical difference between an external and an internal thread on an elbow, which type is used when, what it means for the selection of the opposite part, what are typical situations from the installation practice and how to avoid the most common mistakes. We will explain it from the basics – so that it is clear even for someone who is dealing with their first major installation, but also in sufficient depth for an experienced plumber who wants to have the terminology firmly in mind.

Basic terminology: AG and IG – what do these abbreviations mean

The term AG comes from the German "Außengewinde" – external thread. The term IG is "Innengewinde" – internal thread. This abbreviation practice is so deeply rooted in the plumbing practice in Slovakia and the Czech Republic that you will find it practically in all technical catalogs, on material labels and in project documentation. If you order an elbow or another shaped part and in the description you see "1/2" AG × 15 mm", it means that one end of the elbow has an external thread of size 1/2 inch and the other end is a smooth crimped or push-fit connection for a pipe of diameter 15 mm.

The threads on plumbing fittings are in the vast majority of cases Whitworth pipe threads according to the ISO 7/1 standard (tapered – marked R/Rc) or ISO 228 (cylindrical – marked G). In practice for heating and cold and hot water systems, you will most often encounter:

G 1/2" (1/2 inch) – the most common size for radiator valves, drain cocks, air vents
G 3/4" (3/4 inch) – common for boiler groups, distributors, pumps
G 1" and larger – main lines, boiler inlet/outlet nozzles

An external thread (AG) is a nut cut into the outer surface of the pipe or the neck of the fitting. An internal thread (IG) is a nut cut into the cavity – into the opening of the neck. In order for the connection to work, one AG and one IG thread of the same size and type must always be used. This is an absolute basic condition – it is not possible to pair AG 1/2" with AG 1/2", or IG with IG.

Where the thread is located on the elbow – the geometry of the reduced elbow

A standard elbow (L piece) has no thread at all – both ends are smooth and intended for direct connection to the pipe by crimping, pushing or welding (depending on the material). A reduced elbow has one end larger and the other smaller – this reduces the diameter without the need for a separate reducer. An elbow with a threaded end has at least one end fitted with a thread instead of a smooth neck.

Here comes the first practical question: when the manufacturer says "threaded elbow", does it mean an external or internal thread? The answer depends on the specific product and its purpose. Take a look at Threaded elbow 1/2" x 15 – this elbow has a threaded neck (1/2") on one end and a smooth crimped end for a 15 mm pipe on the other end. The term "threaded" in the context of HEPWORTH systems typically refers to the end that can be screwed to the opposite threaded part – i.e., it is a fitting combining the crimping system with a threaded connection.

Why the type of thread matters – a practical view from installation

Imagine this situation: you are installing a radiator valve on a finned radiator in the bathroom. The valve has an external thread G 1/2" AG at the inlet. You therefore need a mating part with an internal thread G 1/2" IG. If you mistakenly bought an elbow with an external thread as well, you cannot connect it directly – you either need a coupling (with IG on both sides), or you have to return the whole part.

This situation is not exceptional. In practice, I have seen many times how a plumber came to a site with a whole bag of fittings, a third of which was unusable precisely due to the mix-up between AG and IG. Especially when working with HEPWORTH system fittings, where the crimping system and threaded ends function as one unit – it is important to have a clear head about what connects to what.

A second typical scenario: reconstruction of an older pipe layout, where the original pipe was steel threaded. The new layout is made of HEPWORTH plastic pipes, but the connection to the existing valves and taps remains the original – threaded. Here, the combination of crimping and threaded systems is not only common, but also necessary. And precisely at this transition point, the type of thread (AG or IG) plays a key role: it must match what is on the other side.

External thread on an elbow – where to find it and when to use it

An elbow with an external thread (AG) is screwed into a part with an internal thread. Typical use cases in practice:

Connecting to a collector valve or a regulating valve, which has an inlet with an internal thread (IG). An elbow with AG is inserted into the valve opening and sealed with Teflon or hemp with paste.
Connecting to a ball valve with an IG inlet – the same logic applies. Valves are typically supplied with IG on both sides, so the supply pipe must have AG.
Direct installation into a boiler's threaded port – most boilers have threaded ports with internal threads, so the pipe layout leading into them must be terminated with an external thread.
Connecting to a safety valve, air vent or pressure gauge – these fittings are mostly IG, so they require a mating AG part.

An external thread on a fitting is stiffer and less prone to damage to the thread when unscrewing repeatedly, because the thread is on a more solid and thicker material. On the other hand, if the thread is damaged or stripped, it is much harder to repair than an internal thread – it cannot be easily re-tapped.

Internal thread on an elbow – where to find it and when to use it

An elbow with an internal thread (IG) accepts an external thread from the mating part. Typical use cases:

Connecting to an external thread on a pipe or nipple – if the pipe or extension part (nipple) ends with an external thread, an elbow with IG is screwed onto it.
Connecting two elbows via a nipple connector – two IG ends and a nipple connector AG-AG, a common configuration in tight spaces.
Connecting to an external device with an external thread – for example, a circulation pump, expansion tank or flange with an AG inlet.
Extending the pipe layout via threading – an internal thread on the elbow allows for easy screwing of another part without the need for a coupling.

An internal thread is physically more protected – it is located in the depth of the port and external mechanical damage reaches it more difficultly. The disadvantage is that when sealing with hemp, it is harder to check how much sealing material is inside the thread, which at unprofessional installation leads to leaks.

Reduced elbows and the dependence of thread type on the combination of dimensions

Reduced elbows combine two different diameters in one part – thus changing the direction and the pipe diameter at the same time. This is an elegant solution that saves space and eliminates one connecting element. In the category reduced elbows you will find various combinations, where one or both ends can be threaded.

For example, in the combination of a 15 mm pipe and a 1/2" thread: the crimping end of 15 mm goes directly onto the pipe, the threaded end of 1/2" AG or IG goes onto the fitting. If you need to connect a valve with an IG inlet, you need AG on the elbow side. If you connect a nipple or extension with AG, you need IG on the elbow. This seemingly trivial decision determines whether your entire installation will fit together or not.

In practice, it looks like this: the designer determines the pipe layout, the material list lists the fittings, and the installer buys them. Problems arise when the installer buys the materials "by eye" without a plan – and precisely the type of thread on one piece in the whole chain can cause the installation to be impossible to complete. More about how to properly dimension transitions between pipe diameters can be found in the article Dimensioning elbows when transitioning between different pipe diameters in this Knowledge Center.

Specific products and their thread configuration

Let's look at a few specific products and show how the thread appears in their identification.

Elbow with crimping 1/2″ × 15 is a typical example of a combined fitting: one end is crimping for a 15 mm pipe, the other end is threaded with a 1/2" crimping. This elbow is used for the transition from HEPWORTH plastic pipe directly to a fitting with an internal or external thread – depending on what type of thread is applied to this piece. Always check before purchase what type of thread (AG or IG) is on the threaded end, and verify what is on the opposite fitting.

On the other hand, K-kus 15 × 15 and K-kus 22 × 22 are classic 90° elbows without any threading – both ends are smooth, pressed, and intended for pipes of the corresponding diameter (15 mm or 22 mm). Threads do not come into play here, as these elbows are used to change direction within a plastic piping system without transitioning to a threaded system.

Interesting are also 45° Elbow – 15 × 15 and 45° Elbow – 22 × 22: also threadless, but with a different bend angle. The question of AG/IG does not arise here, as the thread is not part of the component at all. For more information on selecting the right elbow angle, see the article 45° Elbow or 90° Elbow – When to Use Which Type.

Sealing a threaded connection – the difference between AG and IG when applying sealing material

Correct application of sealing material depends on which thread the seal is applied to. The rule is: sealing material should be applied to the external thread (AG). The reason is simple – when tightening, the sealing material is pressed into the gaps of the thread between AG and IG. If you were to apply the sealing material to the internal thread, a large portion would be pushed out during insertion, and the connection would not seal reliably.

For hemp sealing: wrap the hemp yarn in the direction of the thread (in the direction of tightening), and apply a sealing paste. The number of wraps depends on the thread diameter – for 1/2", 8–12 wraps are typical, for 3/4", it is 10–15. For Teflon tape, the same principle applies – the tape is wrapped around the AG thread in the tightening direction so that it is pulled into the thread during turning, not pushed out.

A common mistake beginners make when installing an elbow with an IG thread is applying the sealing material to the IG (internal thread of the elbow) instead of the AG part that is inserted into it. In this case, the sealing material is improperly distributed, the connection does not seal, and after pressurizing the system, it leaks at the joint. The topic of leaks and their solutions is discussed in more detail in the article Connection Tightness – Most Common Problems When Installing Reduced Elbows and also in Why an Elbow Leaks and How to Fix It Without Replacing the Entire Joint.

Material differences and their impact on thread type

Threads on fittings differ not only in orientation (AG/IG), but also in the material from which they are made. In HEPWORTH heating systems, you typically encounter:

Brass threaded ends on a plastic body – the most common combination. Plastic (polypropylene or PEX) forms the body of the fitting, while brass forms the threaded insert. Brass withstands higher torque during tightening and is more resistant to corrosion over time. The thread (AG and IG) is very precise here, and with proper sealing, it is extremely reliable.
Plastic-only threads – less common, suitable for lower pressures and temperatures. Here, you need to be careful when tightening – plastic can easily be overstretched and the thread can be stripped.
Steel and stainless steel threads – used in industrial or high-temperature applications. Less typical for standard heating systems.

A brass AG thread on a plastic fitting is strong, precise, and durable. A brass IG thread is equally reliable, but if the opposite AG part is over-tightened, the brass insert can be damaged, especially if the fitting is exposed to long-term vibrations or thermal shock. Always tighten the thread with a wrench, but not with excessive force – when using hemp or Teflon tape, a hand-tight plus 1–2 turns with a wrench is sufficient.

Combination of AG and IG on one elbow – what it brings

There are also fittings that have AG on one end and IG on the other – this is a so-called reducing nipple configuration built into the elbow. This configuration is useful when changing the direction of the pipe run and also transitioning between two threaded systems of opposite polarity. Practical example: a boiler has an IG 3/4" outlet, and a manifold also has an IG 3/4" inlet. Between them, you need to make a 90° turn, so you use an elbow with AG 3/4" on one side (screwed into the boiler) and IG 3/4" on the other side (screwed into the nipple outlet of the manifold with AG).

This configuration is less common in standard product ranges, so in practice, it is often solved with two elbows and a nipple between them – which is one more part, but provides greater flexibility in positioning and direction. If you are working in a space with limited room, I recommend also reading the article How to Determine the Correct Diameter and Angle of an Elbow for Your Pipe Run.

Practical tips: How to identify the type of thread before purchasing

If you are going to buy a threaded elbow and are unsure which type of thread you need, I recommend the following procedure:

Determine what is on the opposite side – if the valve into which the elbow will be threaded has an internal thread (IG), you need an elbow with an external thread (AG). And vice versa.
Measure the thread diameter – a 1/2" thread has an outer diameter of approximately 20.9 mm, a 3/4" thread has approximately 26.4 mm. Measure the diameter using a caliper: if you measure the outer diameter of the thread on an existing part, you get the AG dimension; if you measure the diameter of the opening with an internal thread, you get the IG dimension.
Use a visual inspection with the naked eye – an AG thread is visible as a protrusion with teeth on the outside. An IG thread is a cavity, where the teeth are inside and not visible from the outside, only the opening.
Check the product's technical specifications – every serious manufacturer or seller explicitly states AG or IG for threaded fittings. If it is not mentioned, ask – it is a legitimate and important question.

Special situation: you have an old fitting from a pipe system and want to buy a replacement. Before visiting the store, take photos of the fitting from multiple angles, especially a close-up of the threaded end. A good photo will show whether the thread is external (AG) or internal (IG). Based on this, the store employee can quickly and correctly advise you.

Pressure and temperature parameters – does the type of thread affect them?

The type of thread (AG or IG) does not directly affect the maximum pressure or temperature for which the fitting is certified. These parameters are determined by the material of the fitting, the quality of the thread insert, and the sealing connection. HEPWORTH system fittings for heating are typically certified for operating pressure of 6–10 bar at temperatures up to 95°C, depending on the specific product.

What does affect the tightness and long-term reliability of the connection is the precision of the thread manufacturing – and here the difference between a high-quality fitting and a cheap unknown product is crucial. The pitch accuracy and tolerances must meet the ISO 7 or ISO 228 standard. If the tolerance is greater, the sealing material must fill a larger gap, and the long-term tightness is less certain – especially during temperature cycles, when the material alternates between stretching and shrinking.

For more information on selecting the right fittings for the HEPWORTH system, see the article How to choose the right elbow for HEPWORTH plastic pipe.

Summary of key differences – a clear table

Property	External thread (AG)	Internal thread (IG)
Location of the thread	On the outer surface of the neck	Inside the bore of the neck
Opposite part	IG of the same size	AG of the same size
Where to apply the sealing material	On the AG thread (before tightening)	On the AG part that enters the IG
Visibility of the thread	Fully visible from the outside	Hidden inside the neck
Protection against damage	Less protected, more mechanically durable	Better protected by the bore
Typical use	Entry into valves, boilers, taps with IG	Accepting nipples, extensions, pipes with AG
Repair of a damaged thread	Difficult (chasing AG is not easy)	Possible with a tap (threading tool)

Reduced elbow or separate reducer – how does the type of thread affect this

There is another dimensional consideration: should a reduced elbow (a single part that changes both direction and diameter) be used, or a standard elbow plus a separate reducer? This topic is discussed in more detail in the article Reduced elbow or separate reducer – which is more cost-effective. From the perspective of the thread, however, it is important to note this: every additional connection is a potential risk point – a place where a leak can occur. If a reduced elbow with a thread solves the transition with one part instead of three, it is clearly an advantage in terms of the reliability of the system.

Typical scenario: a transition from a 15 mm plastic pipe to a 1/2" threaded fitting at a 90° angle. You can solve this as:
Variant A: 90° elbow 15 × 15 + direct reducer 15 mm → 1/2" AG = 2 pieces, 2 connections
Variant B: Elbow with a reduction 1/2" × 15 = 1 piece, 1 dependency on the type of thread

Variant B is cleaner. The condition is that the type of thread (AG or IG) on the elbow matches what the fitting requires. If the elbow has AG 1/2" and the fitting has IG 1/2", it is perfect. If the elbow has IG 1/2" and the fitting also has IG 1/2", you need to add an intermediate piece – and we are back to variant A, just with a different arrangement of parts.

FAQ – Most frequently asked questions about external and internal threads on elbows

How can I tell if an elbow has an external or internal thread, if I don't have the technical sheet?

Simply: look at the threaded end of the elbow. If you see the thread teeth on the outer surface of the protrusion, it is AG (external thread). If you see a hollow hole with grooves on the inner wall of the hole, it is IG (internal thread). If you have a caliper, measure the outer diameter of the protrusion (AG) or the inner diameter of the hole (IG) and compare it with the Whitworth dimension table: 1/2" AG has an outer diameter of about 20.9 mm, 3/4" AG has about 26.4 mm.

Can I connect two elbows, both with external threads?

No directly. Two AG threads cannot be screwed together – there is nothing to screw into. You need to insert a coupling (nipple) with IG threads on both sides, or another intermediate piece with IG on both ends. An alternative is to use an elbow with IG on one end instead of AG.

Does it matter in which direction the elbow is turned when screwing it in?

With standard Whitworth threads, the rule of right-hand threads applies – you screw in the direction of the clock (when looking down at the thread). You unscrew in the opposite direction. This applies equally to AG and IG, because they must be paired with opposite orientations. Exceptions (left-hand threads) do not occur in standard plumbing practice for heating.

Can I use Teflon tape instead of hemp sealing on an elbow with an AG thread?

Yes, Teflon tape (PTFE tape) is a full alternative to hemp sealing with paste. It is applied in the same way to the AG thread, in the direction of tightening, in layers (typically 3–5 layers for a 1/2" thread). The advantage is a cleaner installation and easier application. The disadvantage is that Teflon tape is more suitable for threads that are not expected to be repeatedly unscrewed – with each opening of the connection, the tape is damaged and must be replaced. For threaded joints that will be opened for service, it is better to use hemp with paste.

What happens if I combine a metric thread M with a pipe thread G on an elbow?

This is a relatively common and dangerous mistake. A metric thread M (e.g., M20) and a pipe thread G 1/2" have similar diameters, but different thread pitches. It may seem at first that they can be screwed together, but the threads damage each other and the connection will never be tight. The result is damage to the threads on both parts and the need for replacement. Always check whether it is a G (Whitworth pipe) or M (metric) thread – these are incompatible systems.

How to proceed if I don't know what thread the fitting has to which I am connecting the elbow?

If you have physical access to the fitting: take a photo of the threaded inlet of the fitting and use a caliper to measure the inner diameter of the hole (if it is IG) or the outer diameter of the protrusion (if it is AG). Compare with the table: G 1/4" IG = hole about 13.2 mm, G 3/8" IG = hole about 16.7 mm, G 1/2" IG = hole about 18.6 mm, G 3/4" IG = hole about 24.1 mm. If you don't have the fitting at hand, look for the manufacturer's label or technical documentation – the thread is always listed there. In the worst case, bring the elbow to the fitting and try to manually fit it (without tightening) – if the thread starts to turn without resistance for the first two turns, it probably fits; if it resists immediately, it does not fit.

Conclusion: A Small Detail, A Big Impact

The question of external vs. internal thread may seem at first glance like a minor technical detail – a matter hidden in two characters of the product code (AG or IG). In reality, however, it is one of those details that can determine whether the entire installation goes smoothly the first time, or turns into a several-hour effort to figure out why one particular part just won't fit. Experienced plumbers have these things intuitively ingrained – but even they occasionally stop in front of the shelf to double-check whether the elbow requires AG or IG. It's not a sign of shame. It's a sign of professionalism.

Before placing any order for threaded fittings, ask yourself one question: what is on the other side? If it's IG, you need AG. If it's AG, you need IG. That simple. And if you're unsure, it's always better to ask before confirming the order – you'll save time, avoid the cost of return shipping, and unnecessary stress on the construction site.

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