Pressing fittings with internal and external threads: how to properly connect the pipework

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Pressing fittings with internal and external threads: how to properly connect the pipework

Every plumber who has completed at least a few dozen jobs with PEX or PEX/AL/PEX piping knows that the biggest problem when designing a system is not the pipe routing itself, but the points where the system "switches" – where the plastic pipe has to connect to a threaded fitting, a strainer filter, a circulation pump, a boiler valve assembly, or a classic steel threaded pipe. This is exactly where PEX/AL/PEX press fittings with an internal or external thread come into play – and this is exactly where mistakes are most often made, which then show up as leaks, misaligned fittings, or mechanically overloaded joints.

In this article we will look at the whole topic systematically: what an internal thread (IG) and an external thread (AG) are, where each type belongs, how to properly size transition points, what typical situations look like in practice, and what to do to ensure the result is truly tight, strong, and reliable in the long term.

Internal and external thread: basic differences and labeling

Before we get into specific situations, let's review the terminology, as there is quite a lot of confusion about it even among experienced tradespeople.

Internal thread (IG – Innengewinde, or abbreviated as VZ): the thread is cut inside the fitting. This means you screw the external thread of a fitting into it – e.g. a ball valve with an external thread of 1/2", a test valve, a reducer, a nipple, or another component. In Slovak this is also called a "threaded socket" or simply "female". On diagrams and in catalogs it is marked F (female) or with the abbreviation VZ.

External thread (AG – Außengewinde, or abbreviated as VoZ): the thread protrudes outward from the fitting like a peg. You screw a fitting or component with an internal thread onto this thread. We call it "male". On diagrams M (male) or VoZ.

For a correct connection, a simple rule applies: male always into female – the external thread of the fitting into the internal thread of the fixture, or the internal thread of the fitting onto the external thread of the fixture. If you mix this up, you will either get an unstable joint, or the components simply won't physically connect.

When to use a fitting with an internal thread and when with an external thread

This is a question we get on our customer service line quite regularly – and we always answer it the same way: it depends on what is on the other side of the joint. You always need to check what type of thread the fixture or component you are connecting to has.

Typical cases for a fitting with internal thread (VZ / IG)

Connecting ball valves – most common ball valves in a system have an external thread of 1/2" or 3/4" on both sides. A fitting with IG mounts directly onto them.
Thermostats and valve heads – connecting a radiator thermostatic valve, where the valve body has an external thread.
Strainer filters – a screw-in filter with an external thread: a PEX fitting with IG on the inlet and outlet.
Circulation pumps – most hot water circulation pumps have an external thread of 1 1/4" or 1 1/2"; here you need a corresponding larger-diameter fitting with IG or a reducing fixture.
Heat meters (calorimetric flow meters) – usually external thread on both the inlet and outlet.

Typical cases for a fitting with external thread (VoZ / AG)

Fixtures with internal thread – for example end pieces, a manifold with IG connection, pump groups with an IG inlet.
Underfloor heating – manifold – many manifolds have IG for connecting PEX pipes, so the fitting on the PEX side must have AG.
Steel threaded piping – when extending an existing steel system with a PEX branch, where the steel pipe has an internal thread (socket), you use a PEX fitting with an external thread.
Test fixtures and pressure gauge connections – adapters for connecting a test pressure gauge usually have IG, so the PEX branch must have AG.

Thread sizes and how they connect to PEX pipes

A very important point: threads on fittings are always pipe threads according to ISO 7/1 (the so-called Whitworth pipe thread, marked R for external and Rp for internal). These threads are specified in inches: 1/4", 3/8", 1/2", 3/4", 1", 1 1/4", 1 1/2", 2".

The most common combinations in practice for residential and apartment installations:

PEX pipe diameter	Common fitting thread	Typical use
16 mm	1/2" (most common)	Underfloor heating, radiators, hot water
16 mm	3/4" (less common)	Transition to a larger fixture
20 mm	3/4"	Distribution branches, boiler connection
25 mm	1"	Main distribution circuit, DHW cylinder
32 mm	1 1/4" or 1 1/2"	Connecting larger boilers, industrial piping

Important: the inch size of the thread does not correspond to the pipe diameter. A 16 mm PEX pipe therefore does not have a 16 mm thread – it has a 1/2" thread, which is nominally 15.88 mm in outer thread diameter (R 1/2"). This needs to be kept in mind when choosing fixtures.

Threaded elbows: when and where to use them

Elbows with internal or external threads are one of the most widely used groups of fittings in any installation. They allow you to change the direction of the pipe while also creating a transition point for a threaded connection. There are several variants, each with its own specific purpose.

Standard 90° elbow with internal thread

Press elbow 90° PEX/INTERNAL THREAD (16x1/2") is a classic choice in situations where you need to bend the pipework by 90° while also connecting to a fixture with an external thread of 1/2". A typical practical situation: the radiator supply pipe runs along the floor or in a pre-wall installation and needs to be turned upward to the thermostatic valve. The elbow is pressed onto the pipe and screwed onto the valve body.

The advantage of this solution over a separate straight transition fitting combined with a copper or unthreaded PEX press elbow is the cleanliness of the installation – one component handles both the turn and the transition point, with no extra fitting in the line.

Elbow with external thread

Similarly, you would use a press elbow PEX/EXTERNAL THREAD (16x1/2") where the fixture or opposing component has an internal thread. A practical example: an underfloor heating manifold has an internal thread of 1/2" on each port. You direct a PEX pipe branch to each port using this elbow – you screw the external thread of the elbow into the internal thread of the manifold port, and press the pipe onto the other end.

Caution: When screwing the external thread into the manifold, don't forget the sealing – either sealing tape (Teflon) or hemp fiber with paste. Conversely, the internal thread of a fitting (where the thread is in the fitting itself) is sealed the same way: tape or hemp on the external thread of the fixture you're screwing into the fitting.

Wall elbow – for sanitary connections

A special category is the wall press elbow PEX/INTERNAL THREAD (16x1/2"). This elbow additionally has a mounting plate or a reinforced flat back that allows it to be fixed into a wall before plastering or tiling. The internal thread then protrudes from the wall and serves as a connection point for a faucet or angle valve.

From practice: This is the number one solution when installing angle valves for toilet tanks, or connections under a sink, when the supply pipes are routed in the wall. The elbow is fitted before plastering the wall, fixed to the mounting surface, the pipe is insulated and plastered over – leaving only the connection opening with an internal thread in the wall, at exactly the right height and position.

Working with reducing elbows for transitions to a different diameter

We don't always run pipework in a single diameter from the boiler to the last radiator. In practice, we often see that the main branches run in 20 mm diameter and the branches to individual take-off points are 16 mm. Exactly at such a branch or direction change point, it makes sense to use a reducing elbow.

Press elbow 90° reducing PEX/PEX (20x16) solves both functions at once – it changes the direction by 90° and at the same time reduces the diameter from 20 mm to 16 mm (or the other way around, depending on the direction of installation). A practical example: the main riser route runs in 20 mm, and at the end of the route the last branch to a radiator goes off in 16 mm while also changing direction. One reducing elbow replaces a tee + reducer + elbow – that is, three fittings instead of one.

When planning your pipework, always consider whether it's worth thinking through the layout more carefully before ordering fittings. We cover this in more detail in a separate article Reducing tees and elbows: when and why to use them in a system.

Sealing threaded joints: nothing works without this detail

A press joint is tight on its own – tightness is ensured by an EPDM or silicone O-ring that gets firmly compressed during pressing. But a threaded joint – i.e. the point where the fitting is screwed onto a fixture – has to be sealed manually.

The following methods are most commonly used:

PTFE tape (Teflon): Wrap it in the direction of the thread (i.e. in the direction of screwing) onto the external thread. Use 2–5 layers depending on the quality of the tape and the size of the thread. Suitable for water, steam, and most gases (depending on the tape's certification).
Hemp fiber with sealing paste: A proven old method that provides very reliable sealing in heating systems. The fiber is wound onto the external thread and then coated with paste. Advantage: it fills any thread irregularities better.
Liquid sealants (anaerobic): For example Loctite 577 or Henkel Tangit Uni-Lock. Suitable where higher temperature resistance is needed or where there is a risk of vibration. Not suitable for contact with drinking water unless certified.

Practical tip from the field: When screwing a fitting with an external thread (e.g. an AG elbow) into a manifold, try to achieve the correct rotational angle. Wrap the Teflon, screw it in by hand until it stops, then tighten with a wrench – no more than 1 to 1.5 turns. More than that is counterproductive and can crack the body of the fitting or the fixture. Caution: press fittings made of copper or brass have fixed tightening torques – do not exceed them. Brass bodies are relatively brittle if the thread cracks.

Installation procedure for a threaded press elbow: step by step

Based on experience from many jobs, we recommend the following procedure:

Measure and cut the pipe – use a pipe cutter; the pipe must be cut perpendicular, without any angle or deformation.
Align and calibrate the pipe end – for PEX/AL/PEX pipes it is essential to use a calibrator (expander or a setting clamp) so that the end of the pipe is round and has the correct diameter. A deformed pipe end = a leaking joint.
Fit the press ring (sleeve) – before inserting the fitting, slide the press ring (stainless steel or nickel-plated ring) onto the pipe. A common mistake: forgetting to fit it and then having to remove the fitting again.
Insert the fitting into the pipe – the fitting must go in all the way, which you can check via a small inspection window or a stop point. The pipe must be visible in the fitting's inspection window (if it has one).
Slide the press ring into position – move the stainless steel ring onto the body of the fitting so that it aligns with the press groove.
Press the joint – using a press tool (manual or electric) with the appropriate jaw for the given diameter. The jaws must be certified for the given fitting – different manufacturers have different profiles (TH, U, V profile).
Check the press visually – the ring must be evenly deformed around its entire circumference, with no gaps.
Seal the thread and screw onto the fixture – PTFE tape or hemp fiber on the external thread, tighten with a wrench using the correct torque.

You can find more details on tools and procedure in the article Installing PEX press fittings step by step: tools and procedure.

Transition points in practice: three typical job scenarios

Scenario 1: Bathroom renovation – connecting PEX to an existing steel system

The customer has an old steel cold water system with a 3/4" external thread on the inlet valve to the bathroom. They want to extend the system with PEX and connect new fixtures. Solution: a fitting with an internal thread of 3/4" (socket or straight IG coupling) onto the existing external thread of the steel pipe. From there, PEX 20mm continues with press fittings. At the end, where 1/2" external-thread angle valves are connected, transition fittings with IG 1/2" are used.

A typical mistake on such jobs: the customer buys a fitting with AG (external thread) and tries to fit it onto the external thread of the steel pipe – of course it doesn't fit, since both have an external thread. Result: an unnecessary trip back to the store.

Scenario 2: New building with underfloor heating – connecting the manifold

An underfloor heating manifold has an internal thread of 1/2" on each port. We need to connect a 16mm PEX pipe to each port. Here we use a straight transition fitting with an external thread of 1/2", or, if we also need to change direction immediately, a press elbow with an external thread of 1/2". We screw the AG of the fitting into the IG port of the manifold, tighten with a wrench, press the PEX pipe on, and we're done.

On the other side of the circuit, when connecting back to the manifold (return branch), the procedure is the same. The manifold also has a row of IG ports on the other side, so again AG fittings are used.

Scenario 3: Installing a bathroom faucet – wall elbows

When installing a shower corner or bathtub faucet, two connections need to be built into the wall – cold and hot water. The pipes are routed in a pre-wall installation or a groove in the wall, in 16mm PEX diameter. At the correct height and with a mutual spacing of 15 cm (standard for most faucets with a 15 cm spacing), wall press elbows with IG 1/2" are fitted. These elbows are fixed into the pre-wall structure, the pipes are pressed on, and the wall is closed up. Only the internal threads protrude from the wall, onto which the faucet's angle valves or the faucet body itself are then screwed. This is a standard and very elegant solution for all types of faucets with an external thread of 1/2".

Most common mistakes when working with threaded fittings

Based on service work and customer feedback, we can compile a list of recurring mistakes:

Confusing IG and AG: Ordering a fitting with the opposite thread to what is needed. Always think through what is on the other side of the joint.
Wrong thread size: Buying 16x1/2", but the fixture has 3/4". Always verify beforehand what thread the fixture you are connecting to has.
Missing thread sealing: The pressed part is tight, but the thread leaks because they forgot the PTFE tape or hemp fiber.
Overtightening: Cracking the fitting body from over-tightening. Brass is strong, but not unlimited. 1–1.5 turns after reaching hand-tight resistance is enough.
Pressing without a calibrator: An oval pipe end → uneven compression of the O-ring → leakage. You can find more about mistakes in the article Common mistakes when pressing PEX fittings and how to avoid them.
Wrong press tool jaw: Different fitting manufacturers require different jaw profiles. The TH profile (Viega, Uponor) is not compatible with the V profile of other systems.
Rotating the elbow after pressing: Some plumbers try to rotate the elbow into a better position after pressing it – this is not allowed; a pressed joint must not be mechanically loaded or rotated.

Fitting materials and their effect on connecting to threaded fixtures

Most PEX press fittings for residential and apartment installations are made of brass (CW617N or CW614N); some premium systems use red copper or stainless steel. The fitting body usually has a nickel or tin surface finish, which helps corrosion resistance and makes it easier to identify pressed joints (a pressed stainless steel ring stays shiny, which serves as a visual check).

When connecting a brass fitting to a steel fixture (e.g. an old steel valve), you need to consider galvanic corrosion – brass and steel in the same circuit with aggressive water can cause accelerated corrosion damage to the steel component. In such cases, it is recommended to insert a dielectric insulator or use a stainless steel transition fitting.

We cover the topic of selecting the right fittings by material and compatibility in detail in the article How to choose PEX/AL/PEX press fittings: type, size, and compatibility.

Sizing transition points and system hydraulics

When designing transition points, it is also important to consider hydraulics – every threaded joint and every fitting represents a local resistance that affects pressure losses in the system. In practice, this mainly matters for long pipe runs or extensive underfloor heating installations, where hydraulic balance is key.

Basic rule: the internal diameter at the thread should not be smaller than the internal diameter of the connecting pipe. For example, a 16mm PEX pipe has an internal diameter of approx. 10–12mm (depending on wall thickness). A 1/2" thread has an internal diameter of approx. 13mm – so the transition point introduces only minimal resistance. A problem arises if you use too small a fitting or a fixture with a restricted flow cross-section (e.g. a ball valve with a small opening).

For larger diameters (20mm, 25mm), the same logic applies: a 3/4" thread for a 20mm pipe, a 1" thread for a 25mm pipe – these combinations ensure a smooth transition without significant narrowing. You can read more about sizing in the article What diameter of press fittings do I need for my system.

Checking and sealing transition joints

After installation is complete, a pressure test must follow. For water systems, a pressure test is standardly performed at 1.5 times the working pressure, at least 0.6 MPa (6 bar), for a minimum of 30 minutes (according to STN EN 806). For heating