difference between fittings with internal and external thread

When installing crimp systems for PEX pipe, the correct choice of fitting is crucial for the long-term functionality and safety of the entire heating system. A common mistake among beginners is confusing internal threads (female thread) with external threads (male thread), which can lead to incompatibility when connecting to plumbing fittings or pipe runs.

Physical difference in thread construction

The basic difference lies in whether the thread is located on the inside of the coupling or on its outer part. Inside is the female thread, which is used to screw into another part with a male thread, while the external thread is male and is used to insert into a hole with an internal thread. In the PEX system, these types are used for completely different purposes, and swapping them means the connection simply cannot be combined.

Where internal thread is used in practice

Internal thread is the standard for most direct connections between two pieces of PEX pipe, where one piece is inserted into the other and compressed using a crimping tool. These fittings are designed to create a tight connection without the need for further tightening, as the crimping ring ensures pressure on the pipe walls. If you need to connect two pieces of pipe of the same thickness, such as 90° Press Elbow - for PEX pipe - 17x17, you will always use a fitting with an internal thread.

• Direct connection of two pieces of PEX pipe of the same diameter.
• Creating corners and changing the direction of the pipe using elbows.
• Creating tees to divide the flow of heat at one point.
• Connecting different materials, where one side is crimped and the other is threaded (or vice versa).

When to choose external thread and its specifics

External thread is primarily used for the transition from a crimping system to traditional plumbing components that have an internal thread, such as radiators, taps, or valve bodies. This type of fitting allows for a smooth connection of modern PEX pipe to older or hybrid systems where a threaded connection is required. For example, if you need to connect a PEX pipe directly to a thermostatic valve with an internal thread, you will need T-piece PRESS - for PEX pipe - 17x1/2"Fx17, which has an external thread on one side.

It is important to note that the external thread must exactly match the standard (e.g., G 1/2", G 3/4"), otherwise the threads will be damaged or there will be a leak. When selecting, always verify that the component you want to connect has an internal thread that will "surround" your external thread.

Technical parameters and thread measurement

Measuring an external thread with a ruler is a common mistake that leads to the purchase of the wrong part. For an external thread, the outer diameter of the thread is measured, while for an internal thread, the diameter of the hole is measured. Standard metric and imperial threads have precise tolerances that must be followed to ensure the crimped connection is tight and secure after mechanical tightening. An incorrect thread will either prevent connection or cause a water leak under pressure, as the sealing is not sufficiently compressed.

Practical example from installation practice

Imagine a situation where a plumber wants to connect a new radiator to an existing system with PEX pipe. The radiator has standard threaded holes with internal threads. If the plumber mistakenly selects a fitting with an internal thread, he will not be able to fit it onto the radiator, as he would have to "insert" the thread into the threaded hole, which is physically impossible. The correct procedure requires a fitting with an external thread, which is screwed into the radiator. Conversely, when connecting two pipe sections, he must use Press Insert - for PEX pipe - 17x17, which has an internal thread on both sides.

In practice, I have seen many cases where customers had problems with leaks precisely due to the confusion between these two types. Some tried to force the fittings "tighten" them, which leads to deformation of aluminum or brass parts and subsequent leakage. Always check before purchasing what type of threaded connection the component you are connecting to has.

Difference compared to pure crimping systems

Crimping systems are often considered a separate category where all connections are made only by crimping without threads. However, in the real world of heating, we often encounter hybrid solutions where it is necessary to combine both principles. The crimping system is based on plastic or metal elastic sealing, which is compressed by a crimper, while threaded connections depend on tightening and sealing via rubber sealing rings or tape. Therefore, it is critical to know whether it is a pure crimping connection or a connection that includes a threaded part.

• The press fit is permanent and does not require repeated tightening.
• The threaded connection allows for disassembly and reuse, but requires regular tightness checks.
• Hybrid fittings combine both technologies into one unit for installation flexibility.
• Incorrect selection of the type of threaded part leads to the impossibility of completing the installation.

Visual identification using a thread gauge

Skilled installers on site use a simple yet crucial tool – a thread gauge (often referred to as a "thread checker") – which immediately reveals the type and size of the threads.

Example: Connecting to a radiator head with an internal thread

In practice, this means that if you have a standard connection with an internal thread on the radiator (e.g., G 1/2"), you must use a fitting with an external thread that fits into it and is tightened.

Frequently asked questions

How can I quickly determine whether I have an internal or external thread?

Just look at the end of the fitting: if you see the thread extending outward, it is an external thread; if you see a cavity with internal threads, it is an internal thread.

Can I use an adapter to change the type of thread?

Yes, there are special reducing pieces that allow you to connect two different types of threads, but it is better to purchase the correct fitting from the start to minimize the risk of leakage.

Why is my pressed connection with an external thread not sealing?

You likely used the wrong type of sealing or did not consider that an external thread requires a precise internal thread on the opposite side; otherwise, liquid leakage will occur.

Correct identification of the type of thread is the foundation of professional installation, saving you time and money on future repairs. Thorough adherence to technical specifications and the selection of the right components from our range ensures trouble-free operation of your entire heating system for years to come.

Do you have a question about this topic?

Are you unsure or dealing with a specific situation in your home? Write to us – we are happy to help.