Pipe threads – Standards and standard dimensions

This article examines the standard dimensions of pipe threads and their meaning. Pipes with threads ensure a safe and tight connection between pipe parts and adjacent components. In industry, there are a variety of standards for this, such as DIN/ISO, JIS, BSP, NPT or BSW.

What are pipe threads?

Pipe threads are special thread types that are used to connect pipelines. They are designed to ensure a tight bond under pressure and prevent liquids or gases from escaping. A great benefit is that the connections can be loosened again.

Use of pipe threads

Pipe threads are used in general mechanical engineering for the connection of pipes. The screwing of pipes to a machine is performed by means of threaded pipes. These can be designed to seal by tapered threads, among other things. These are used, for example, for hydraulic lines or drain plugs.

Manufacture of threads

The threads are made either by machining or non-machining methods. In the machining variant, the thread is cut out of the pipe using a special tool cutter. In the case of the non-machining variant, the thread is pressed into the material. This has the advantage that the material fibre remains unbroken and finally has a higher strength. Below are some common manufacturing methods:

• Cutting: Cutting is the most common method for producing pipe threads. It is normally used for metal pipes such as steel or stainless steel. A thread is cut into the pipe using a thread cutter or thread tap. The thread cutter has a specific thread profile that matches the desired thread. Cutting pipe threads requires precision and careful alignment to ensure the thread is cut correctly.
• Rolling: During rolling, the pipe is guided by two shaped rollers that press the thread onto the outer wall of the pipe. This process is often used for pipes made of alloyed materials or pipes with high strength. The advantage of rolling is that the thread is pressed into the material instead of cutting it, which can lead to higher strength and precision.
• Forming: When creating pipe threads, the thread is made by moulding the pipe instead of removing material. This procedure is often used for plastic pipes. In this process, the pipe is heated and pressed into a mould that has the desired thread profile.
• Fitting assembly: In some cases, pipe threads are created directly during assembly. For example, a so-called threaded fitting is screwed onto the pipe.

Why are standard dimensions important?

Standard dimensions are a basis for the cooperation of the various actors in the industry. They affect the following factors, for example:

• Uniformity: Standardisation enables manufacturers, installers and customers to be sure that parts fit together and ensure a secure connection.
• Safety: A correct and tight connection prevents leaks that could lead to hazards.
• Cost-effectiveness: Mass production of standardised parts results in lower production costs.
• Efficiency: Standard dimensions can increase production efficiency by enabling work with standardised tools and machines. This reduces the effort involved in adapting and converting production processes.
• Communication and documentation: Standard dimensions facilitate communication and documentation in technical drawings, construction plans, technical specifications and other technical documents. This prevents misunderstandings and misinterpretations.

Common standards for pipe threads

In Germany, there are several standards for pipe threads, which are often used in European industry. The German Institute for Standardisation (DIN) determines these. It is important to know the standard used, since e. g. BSP-threads and NPT-threads are not compatible with each other:

• Metric ISO- thread (ISO 68-01): A cylindrical standard where the outer and inner diameters are precise to the millimeter. Primarily standard in countries with metric systems.
• BSP (British Standard Pipe): A widely used cylindrical standard available in two variants: BSPT (British Standard Pipe Taper) and BSPP (British Standard Pipe Parallel). The former is a conical pipe thread and the second is a straight pipe thread, i.e. the thread diameter remains the same over the entire length.
• NPT (national pipe thread): The NPT-thread is a U.S.-American standard, which is characterized by its conical exterior or inner diameter.
• Whitworth-thread (BSW and BSF): Originally developed in the U.K., the Whitworth-pipe thread is still in use in some areas such as the U.K. automotive industry and in historical applications.

The term "inch thread" is also often found. Inch threads are a general term for threads measured in inches.

How do you determine the size of a pipe thread?

In order to determine the size of a pipe thread, you must first take a closer look at the differences between inch threads and metric threads:

The following parameters apply to metric threads:

• Thread diameter: For metric threads, the diameter of the thread is measured in millimetres (mm). This diameter is often referred to as the “nominal diameter” and is the key measure for metric threads. If a metric thread has a nominal diameter of 10 mm, for example, the external diameter of the thread is also 10 mm.
• Thread pitch: The thread pitch indicates how many threads per millimetre are present. A thread pitch of 1.5 mm means, for example, that there are 1.5 threads per millimeter. The thread pitch determines the fineness of the thread.

The thread pitch is also specified for inch threads. Here, however, it refers to the number of threads per inch. Another important characteristic for inch threads is the length measurement. This refers to the physical length of the thread or pipe measured in inches (or fractions of inches). This measure indicates how long the thread or pipe is in its entirety, regardless of the number of threads. A pipe with a length measurement of 6 inches can, for example, have a physical length of 6 inches, regardless of the number of threads.

It should be noted that the actual size of a pipe may vary slightly from the nominal width, especially in old pipes or in different countries with different standards.

When converting from inches to the metric system, the following values apply:

• 1.00 inch = 2.54 cm = 25.4 mm

Standard dimensions

Note: The nominal diameter of a conical external thread is given here. In the case of a conical internal thread or parallel internal thread, R must be replaced by Rc or Rp. (See*) Conical thread: Length from the position of the plug gauge diameter point to a minimum diameter point. Parallel internal thread: Length from the pipe or pipe connection end.