Do you think that the thread size of the valve, heat exchanger, elbow, or any hydraulic device with threaded connections does not meet the standards? Is it not the size it was supposed to be?
How many mm should a 1-inch thread be? And how many mm should a 1 1/4-inch thread be?
You will find answers to these and other questions in this article, and you can check any thread—internal and external—for size.
In one table, we present British pipe thread sizes in metric terms (i.e., in millimeters) and inch terms (i.e., in "inches"—a commonly used measure in hydraulics). Many people believe that an inch in hydraulics corresponds to the same assumption as in mathematics and that such a thread must be 24mm. This is not true — 1-inch threads have been adapted over the years to pipes of such diameter. Their actual size—as well as the size of threads in any other dimension—can be found in the metric thread table (inch thread table) below:
The above inch thread table for pipes refers to BSP threads, a standard commonly used in Europe. This standardized thread size was created in the 19th century to ensure compatibility of hydraulic equipment across different countries. It was developed in Great Britain and is known as British Standard Pipe (BSP). Thanks to the above inch thread table for pipes, you can check any thread in metric terms—i.e., its size in millimeters.
The best way to measure the thread size is with calipers. You need to measure the diameter, which is the longest axis across the thread. If you don't have calipers, you can use a regular ruler.
After measuring the thread, once you know its size in millimeters, you can identify its size in inches.
Other widely used thread standards include NPT-thread or, for example, SAE. They also use inch-based sizing, but they differ from the commonly used BSP threads. Of course, this also depends on the geographic region – for example, NPT threads are the most widely used in the USA and Canada.
A 1-inch thread, along with a 3/4-inch thread, is one of the most commonly used sizes in hydraulics, perfectly fitting the needs of single-family homes. Therefore, the dimensions of a 1-inch thread are frequently searched for on the internet.
A 1-inch thread has the following dimensions: 30.3 mm (internal thread) and 33.3 mm (external thread).
5/4" threads are slightly larger and are used far less frequently in modern construction than they were in the past. Previously, during the widespread use of so-called open systems, which operated gravitationally (i.e., without a pump), they were very common. Nonetheless, they are still used today.
The dimensions of a 5/4-inch thread are 39 mm (internal thread 5/4") and 41.9 mm (external thread 5/4").
A 3/4-inch thread, often simply referred to as a 3/4" thread, has the following dimensions: 24.1 mm (internal thread 3/4") and 26.4 mm (external thread 3/4"). Alongside 1-inch threads, it is one of the most popular sizes used in hydraulics. Its popularity has significantly increased with the advent of modern construction, which relies heavily on smaller buildings and central heating systems based on thinner pipes.
A 1/2-inch thread is one of the smaller sizes, used less frequently in construction than the 1-inch and 3/4-inch threads. However, it is not uncommon and is often used. It is also frequently referred to simply as G1/2.
The dimensions of a 1/2-inch thread are: 18.6 mm (internal thread 1/2") and 21.0 mm (external thread 1/2").
As indicated in the thread size table above, a 1/4-inch thread has the following dimensions: 11.5 mm (internal thread 1/4") and 13.2 mm (external thread 1/4"). Generally, a 1/4-inch thread is too small to provide significant throughput. Nevertheless, it is often used in low-flow faucets or three-phase filters, as well as small UV water filters.
3/8-inch threads are quite small and used sporadically. They are rarely found in central heating (CO) and domestic hot water (DHW) systems, but they do appear. They are widely used, for example, in automation and mechanics.
In millimeters, 3/8" threads have the following dimensions: 15 mm (internal thread 3/8") and 16.7 mm (external thread 3/8").
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