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4 Steps to Measure the Thread Size of Hydraulic Fittings

Depending on their application, hydraulic fittings can be classified into different types. While some hydraulic fittings permit fluid flow, others enable the fluid to switch, mix, and change direction. But overall, a hydraulic fitting should seal the hydraulic hose or pipe to avoid leakage.

You can determine if a hydraulic fitting is suitable for your hydraulic system based on its thread size. For instance, you use an oversized hydraulic fitting on your hydraulic hose. In that case, the fitting will not create the required leak-proof seal. Conversely, an undersized hydraulic fitting will not fit the hydraulic hose.

These are a few reasons why purchasing hydraulic fittings of the right thread size is critical. One way of ensuring you get the correct thread size is by measuring it. So, in this post, we look at how to measure thread size in detail. Here are the steps you need to follow to measure the size of your hydraulic fittings..

1. Identify the thread type

A hydraulic fitting either comes with parallel threads or tapered threads. So, you first need to identify the thread type. It is simple to recognize parallel threads as they are straight. Conversely, tapered threads appear to decrease in diameter towards the bottom of the fitting.

You’ll also find that parallel threads are easier to measure. Since they are straight, they maintain the same major diameter. However, with tapered thread fittings, the threads closer to the fitting head diameter are higher than those at the bottom.

While it is possible to tell the difference between a parallel or tapered thread by looking at it, using a caliper is ideal. Simply slide the caliper point against the threads at the bottom end of the fitting. Then take the measurement.

Next, slice the caliper point against the threads at the head of the fitting. Also, take the measurement and compare it to the earlier one. If the head measurement is higher than the bottom measurement, it is a tapered thread.

You also need to determine if it is a male or female thread. If it is a male thread, you measure the outer diameter. But if it is a female thread, you measure the inner diameter instead. 

2. Measure the diameter of the thread

Once you are sure that you have a parallel or tapered thread hydraulic fitting, the next step is to measure the thread diameter. Calipers work for measuring inner and outer diameters, so they are helpful whether it is a male or female thread. 

If it is a parallel thread fitting, feel free to measure the diameter anywhere along the threads. All the threads will have the same diameter. But if it is a tapered thread, measure the fourth or fifth thread — this will be somewhere in the middle.

It is wrong to measure the diameter of tapered threads at the head or the bottom. Both areas will have significantly varying diameters. But the diameter of the fourth or fifth thread, which is in the middle, will be closer to both the head and bottom area diameter.

3. Confirm the number of threads per inch (TPI)

One of the most crucial measurements regarding hydraulic fitting thread size is the number of threads per inch. It is simple to measure with a ruler or a thread pitch gauge.

If you are using a ruler, place it against the hydraulic fitting. Then count how many threads are there in one inch of the ruler. You should count the raised parts of the threads — the mountains — and not the lower parts — the valleys.

If you are using a pitch gauge, the process is the same. However, when measuring hydraulic fittings with metric sizing, you will need a pitch gauge. For such fittings, the measurement is in millimeters. Note that TPI and metric pitch gauges differ, so ensure you use the right one.

4. Crosscheck the measurements with a standard thread chart

The final step of measuring hydraulic fitting thread size is to consult a standard thread chart. At this point, you know if the thread is parallel or tapered, male or female, and its TPI, pitch, and major diameter.

These are the details you have to check for in a thread chart to determine the exact thread size of your hydraulic fitting. There are many hydraulic fitting thread charts online, which are free to use.

Alternatively, simply write the measurements in a standard form. The diameter comes first, followed by the number of threads per inch, and then the length of the fitting thread area.

Suppose your diameter measurement is ⅛ inch, your number of threads per inch is 15, and the fitting thread area is 2 inches long. In that case, you will write “⅛ inch x 15 x 2 inch.”

What Is Thread Size Measured In?

Hydraulic fittings thread sizes are measured in inches, but that is only the unit of measurement. When measuring hydraulic fittings, the actual inch thread size depends on the diameter and threads per inch.

The diameter refers to the measurement of the distance between the thread’s crests. On the other hand, threads per inch refer to the number of threads each inch features.

There is also metric sizing for some hydraulic fittings. For such fittings, a third factor — pitch — also determines the actual thread size. Pitch refers to the distance between a thread’s crest and another. Also, the unit of measurement for metric hydraulic fitting is millimeters instead of inches.

The threads per inch measurement system is common in North America, particularly the United States. Meanwhile, the metric thread pitch measurement system is more common in other parts of the world.


When you hit the market for hydraulic fittings, you will find ORS, BSP, NPT, and JIC fittings, to name a few. Irrespective of which is compatible with your hydraulic system, you must ensure the thread size is correct.

From the steps explained here, you will agree that measuring the thread size of hydraulic hoses is not difficult. All you need is a caliper and thread gauge, or you can use a ruler. 

Once you have determined the right thread size, purchasing quality fittings is next. For that, reach out to us at Sinopulse. We supply some of the best hydraulic fittings and will be happy to help you purchase the right ones that meet your needs.

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