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How to Easily Identify Thread Sizes in Four Easy Steps

by Jared De Hooge

The identification of thread sizes is a fundamental skill for anyone involved in inspection and quality-control efforts. Not being able to determine the size of a thread makes it impossible to make accurate assessments about its suitability for use. Fortunately, thread-size identification is not a difficult task, and it can be accomplished with a minimum of practice. Below are the essential tools you will need as well as how to perform the necessary measurements.

What you will need

  • Caliper

  • Pitch gauge

  • Thread-identification guide

Step-by-step procedure

1. Make a determination of whether threads are tapered or straight—Tapered threads, also commonly known as dynamic threads, are those those that are sloped from front to back. This slope provides an increasingly tighter fit as the threaded ends are joined together. Straight threads, which are often called parallel threads, are those that have no slope from front end to back end.

Often, distinguishing between tapered and straight threads is a simple matter of eyeing the slope; however, there are instances where taper cannot be identified, if it exists with observation alone. That is why making a measurement is necessary.

To measure whether taper is present, take caliper readings at the first, fourth, and last threads of the male or female connector. Should the caliper readings gradually increase or decrease from front to back, then the threads are tapered. Conversely, if there is no measurable taper, then the threads are straight.

2. Determine the thread diameter - After assessing whether the threads are tapered or straight, the next step is to measure the thread diameter. Thread diameters are measured at the fourth thread when taper exists, but they can be measured at any full thread if there is no taper. To measure the diameter, place the caliper jaws at the crest, or high point, of the threads being measured. Be sure that both jaws are on the same thread to avoid an inaccurate measurement.

3. Assess the pitch of the threads - The last step in measuring threads is to determine the pitch of the threads. To make this job simpler, pitch gauges are available and can be used to make an assessment. These gauges consist of a variety of "saw-tooth" blades that are aligned with the tops of the threads and inserted into the gaps between the crests and roots (bottoms) of the threads. The sides of the saw-teeth will rest perfectly flush against the flanks, which are the interior sides of the threads, whenever an exact match is made.

When you are using a pitch gauge, it is important to be patient and be certain that the correct gauge is matched with the threads. An exact match will not permit much, if any, light to show through the gaps between the gauge and the thread surfaces. If you have any doubts about the pitch size, then repeat the measurements between competing gauge sizes and take note of which one best fits the shape of the threads in its entirety.

4. Identify the thread size - After determining whether taper exists, measuring thread diameters, and finding a match with the pitch gauge, the last step is to identify the correct size by using a thread-identification guide. These references use all three of the above criteria to determine the size of the thread. References are commonly found online, and hard copies also exist if you prefer to use them. Simply cross-reference these criteria in the guide, and the thread size will be readily obtained from the reference.

As a word of caution, take note that diameter measurements are not always completely accurate or in agreement with standards in the identification guide used. In those circumstances, always reference the closest standard during identification procedure. Talk to a company such as West Port Corp for more information.