Step 1 – Determine if the thread is tapered or parallel. NPT/NPTF and BSPT are tapered threads while UN/UNF and BSPP are parallel. Metric Tapered and Metric Parallel speak for themselves. In some cases, Step 1 can be accomplished by visual inspection alone. Tapered threads get smaller in diameter toward the end of the fitting while parallel threads maintain the same diameter from start to finish. If this is not obvious by looking at the fitting, use the parallel jaws of a caliper to make a comparison.
Step 2 – Determine the pitch. This can be deciphered using a pitch gage for comparison or by accurately measuring and calculating the number of threads within a given distance. It is much easier to compare threads against a lighted background with a pitch gage. Because some thread pitches are relatively similar, it is advisable to try a number of gages before deciding which one fits best. The result from Step 2 will narrow down the possible thread forms even more because most have a distinct pitch. Consult the “Step 2” column in Table 1 for possible pitches.
Step 3 – Determine the size. Combining the results of Steps 1 and 2 will determine – or help predict, in some cases – the correct procedure for Step 3. There are two methods for determining the thread size – which to use depends on whether the thread is a pipe thread (NPT/NPTF, BSPT, BSPP) or is not a pipe thread (UN/UNF, Metric Parallel, Metric Tapered). Keep in mind that tapered (as determined in step 1) does not necessarily mean that it is a pipe thread (e.g. Metric Tapered). Likewise, pipe thread can be parallel (e.g. BSPP). For pipe thread, determine the size by comparing it with a nominal size profile, as shown in Figure 1 (a useful tip – pipe sizes up to 2” nominal size can be determined by measuring the actual outside diameter, subtracting ¼” then rounding-off ). For non- pipe thread, the actual size can be determined by measuring the outside diameter (major diameter) with a caliper, as shown in Figure 2.
Step 4 – Designate the thread. Technically, this final step does not pertain to identifying the thread. Rather, it is a method of designating the thread type in an industry standard format for others to understand. Examples of the various formats are shown in the “Step 4” column of Table 1. These typically have an indication of the thread size (whether nominal or actual), the type and – in some cases – the pitch. Your company’s maintenance and repair professionals can put this easy four-step process to use to minimize machine downtime, avoid the expense of acquiring (and returning) incorrect parts and help ensure a safe, accident- free work environment. Summary of How to Identify Threads – In order to differentiate between the various thread types, all that is needed is this reference chart, a caliper and a thread gage. The most important tool is the thread gage (or pitch gage). This tool, which has a “saw tooth” appearance, helps determine the thread pitch. It has a specified number of serrations within a certain distance and is (usually) marked accordingly. For metric threads, the pitch is considered as the distance, in millimeters, between each thread. For all other threads, the pitch is considered as the number of threads per inch.