In order to determine and differentiate between the various thread types, a reference chart (as provided in Table 1 - Below) and a few simple tools such as a caliper and thread gage is all that you'll need. The most important of these tools is the thread gage or pitch gage which will help to determine the thread pitch. A thread gage has the appearance of saw tooth. 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 mm, between each thread. For all other threads, the pitch is considered as the number of threads per inch.

Knowing the correct thread form on a fitting is critical for selecting the proper replacement parts for maintenance and repair. This, in turn, will prevent thread damage during installation and also maintain the pressure holding capacity and seal reliability of the fitting or adapter. Regardless of which thread type you encounter, here are the four simple steps that will help you determine what it is.

Step 1 - Determine if the thread is tapered or parallel
This can sometimes be accomplished by visual inspection. Tapered threads get smaller in diameter toward the end of the fitting while the parallel threads have the same diameter from start to finish. If this is not obvious just by looking at the fitting, use the parallel jaws of a caliper, for example, to make a comparison. The presence of an O-ring or the removal of a tube nut is usually an indication that the male thread is parallel. The result of this first step will eliminate three of the six thread forms. Remember that three are parallel and three are tapered.

Step 2 - Determine the pitch
The pitch can be determined by using a pitch gage for comparison or by accurately measuring and calculating the number of threads that would be 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 close, it is advisable that you try a number of gages before deciding on which one fits best. The result from this step should help you narrow down the possible thread forms even more because most thread forms have a distinct thread pitch. Consult Table 1 (Below) for the possible pitches.

Step 3 - Determine the size
The results of Step 1 and Step 2 will help in deciding how to proceed with Step 3. The possible pitches are shown in the "Step 2" column of Table 1, and by combining this with the decision of Step 1 the correct procedure for Step 3 can be predicted. There are two methods for determining the thread size. It depends on whether it is a pipe thread or not. If it is a pipe thread, the nominal size can be determined by comparing with a size profile as shown in Fig. 4. (A useful tip that can be used to determine pipe sizes up to 2" nominal size is to measure the actual outside diameter and then subtract ¼". Round off for the nominal pipe size). If the thread is not a pipe thread, then the actual size is determined by measuring the outside diameter (major diameter) with a caliper as shown in Figure 5.

Step 4 - Designate the thread
Technically, this final step is not a part of identifying the thread, but rather 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 usually have an indication of the thread size (whether nominal or actual), the thread type and the pitch, in some cases.

Of course, the best way to benefit from this easy four-step process is to put it to use. Now is a great time to grab a random selection of threads and test the method. As with most skills, your level of "expertise" in identifying threads will increase with practice. And when the time comes to make a thread ID, you'll be grateful for the hands-on skill and knowledge.

Table 1
Summary of How to Identify Threads

*For JIS (Japanese Industrial Standards), the thread can be identified similar to BSPP and BSPT but defined with PF and PT, respectively. For example, PF 1/8 and PT 1/2.

Helpful Hints for Threads in General

• British taper pipe threads (BSPT) and American pipe threads (NPT/NPTF) have some similarities and some differences but there are two sizes, 1/2 and 3/4, that are very difficult to distinguish with the naked eye. These two have the same nominal size and the same pitch (14 threads per inch), and sometimes can only be differentiated by deductive reasoning.
• It is advisable to use a pitch gage set that is specific for hydraulic tube fittings and adapters. This way, you only have a limited number of gages and won't have to contend with the many different gages that have pitches that are too close together. There is only a limited number of different pitches that are used on hydraulic fittings as opposed to the vast amount that are possible on fasteners and other threaded components.
• Standard thread diameters normally do not measure any larger than their stated sizes. If you are measuring accurately, with a caliper for example, it is normal for the measured diameter to be slightly smaller than the actual thread size.
• Internal threads should never be identified by this method because it is difficult to verify the proper fit of the thread pitch gage. If possible, find the mating external thread and then follow the process to identify this thread.
• The term "Pipe Thread Sealant" is not really correct. It should be called "Taper Thread Sealant." It is taper threads that require sealant and not all pipe threads are tapered.
• Dryseal pipe threads were designed to function without any added sealant.
• It is not unusual to have a tapered external thread connecting to a parallel internal thread. For example, BSPT male works well in BSPP female, as long as the appropriate sealant for tapered threads is being used. Never connect parallel male into tapered female though.
• NPSM threads are actually parallel but with the same pitch as NPT/NPTF. These threads are typically only found inside swivel nuts and are for holding-power only. Sealing is accomplished by the contact of the 30° internal chamfer of the male pipe thread and the seat inside the swivel.
• Specific thread sizes are assigned to the various tube fittings sizes and styles. These can be found within tables of the pertinent industrial standards such as SAE J514, SAE J1453, ISO 6149, DIN 2353, etc.