Determining the correct thread for a hydraulic fitting sounds simple. But a burgeoning global marketplace for hydraulic equipment has produced a proliferation of fittings and connectors. This makes correct identification of threads an increasingly difficult and frustrating task, even for the most-experienced technician and engineer.
Most people in the fluid-power industry are aware of American pipe threads (NPT/NPTF) and what are often called SAE or Unified threads (UN/UNF). But there seems to be an endless number of other unfamiliar threads, usually classified as "metric," regardless of their actual form. In reality, there are true metric threads as well as BSP (British Standard Pipe) threads.
For identification purposes, hydraulic tube fittings and connectors can be divided into six different thread types: UN/UNF, NPT/NPTF, BSPP (BSP, parallel), BSPT (BSP, tapered), metric parallel, and metric tapered. Three are parallel (UN/UNF, BSPP, metric parallel) and three are tapered (NPT/NPTF, BSPT, metric tapered). Three are pipe threads (NPT/NPTF, BSPT, BSPP) and three are not (UN/UNF, metric parallel, metric tapered). Keep in mind that tapered does not necessarily mean it is pipe thread (for example, metric tapered). And pipe threads can be parallel (for instance, BSPP).
Determining the correct type of thread is critical for selecting the proper mating or replacement fitting. First and foremost it is a safety issue. Correct assembly maintains pressure and reliably seals a fitting or adapter. It also prevents costly and time-consuming thread stripping and damage. Regardless of thread type, using a few simple tools and the following steps will result in proper identification.
1. Determine if the thread is tapered or parallel. Sometimes visual inspection is sufficient. Tapered threads get smaller in diameter toward the end of the fitting while parallel threads have the same diameter from end to end. If this is not obvious just by looking at the fitting, use the parallel jaws of a caliper, for example, to make a comparison. An O-ring or tube nut usually indicates parallel threads.
2. Determine pitch. Pitch is typically defined in terms of threads per inch (TPI) or distance between threads. For metric threads, pitch is the distance between threads in millimeters. Use a pitch gage to compare threads, or accurately measure and calculate the number of threads within a given length. It is much easier to compare threads against a lighted background with a pitch gage. Because some thread pitches are nearly identical, try several gages before deciding on the one that fits best.
3. Determine size. The two methods for sizing threads depend on whether the threads are pipe or non-pipe. For pipe threads, determine nominal size by comparing the part against a size profile. A good rule of thumb for pipe sizes up to 2-in. nominal is to measure the OD and subtract 1 /4 in. Round off for the nominal pipe size.
For other threads, determine actual size by measuring the OD (major diameter) with a caliper.
4. Designate the thread. Technically, this final step is not part of the identification process but, rather, a method of designating thread types in a standard format. It usually indicates thread size (nominal or actual), type and, in some cases, pitch.
Grab a handful of "mystery threads" and practice the four-step ID process. It's kind of like learning how to change a tire — a useful skill to have when the need arises.
Helpful hints for thread ID
British taper pipe threads (BSPT) and American pipe threads (NPT/NPTF) have some similarities and some differences, but there are two sizes, 3 /4, that are difficult to distinguish with the naked eye. The two have the same nominal size and pitch, and sometimes can be differentiated only through deductive reasoning.
Standard thread diameters are normally no larger than their stated sizes. Gauged accurately, with a caliper for example, the measured diameter is normally slightly smaller than the actual thread size.
Internal threads should not be identified using this method because it is difficult to verify the proper fit of the thread pitch gage. If possible, find the mating external thread and follow the four-step process to identify it.
It is not unusual to have a tapered external thread connected to a parallel internal thread.
NPSM threads are actually parallel but have the same pitch as NPT/NPTF. These threads are typically used inside swivel nuts for holding only. Contact between the 30° internal chamfer of the male pipe thread and the seat inside the swivel provides the seal.