Name that thread

Four basic steps identify threads on industrial hydraulic fittings. Hydraulic fittings have either tapered or parallel threads. But tapered ones are not always pipe threads, and pipe threads can be parallel. Using a thread-pitch gage is the easiest way to determine pitch. Use calipers measure nonpipe size. Determine pipe-thread size by comparing threads against nominal-size profiles. 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