Naming a Thread

Determining the pitch of hydraulic fittings can be facilitated by utilizing a thread-pitch gauge. It is important to note that hydraulic fittings come with either tapered or parallel threads, and while tapered threads are not always synonymous with pipe threads, it is possible for pipe threads to be parallel.

For non-pipe size calculations, calipers can be employed to accurately measure threads against nominal-size profiles, aiding in the identification of the pipe-thread size.

Although identifying the appropriate thread for hydraulic fittings may initially seem straightforward, the proliferation of fittings and connectors in the global marketplace has made this task increasingly challenging and frustrating, even for experienced technicians and engineers. Understanding thread types, including American pipe threads (NPT/NPTF) and SAE or Unified threads (UN/UNF), is essential in the fluid-power industry. Additionally, there exists a wide array of other less recognized threads, often classified as "metric," encompassing genuine metric threads and BSP (British Standard Pipe) threads.

To simplify the process of identifying hydraulic tube fittings and connectors, they can be categorized into six distinct thread types, with attention to their classification as either parallel, tapered, pipe thread, or non-pipe thread:

UN/UNF - parallel
NPT/NPTF - tapered, pipe threads
BSPP, BSP - parallel, pipe threads
BSPT, BSP - tapered, pipe threads
Metric parallel
Metric tapered

Properly identifying the thread type is crucial for selecting the appropriate mating or replacement fitting. This ensures safe assembly, reliable pressure retention, and effective sealing of the fitting or adapter, thereby mitigating the risk of costly and time-consuming thread stripping and damage. Several simple tools and steps can be employed to aid in the identification process, irrespective of the thread type:

Determine if the thread is tapered or parallel, using a caliper to make comparisons if not immediately evident.

Ascertain the pitch, usually defined as threads per inch (TPI) or the distance between threads in millimeters for metric threads. A pitch gauge or manual calculation can be used for comparison.

Identify the size, considering if the threads are pipe or non-pipe. For pipe threads, nominal size can be determined by comparing the part against a size profile, while for other threads, the actual size can be calculated using a caliper to measure the OD (major diameter).

Assign the thread type in a standardized format, indicating the thread size (nominal or actual), type, and sometimes pitch.

A few tips for thread identification:

Certain sizes, such as ¾, can be challenging to identify visually but may be either British taper pipe threads (BSPT) or American pipe threads (NPT/NPTF) with identical nominal size and pitch.

Standard thread diameters are typically slightly smaller than their stated sizes when accurately gauged with a caliper, for example.

For internal threads, it is best not to use the thread pitch gauge, and instead, find the mating external thread and follow the four-step process for accurate identification.

Tapered external threads are not typically connected to parallel internal threads.
NPSM threads are parallel but have the same pitch as NPT/NPTF and are usually used inside swivel nuts for holding only. The seal is achieved through contact between the 30° internal chamfer of the male pipe thread and the seat inside the swivel.