Compression fittings comprise of all kinds of fittings that use compressive force to connect a fitting to a vessel. This article seeks to explore the NPT Standard of compressed air fittings so as to identify fittings and also guide your selection of the proper fitting for particular applications.

The NPT Fitting Standard

NPT is a dimensional standard in the industry which is an acronym for National Pipe Taper. A threaded boss refers to a hole in an air tool or device into which a fitting can be threaded or inserted. The boss could be an air valve port, an air tool, an air manifold, an air cylinder port or any other type components for compressed air.

If a particular air fitting is manufactured to a standard specified by NPT, it normally has dimensional compatibility with other threaded fittings from all other manufacturers that have been made to NPT standards. In other words, it is safe to say that it is possible to interconnect all NPT air fittings.

It is also possible to replace a fitting from one manufacturer by a similar-sized NPT fitting from another manufacturer, with both – if they are of the same size – fitting the same threaded hole in an actuator, air cylinder, valve, air tool handle or whatever type of application.

NPT Fitting Sizes

It could be a bit confusing trying to figure out fitting sizes. It’s wrong to assume that a 1/4” NPT fitting thread is close to 1/4 inch in diameter. When it comes to the sizing of NPT fittings, the 3/8”, 1/4”, 1/2″ etc. are just reference sizes. The real size of each one is bigger than the fraction size they are meant to be.

The fact that the threads of various air fittings with equal NPT sizes connects does not necessarily mean that all the fittings will look alike.