Compressed Air Fitting

On these pages I'll take a look at all kinds of compressed air fittings in an effort to make fitting identification, and how to select the right fitting for a specific application, easier for you.

Once you have reviewed the information here, if you have not found the information on air fittings you would like, visit here and ask question on the Fitting forum page.

The NPT Fitting Standard

A thread boss is the hole in the device or air tool into which you insert or thread a fitting. The boss could be an air tool, an air valve port, an air cylinder port, an air manifold, or any number of other compressed air components.

NPT stands for National Pipe Taper.

If a specific air fitting is manufactured to the NPT standard, it will have dimensional compatibility with other NPT threaded fittings of all other manufacturers. Which is another way to say that all NPT fittings are interconnectible.

One manufacturers fitting can be replaced with the same NPT size fitting of another manufacturer, and both - assuming they are the same size - will fit the same threaded hole in your air cylinder, actuator, air tool handle, valve...or whatever the application is.

Different NPT Fitting Sizes

Fitting sizes can be a bit confusing! Do not make the assumption - as I used to - that a 1/4" NPT fitting thread is anywhere near 1/4 inch in diameter! Not even close.

Visit your favorite hardware or plumbing store and ask for a 1/4" fitting. The fitting they will show you will likely be a 1/4" NPT. You will look at the diameter of the thread and say to yourself, this ain't no 1/4" fitting. It will be. The actual diameter of the 1/4" NPT thread will be just over 1/2".

The size of an NPT fitting, teh 1/4", the 3/8", 1/2" and so on are reference sizes. The actual size of each is much larger than the fraction size they are supposed to be.

NPT Fitting Size Chart

To help you get a grip on the actual sizes of NPT fittings, see the chart below. It shows the comparison between NPT fitting sizes and their actual inch equivalents.

On this chart I have listed the NPT thread size first, and then what that thread size is in actual (decimal) inches, and then the closest common size fraction to that NPT size.

NPT Fitting size chart

Sizing The Hole or Fitting Port

To determine the size of a threaded air port in an air tool handle, air cylinder, an valve, or the female thread in any compressed air component, you will want to measure the width of the hole in inches. That's the distance across the middle of the opening.

Compare that result to the fractional sizes on the chart to help guide you to the right male-threaded NPT fitting size to use.

NPT, as an acronym for National Pipe Taper, and is an industry dimensional standard. The threads of different fittings with the same NPT size will connect, but that does not mean that all of the fittings will necessarily look the same.

Fittings And What You Need To Know

What do you need to know when you are trying to determine what fitting to use and where to use it?

You need to identify the...

  • Port size / fitting thread size into which the male thread is screwed
  • Size of tube or hose that you want to attach to the fitting; and note, tube and hose do NOT connect the same way
  • Style of fitting; whether it is a straight, elbow, Tee , run Tee etc.)
  • The way the hose or tube is to be connected to the fitting

Fitting NPT Port Sizes

You will commonly see the following in NPT thread sizes;
  • 10-32
  • 1/8 NPT
  • 1/4 NPT
  • 3/8 NPT
  • 1/2 NPT

There are certainly much larger NPT sizes. The sizes listed above are the commonest fittings in industrial compressed air applications.

How Fittings Seal

When you thread the fitting into the hole in the valve, or cylinder, or air tool, the NPT type thread seals by threading into - penetrating the thread boss (the threaded hole) until it is tight. That typically means that sufficient threads on the fitting are interlaced with sufficient threads in the boss to generate a seal.

To aid in the thread penetration, and to help render the thread connection leak proof, it is common for folks to select Teflon tape as a sealing aid. They coat the male thread with the tape, allowing those male threads to turn deeper into the female boss than would be possible without the thread, and improving thread-to-thread penetration distance and the resulting tighter sealing.

Is Teflon Tape A Problem on Fittings?

Some compressed air professionals prefer not to use this tape. It is too easy for pieces of the tape to overlap the end of the fitting if not carefully applied. The overlapping tape can impede the air passageway.

Since compressed air flows very rapidly, sometimes pieces of the Teflon tape can break off in the air turbulence inside the fitting and create problems downstream by lodging in other air components. For example, pieces of tape can lodge in smaller air orifices and block them, or prevent another sealing surface from closing properly, actually generating a leak.

Yet Teflon tape is convenient, inexpensive and relatively easy to use. If you do use it, just be careful to keep the tape wound well back from the end of the fitting thread, and do not let the Teflon tape overlap air passages when the fitting is threaded into the hole.

Wind the Teflon tape onto the male threads against the thread, so when the fitting is threaded into the port, the Teflon tape would tend to tighten on the threads rather than loosen.

Doped Air Fittings

Lately it seems air fittings come complete with a paste sealant or other similar type of sealing compound already dried on and coating the fitting threads.

Like the Teflon tape, this sealing compound allows the NPT threads to seat further into the boss thread, allowing greater thread-to-thread contact inside, and filling micro-voids and unevenness in the threads to improve the seal. This is a nice feature of modern air fittings, and is both readily available and well accepted.

Liquid Thread Sealant

Alternatively, you can purchase liquid thread sealant to add to the threads of your fittings before you thread them into a boss. Using the liquid on smaller fittings is problematical if care is not taken to keep the sealant from overlapping and blocking the actual air passage.

The Straight Fitting

Just as its name suggests, the straight fitting accepts an air line (tube or hose) on one end, and on the other there is an NPT male or female thread that screws into or onto a boss.
Straight Air Fitting

The Elbow Fitting

Elbow fitting

Elbow fittings are those that allow the supplying line to connect to the air fitting tangentially, out to one side, rather than straight in the end.

To make orientation of the air line easier, modern elbows are swivel type. After the thread is screwed tight into the port, the air line connection can be rotated to allow the air line to align in the most convenient direction.

A swivel elbow is NOT normally a rotary device. The purpose of the typical elbow fitting is NOT to allow continuous movement of the swivel as the equipment moves. Use of a standard moveable elbow fitting as a rotary union will result, fairly quickly, in fitting failure.

If the elbow fitting is to be swiveling continuously, by a reciprocating part or some such on a machine, install a purpose-designed rotary union , NOT the swivel elbow fitting.

Rotary Union

For applications that require a 90 degree elbow that swivels continuously or fully rotates on a continuous basis, a rotary union can be used.

For example, rotary unions are commonly used on index tables, where there is continuous rotary movement of the air supply line to the table.

I recommend you select a quality built Rotary Union, one with ball bearings in their construction to allow easy, continuous rotation, one that has the proper seals necessary to ensure reliable, often high speed, continuous rotation without fitting failure or air leaks.