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

The NPT type thread seals by penetrating the thread boss until tight, and sufficient threads on the fitting are interlaced with the threads in the boss to generate a seal.







To aid in the sealing process, and to render the thread leak proof, it's common for folks to select Teflon tape with which they coat the male thread, thus allowing the male threads to turn deeper into the female boss and improving thread-to-thread sealing.

Industrial compressed air professionals prefer not to use this tape as it’s easy for pieces of the tape to overlap the end of the fitting and impede the air passageways, or perhaps break off in the air turbulence inside the fitting and create problems downstream in other components. 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 and easy to use, so if you do opt to use it, be careful to keep the tape threaded back from the end of the fitting threads and don’t let it 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, rather than loosen.

The norm of late seems to see standard NPT fittings coming complete with a Teflon or similar type of sealing compound already coating the threads. This sealing compound allows the NPT threads to seat further into the boss allowing greater thread-to-thread contact, and also improves sealing by filling voids and unevenness in the threads. This is a nice feature of modern air fittings, and is both readily available and well accepted.

Alternatively, you can purchase liquid thread sealant to add to the threads of fittings. This type of sealant is useful for larger threads. Of course, it's suitable for smaller NPT threads too, however the application of the liquid on smaller fittings is problematical if care isn't taken to keep the sealant from overlapping and blocking the actual air passage.

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

Careful! A swivel elbow is not a rotary device, and it's purpose is not to allow continuous movement of the swivel as the equipment to which the air line is connected moves. In this instance, the swivel portion of the elbow fitting will fail.

If you need to connect an air live to or from a moving component, make sure you select a Rotary Fitting that is designed to allow continuous movement of the swivel part of the fitting.

Metric Port Sizes

Common classification for METRIC fitting sizes are “G” type, and “R” type threaded fittings.

The “G” type will offer the same metric sizes as other metric fittings. The sealing mechanism of the "G" type fitting will be with a “sealing ring” or ‘O’ ring placed over the thread of the fitting and butting up to the base of the wrench flats. In other words, you will screw this fitting thread right down into the boss until the sealing ring or ‘O’ ring “squishes” between the body of the fitting wrench flats and the boss into which you are threading it. Pressure on this seal is what contains the compressed air.

“G” style fittings come in a variety of thread sizes, the commonest of which are: