Metric Fittings

While fittings are fittings, and the threads on metric fittings look the same as those on NPT fittings, and a 1/4" NPT fitting will have a similar diameter to a 1/4" Metric fitting, I assure you that they are not the same.

Metric fittings have a different thread configuration, with differences in the thread tip-to-tip distance.

As a result, metric thread sizes are incompatible with their NPT counterparts.

Since there is enough “slop” in a couple of the metric sized fitting threads they can actually be screwed into an NPT threaded boss. This produces mixed results with low to high level leakage depending on the thread sealant selected.

At this time, there doesn’t appear to be a true “universal” thread which will fit metric and NPT ports of all sizes with complete satisfaction.

Some firms suggest they offer a universal fitting. Do check and be sure before "buying into" this concept. If you find a true “universal” fitting, do let me know and I’ll note it here.

So do yourself a huge favor and keep your metric air fittings stored away from your NPT air fittings.

It's not always easy to tell them apart.


Metric Fitting Classifications

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


'G' Type Metric Fittings

The sealing mechanism of the "G" type fitting will be a “sealing ring” or ‘O’ ring placed over the thread of the fitting and butted up to the base of the wrench flats.

The threads will be shorter in length than other types. The reason for this is that the "G" type fitting is designed to screw right into a boss.

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:

• M3 (3 millimeter)
• M5 (5 millimeter)
• 1/8
• 1/4
• 3/8
• 1/2

The “R” Type Metric threaded fitting has a taper, similar in concept to the National Pipe Taper thread. However, these are not tapered NPT threads, they are metric, and trying to use an “R” tapered metric thread in an NPT port can create problems from leaking joints to stripped bosses.

Since the “R” threaded fitting has a taper, it seals by penetrating into the threads of the boss until sufficient threads are in contact with each other (the fitting is tight) to prevent the egress of compressed air.

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

• M3 (3 millimeter)
• M5 (5 millimeter)
• 1/8
• 1/4
• 3/8
• 1/2

Of course, as with other styles of compressed air fittings, larger fitting sizes in the 'R' style are available. The sizes shown are the most common for industrial compressed air valves and cylinders.

Fittings, whether metric or NPT have to connect to something...right?

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