Compressed Air Metric Fittings – Guide, Types & Sizes

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Though 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, they are not interchangeable.

Metric fittings have differences in the tip-to-tip distance of the thread. As a result, metric thread sizes are incompatible with their NPT same-sized counterparts.

Metric Fitting Thread Slop

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.

Universal Fitting

Metric Fittings for compressed air
Metric Fittings

At this time, there does not appear to be a true universal thread which will fit metric and NPT ports and provide an acceptable level of thread sealing to prevent compressed air leakage.

Some firms do suggest they offer a universal fitting. 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. I have seen some that claimed to be, but they did not provide adequate thread-depth penetration or sealing.

Separate Metric Fittings Storage?

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

It is not always easy to tell them apart. Some manufacturers use different color release-rings for their metric fittings. Unless someone clues you in to the color guide though, how will you know which are which?

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 on this style will likely be shorter in length than other types of metric fittings. The reason for this is that the G-type fitting is designed to screw right down into a boss.

In other words, you will screw this fitting thread right down 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, sealing in 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

Larger sizes in the G-style metric fitting are certainly available, yet the sizes shown seem to be the highest volume in industrial compressed air applications.

R-Type Metric Fittings

The R-type metric threaded fitting has a taper, similar in concept to the National Pipe Taper thread (NPT). However, these are not tapered the same way as the NPT threads are, they are metric. Trying to use an R-type tapered metric thread in an NPT port can create problems such as badly leaking joints to stripped bosses.

Since the R-type threaded fitting has a taper, it too 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 escape of compressed air.

R-type metric 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

As with other styles of compressed air fittings, larger fitting sizes in the R-type are available. The sizes shown above are the most common for industrial compressed air valves and cylinders.

You might find this Metric Fitting Hole Size chart useful, too.

Additional Reading:

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