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On earlier pages we've talked about compressed air fittings in general. That fitting information can be found here.
And now we carry on with...
Note that the fractional sizes of metric fittings, while similar in appearance and O.D. to their NPT counterparts, have a different thread configuration, with differences in the thread tip-to-tip distance.
Most metric thread sizes are incompatible with their NPT counterparts. Since there is enough “slop” in a couple of the metric sized fitting threads that 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.
R TYPE METRIC FITTING
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, 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 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 egress of compressed air.
“R” metric style fittings come in a variety of thread sizes, the commonest of which are:
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.
On these compressed air fittings pages we've written about the peculiarities of the actual thread size for an NPT fitting and compared the actual thread sizes to the NPT "inch" sizes that refer to them, noting that there’s not a lot of correlation.
We touched on the common sizes of imperial and metric fittings, and the methods of sealing their threads into the boss, with advice to be careful about trying to use metric fittings in NPT bosses and vice versa.
We thought it might be useful now to provide a bit more information about the compressed air air-lines themselves. After all, it’s the air line that brings the air to the fitting and connects the supply of air, through the fitting, into the application.
more fitting information here.
