Most other plumbing fittings have straight or parallel threads, but NPT fittings have a tapered thread profile. The outside diameter of the male thread tapers down from the base, so the end of the fitting has a smaller OD than the base of the fitting.
The matching female thread has a larger OD at the opening, tapering down as it goes in. NPT fitting sizes refer to the nominal ID of a standard cast-iron pipe.
This is where the confusion comes in. Most plumbing fittings are named for the thread size, but NPT sizes are not. Use the chart below to positively identify NPT fitting sizes. The tapered thread seals by means of an interference fit between the male and female threads. This means that NPT fittings should never bottom out in the port.
The fitting should screw in only partway before jamming. If an NPT fitting screws all the way into a female port without binding, the threads are either mis-tapped or worn out, or the female part may have cracked. After final tightening using the "Turns Past Finger Tight" method, 3 to 6 full threads should be engaged. To avoid undersized threads that bottom out without sealing, manufacturers often make the male threads slightly oversized and female threads a little tight.
This can lead to a fitting which does not screw in far enough for a secure connection. See the table below for recommended thread engagement. Too-tight fits are easily remedied by repeatedly hand-tightening the fitting into the port, turning just a little farther each time.
This burnishes the threads and allows more thread engagement. Be careful not to overtighten or force the fitting, as this can crack the female part. The idea is to polish or burnish the threads, not to change the thread size. Once you are satisfied with the thread engagement, you need to disassemble the fittings one last time.
Clean any grit off the threads and apply a thread sealant. Because NPT fittings seal at the threads instead of at a flare or at the hex, thread sealant is the only type of gasket you can use. LT instead of plumber's tape. However, they have been used in hydraulic systems for many years. Pipe threads in general are not recommended for high-pressure applications as they tend to leak more than any other style of connection. As previously mentioned, NPT requires some kind of sealant placed on the threads before assembly.
NPT threads are more likely to leak than their Dryseal counterparts; however, either thread type will leak if undertightened. Also note that overtightening can crack the female port. Measuring for a leak-free seal In the imperial system, TPI is measured by how many crests threads there are per inch; the metric system measures the distance between two crests. Using the trial-by-error method, try different thread gauges until you determine the tightest fit; engage as many threads as possible—the more threads engaged, the more accurate the reading.
Hold the fitting and thread gauge up to the light, looking for gaps between the gauge and thread. This is easier to see on a male connection than a female one.
Male thread diameter is measured on the OD; female thread diameter is measured on the ID. The Hose Safety Institute Handbook recommends holding the caliper at a slight angle for a more accurate male reading, and holding it perpendicular to the thread for a more accurate female reading. If the pipe or fitting connection seals on a flared surface or inverted angle seat, determine that angle of seal with a seat angle gauge on the male connection. Place the gauge on the sealing surface; if the centerlines of connection and gauge are parallel, the correct angle has been determined.
For a female connection, insert the gauge into the connection and place it on the sealing surface. As with the male connection, if the centerlines are parallel, the correct angle has been determined. References Eaton Hydraulics. Eaton Hydraulics. One thing no one seems to mention either is that there are hydraulic npt fittings with somewhat of an inverted flare incorporated into the end of the hose end which mates to a protruding nipple in the mating fitting.
This makes a npt which seals better for hydraulics nut I can find no information on this standard.
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