View Full Version : AN/NPT Basics

07-23-2010, 07:53 AM
FAQ: Plumbing Basics

It has come to my attention over the years that people really have no knowledge of AN/NPT fittings, for one reason or another people want to switch to them and end up making a thread asking for people to give a detailed list of parts and such to get it done. Hopefully this FAQ will help you understand the basics to put your own stuff together.

Pipe Threads:
Pipe thread fittings can come in two ways, straight or tapered. In the short time I’ve been dealing with automotive stuff, you’re going to be dealing with tapered threads. These seal by putting Teflon tape on the threads and screwing the fittings in. Again, remember they are tapered so they will only go in so far depending on how deep it was tapped. A common mistake is to over-torque the fitting and end up stripping the threads. It is completely normal for the fitting to stick out a bit.

Use the following as a reference for tapping and installing NPT fittings (http://www.sizes.com/materls/pipeThrd.htm):

Hose Sizing:
Hose sizing corresponds to the OD of a metal pipe it’s most like in a fraction of 16. So a -6 (read dash 6) corresponds to a metal pipe of 6/16” OD, or rather 3/8”. However, since wall thickness may vary, the nominal ID may not always correlate.

Army-Navy (AN) and Joint Industry Conference (JIC):
These mil-spec fittings call for a flare on the ends of fittings and hose ends. Be aware that there are two types of flares, AN/JIC fittings have a 37* flare, SAE fittings have a 45* flare. Do NOT mix and match these fittings. These seal when the two flares are pressed up each other. So understandably, if one is 45* and the other is 37*, It’s going to leak. It is also worth mentioning that these fittings are to be fitting DRY…Teflon tape should not be used and if you HAVE to use it to get it to stop leaking, you need to address that issue.

O-Ring Boss:
Sometimes a part you buy may call for an ORB (aka port) fitting instead. These are straight cut fittings with the same threading as AN/JIC fittings. The difference between the two is that while AN fittings seal via the flare, ORB fittings create a positive seal via an o-ring. Most times you’ll come across these fittings on a fuel pressure regulator or fuel rail. I have read write-ups in the past where people said to use AN fittings with some Teflon tape on the threads…this is NOT the right application. If a part calls for an ORB, use the correct fitting and install it dry.

Use the following image as a reference to what these fittings look like. (http://www.kurthydraulics.com/threads.php) Note: Pictured on the right is an O-Ring Face Seal. I haven’t come across it in an automotive application but essentially it is a combination of AN/SAE and ORB fittings in that an o-ring is placed in a groove of a male end and seals up by pressing itself against the flat side of a female end.


---------- Post added at 02:53 AM ---------- Previous post was at 02:52 AM ----------

Assembling a hose with a cutter style hose end:
The following illustrates an assembly using Earls Prolite 350 nylon braided hose with their Anu-Tuff line of swivel seal hose end. Note: Pictures were taken out of order, so you may notice a straight hose end in one picture and 90* in others.

Cut the hose. I highly recommend the hose cutter Earl’s sells (PN: D022ERL). It cuts prolite hose like it’s butter. If you’re cutting steel braided lines, I’ve used a dremel with cut off tool before, but a sharp chisel and sledge hammer works much better as well as a large wire cutter.

Unscrew the socket off the hose end

Place the hose inside the socket. Push It in until just below the threads.

Mark on the hose where the socket ends. If using steel braid a marker will do, since I’m using nylon braid I used some tape.

Place the other part of the hose end into a vise and lubricate where the socket will go. I use whatever I have lying around. Do not overtighten the vise, it doesn’t need to be super tight.

Screw the socket back onto the hose end but turning the HOSE by hand, not the socket. Go as far as you can go.

When you get to the point where you can’t turn it by hand anymore, grab a wrench and finish it off. There is no need to spend $$$ on AN wrenches…regular imperial sized wrenches work just fine. Once the socket is on, check the marking on the hose you made earlier. If the mark is more than 1/16” or so, then redo the assembly.

Pressure Testing:
You may want to test these lines on a bench as opposed to installing them on the car, running the engine, and then checking for leaks. Because then, if they do leak, you’ll just have to take it back off to redo it. Don’t buy into the super expensive pressure test kits/manifolds out there. Unless you do this stuff for a living and you use it constantly…and not only that, you make so many lines that you use ALL the sizes (-3 to whatever the largest they do…-16 or -20 I think)…it really is a waste of money.
All you really need to pressure test your line is to plug one end of it, and adapter the other end to your air compressor (though, I hear a lever type like a bicycle pump works just fine). Below is a cheap alternative I use for my -6 lines.

Connecting an AN line to a hardline:
To Be Continued…until after I get a flaring tool.

07-23-2010, 08:30 AM
Do you ever put a pressrized line in water to look for bubbles?

07-23-2010, 05:30 PM
I usually have a spray bottle hanging around the garage somewhere with some soapy water in it and just use that.

07-23-2010, 06:17 PM
Handy right up. Thank you.

joe's gt
07-24-2010, 07:03 AM
Great post. Always wondered what the "-6" number represented as well as "AN".

08-03-2010, 07:34 PM
i have the joy of also working with bpt fittings. stupid BW turbos.

08-04-2010, 12:10 AM
The 3S and 5S uses british pipe threads as well on a couple places. But essentially BSPT and NPT work the same in in principle so I didn't bother posting it.

mcmaster-carr has a very nice diagram of NPT and BSPT adapter fittings.