ok, been trying to educate myself on NPT vs AN threads. was reading this article:

https://www.aeromotiveinc.com/an-vs-npt/

the one thing I dont think I fully understand is if you can thread an AN fitting into a port that is already threaded for NPT?

the article above does mention " NPT ports are commonly adapted to AN lines"

but it doesnt make sense to me. if -8AN uses 3/4"-16 TPI pitch how does that equate to 3/8" NPT?

or am I reading something wrong? I'm mainly curious as I have a 3/8" NPT tap but I dont have a 3/4"-16 tap.

I do understand that -8AN is equal to 1/2" as far as inside of the pipe/hose is concerned.

 

NPT is national pipe taper, it would need an adapter as AN is straight thread and afaik is measured by the pipe inner diameter, -AN is measured by tube outer diameter which is not relatable to inner diameter. They are two completely separate systems of measurement. to adapt an npt port to AN fitting you would best use an adapater fitting that has a 3/8 npt thread on one male and the correct sized AN fitting on the other male end.

 

Both AN and NPT sizes are the approximate internal measured size of the hose fitting. Obviously, the external thread size would be larger on either one. The big difference is that NPT is a tapered thread that seals on itself whereas the AN fitting seals between the cone shaped tip of the male fitting and the corresponding reverse cone face inside the female hose fitting. To confuse things, one can also get AN "Port" fittings for fuel rails, fuel filters and fuel regulators that have the same AN thread sizes and pitch as their male AN fitting counterparts, only they seal by O-ring or crush washer between the fitting and the flat surface in which they are screwed into.

Simply, you cannot interchange AN male and NPT male products with their opposing female receptacles in any similar thread size and pitch.

If you have a female NPT hole, you will buy an adapter fitting from XX NPT male to YY AN male (which corresponds to the AN sized hose you intend to use).

 

OK, so AN only for a fuel rail then. how do you properly measure the rail to determine the size AN fitting you need? So I just mic the inside of it and go from there? I think I need -8AN because I think the inner diameter of the fuel rail is 1/2" but will check in a little while.

 

If this discussion continues in this great direction of education and use, I'll make this a sticky. So please, post up photos and examples so I can make this a great sticky thread for all to share.

 

ok so I mic'd the inside diameter of the fuel rail and its definitely 1/2" as it mic'd to .495

so does that mean I need -8AN fittings and then use a 3/4"-16 tap? I'm thinking this is so the inside diameter doesnt reduce at the connector and stays 1/2" all the way to the fuel line itself.

I'm wondering if that even matters for 2 reasons for me.

1. I dont have a way to properly drill out the inside of the rail before I use the 3/4" tap on it
2. the fuel line is going to 3/8" anyway so it will choke down there regardless.

If #2 holds true, then cant I just use -6AN and then tap from there so I dont have to drill the inside of the rail out in my bench vice and hope me "eyeballing" if the drill bit is level or not?

 

wait, what? so there are two different AN types?? AN and -AN??

 

"-AN" and "AN" are interchangeable... they are not different. I will have to measure some fittings to add substance to this thread, but it certainly appears that the outer diameter of the fittings is typically almost double the size of the internal measurement or AN size. I will get some photos and measurements for this thread over the next couple of days and post them up.

 

Okay, there's a handful of things going here you're missing.

AN is the style of connector . . . it means a taper connection of 37* on the face of the taper and was created during WWII for our military (primarily fuel connections). AN is independent of a size . . . it simply means a 37* flared fitting.

The -(number) value is how many 1/16ths the Inside Diameter of the pipe/tube you are using is. For example -8 means eight sixteenths which divides to 1/2. Meaning that a -8 size line has an inside diameter of 1/2".

The thread on the outside of the male AN fitting is standardized to a nominal Unified National Fine (UNF) thread pitch which has a minor diameter large enough to leave a wall thickness of roughly 5/64 (.078") between it and the bore diameter.

A 3/4-16 TAP needs a hole size of 11/16 (.6875") to create the desired thread form. Your fuel rail with an Inside Diameter of .500" would need an Outer Diameter of at least 15/16 (.9375") so you could in theory drill the fuel rail to 11/16" and then successfully tap the fuel rail to 3/4-16 without breaking out of the material since 3/4-16 female has a major diameter of ~.755".

Additionally, straight UNF thread to AN fittings require use of an o-ring to seal. As far as I can tell, there isn't a standardization of the counterbore or chamfer required by the automotive aftermarket industry. I've personally used a 60* chamfermill around .080" depth with success. I've also seen counterbores with straight corners but I haven't measured one.


Secondly, National Pipe Taper (NPT) is a tapered thread form different from a straight taper like UNF. 3/8" NPT (3/8-18) has a nominal starting minor diameter of .575" which makes it ideal to use for tube/pipe that has a 1/2" Inside Diameter hole like -8 does since you don't have a bunch of wasted material between the through hole size and the threads that hold it on.

 

In this case I think pictures would be best to explain the difference between NPT and AN. Top picture is NPT thread. Notice how the thread tapers as it is being inserted into the female fitting. Bottom picture is a AN fitting. The thread in an AN fitting is straight and the tip is conical in shape which is how it seals itself when inserted into a female AN fitting.

OP, what is it you are trying to accomplish? What are the parts and corresponding threads that you are trying to make hoses for?

 

this is for an old project of mine that I am finally getting around to finishing up. as in, I started putting this together in 2004 when I lived in atlanta and was single. now I'm in charleston, sc and married with a newborn haha.

Its a ford 2.3t engine with a volvo 16v head on it. We welded the bottom of the volvo intake to a tube of aluminum and then welded on the stock 2.3t throttle body flange. I wasnt thinking and, as you can see, its kind of a close fit with the fuel rail. I literally bought bare fuel rail stock off ebay back in 2008 and then used my dad's drill press (long since broken and sold for scrap) to drill the injector holes. As I see it, I've got two shots at getting the front setup right.



inner diameter of the fuel rail is 1/2" as stated above. I've read you want -8AN for this as thats 1/2" internally but I dont even think that matters as the fuel pressure regulator AND the stock fuel lines on the car are only 3/8" anyway so that will be the choke point.



I'm thinking I should use -6AN as that's 3/8" and I wouldnt have to drill out the damn fuel rail super huge in my bench vice just to get the proper thread tap in to tap the damn thing.

I'm going to use a 90* fitting as well. or should I just use the NPT taps I have on hand and tap it with a brass plumbing fitting? I dont think this will work even with Natural Gas thread tape as the liquid gas would likely eat it, which is why I'm leaning towards AN.

I've just somehow messed with everything on a car EXCEPT AN fittings and when I HAVE messed with them....I've just gotten adapter fittings, never have I had to drill/tap raw fuel rail stock. If I goof on the backside I can flip it around and cut off some rail and try again and then address the backside even if I have to take the fitting and rail to my dad and use his tig to weld the damn things together as most AN fittings are aluminum.

 

You should be fine with -6 unless you plan on pushing big horsepower. For your fuel rail, you could use a port thread fitting (see picture below). The threaded part will come with an o-ring (not shown) which will seal against the fuel rail port. This particular fitting from Brown and Miller (link here) swivels so the attached hose can be oriented properly. If your fuel rail can use -6 then all you would need to do is thread the rail and buy -6 90 degree fittings. But if the port is bigger and you have to use -8, then you would need a -8 to -6 reducer connected to a -6 90 degree fitting.

 

digging further, I've found a good article on NPT (national pipe thread) vs NPTF (national pipe thread fuel) with NPTF being use a lot in marine applications.

National Pipe Thread vs. National Pipe Thread Fuel

looks like a lot of places use liquid sealants when using NPTF
https://www.jamestowndistributors.co...t.do?docId=563
the aviation form-a-gasket isnt that expensive either:
https://www.jamestowndistributors.co...+No.+3+Sealant

I've already got some NPT tap/dies as I bought a set to use when tapping the block for 1/8" NPT for the oiling system as thats what the oil pressure sending unit uses and the volvo head requires its own separate oil feed to the cam tower galleries...not to mention tapping for the oil feed for the turbo...

I might see if I can find a NPTF-3/8" hose barb 90* fitting and try that. also, emailed a seller on ebay that custom machines fittings as well as banjo fittings.

 

my main concern is that if you look at the first pic I posted, how little clearance there is from the front opening of the fuel rail to the backside of the throttlebody mounting plate. I need something tighter than what you posted.

something like this:


or like this:


but then I saw AN fittings on ebay which is what sparked this whole thing as I saw they had o-rings on them which would eliminate any sealing issues...I've just never used them...


that isnt the one I'm thinking of because of the nut portion on the AN side but that was my quick google result with an o-ring on it.

 

I see what you mean. You could chop a little off the fuel rail and then extend the threads to give you more room. Did you look through the Brown and Miller catalog (the link I posted)? They have a bunch of different fittings that may suit your needs. They also have fittings with o-rings on them. The picture I posted doesn't show an o-ring but when you order it comes with it. Matter of fact I'm currently using them on my daily driver with no leaking issues whatsoever.

 

went ahead and ordered a 6AN 90* to 6AN fitting. one with the o-ring for the fuel rail surface as well as that swivels on that side. its 1.2" tall when installed and my clearance between the rail and throttle body flange is .75" but I think I can gain enough by cutting the rail down a bit. Also ordered a 6AN female to 3/8" pushlock fitting for the opposite end. I will wait and see how it works out with the front side before messing with the back side as there's plenty of room back there and if I goof on the front, I can flip it around and try again using the backside haha.




I've got a 9/16"NF18 tap in my tap/die set which, after looking online, "looks" to be the size tap I need as people are telling me its the same as 9/16"-18TPI which is what 6AN calls for.