AN vs. NPT: Understanding Port Threads, Adapter Fittings and Line Sizes - Engine Builder Magazine
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AN vs. NPT: Understanding Port Threads, Adapter Fittings and Line Sizes

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AN specifications are a
popular standard met by all companies that manufacture AN style
performance fuel hose and accessories. For many there has been much
confusion about the subject of AN lines, NPT and ORB ports, and how all
of this works together. Here are the answers for those wanting to know.

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The flare angle used to seal AN connections is required to be SAE,
37 degree, as apposed to the 45 degree flare commonly found on
household plumbing adapters. This angle can be found on the male point
of the port adapter fitting and on the female inside the hose-end nut.
AN port threads are not NPT or “pipe thread” but instead utilize
straight threads (like any normal fastener) and SAE O-Ring Boss (ORB)
technology for sealing. AN lines, ORB ports and the appropriate port
adapter fittings are measured in inch/fractional sizes.

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A dash (–) size in "AN speak” refers to the I.D. of a standard, thin
wall, hard line as the basis to construct a comparable flexible hose
that may be used in it’s place. A 1/2?, thin wall, hard line measures
.500? on the outside diameter (O.D.), has an inside diameter (I.D.) of .440?, and a wall thickness of .030?. An appropriate, flexible
replacement line would be –8 AN, with a minimum .440? I.D. Depending
on line construction, rubber with stainless steel or nylon braid, or
Teflon with stainless steel braid, the line’s wall thickness and O.D.
may vary.

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AN line sizes will have a dash (-) preceding the line size. The
number after the dash refers to the number of 1/16? O.D.,
thin wall, hard line to which the flexible line will compare. For
example, calling for a –8 AN line would mean the engineer or system
designer requires a flexible line, made of certain materials suitable
for the application, that would have the minimum I.D. of an 8/16”
(1/2”) O.D. hard line. The actual line construction is dictated by the
application with regard to line flexibility, vacuum and pressure
capability, abrasion resistance and chemical compatibility, etc.
Regardless, the engineer knows a -8 line of any construction will have
a minimum I.D. equal to 1/2” hard line (.0440”), and be able to support
similar flow rates.

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Here are some of the common army/navy (AN) line and thread specifications:

-04 AN line = 4/16? = 1/4? hard line. –04 AN Port and Fitting thread is: 7/16? -20 TPI.

-06 AN line = 6/16? = 3/8? hard line. –06 AN Port and Fitting thread is: 9/16? -18 TPI.

-08 AN line = 8/16? = 1/2? hard line. –08 AN Port and Fitting thread is: 3/4? -16 TPI.

-10 AN line = 10/16? = 5/8? hard line. –10 AN Port and Fitting thread is: 7/8? -14 TPI.

-12 AN line = 12/16? = 3/4? hard line. –12 AN Port and Fitting thread is: 1-1/16?-12 TPI.

Modern, high performance fuel systems are predominately fitted with
safer, better sealing, higher flowing, AN-ORB ports. These ports
require a straight thread adapter fitting, with a sealing O-Ring
installed over the threads, up to the hex, that disappears into the
port when properly installed. No additional thread sealer is required
or recommended.
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National Pipe Thread (NPT) ports, AN Ports and port adapter fittings:

Over the years, in low-pressure hydraulics, NPT has been a popular
thread for ports and adapter fittings. When NPT ports are used in a
fuel system with AN line, an adapter fitting to convert from NPT to AN
is required. NPT was designed for use with thick walled pipe, typically
black pipe, used in fixed structures like buildings, to handle
distribution of water and natural gas. Black pipe isn’t particularly
bendable, flexible or lightweight and hardly desirable for plumbing a
high performance fuel system. As a result, fittings that adapt NPT ports
to AN line are common to allow flexible AN lines to be utilized in
performance automotive fuel systems.

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Unlike AN thread, which is straight, NPT ports and fittings are both
tapered. NPT male to female adapters start loose, threading easily but
get tight and harder to turn well before the hex touches the port. When
threaded together, the NPT design creates a wedging effect, binding the
thread in order to seal. The use of a thread sealant is common and
required with NPT, as it does not consistently create a positive seal
on it’s own, like an O-Ring configuration. It’s common to see a number
of threads showing on the adapter fitting when NPT is sufficiently
tight, making NPT assemblies bulkier and less clean appearing than a
similar AN assembly.

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NPT ports are commonly adapted to AN lines, but the NPT size
designation is confusing, identifying the pipe I.D. rather than the
O.D. Black pipe has a much thicker wall than hard line, so the
pipe/port O.D. is much larger than the NPT size would seem to indicate.
For example, a 3/8” NPT port will have an outside diameter of 5/8”,
allowing for a wall thickness of 1/8” (0.125”). As a result, NPT port
sizes allow use of a one step larger AN line than their indicated size
would seem to support. As long as the wall of the adapter fitting is
not overly thick, the following NPT Port to AN adapters will provide a
common I.D. throughhole:

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Maximum AN line for NPT port size:

1/4? NPT is compatible with up to -6 AN (3/8? hard line)

3/8? NPT is compatible with up to –8 AN (1/2? hard line)

1/2? NPT is compatible with up to –10 AN (5/8? hard line)

3/4? NPT is compatible with up to -16 AN (1? hard line)

Adapter fittings are available for connecting larger than
recommended AN lines to the above NPT ports. Beware, the inside
diameter of the adapter fitting will necessarily be smaller on the NPT
side, creating a flow restriction that many racers and hotrod
enthusiasts overlook. This is a poor practice and should be avoided,
but when no alternative is available, consider sourcing a steel NPT to
AN adapter from a good hydraulic supplier. Steel adapters will have a
thinner wall than aluminum, due to the increase in material strength,
leaving a larger I.D. to support higher flow on the too small, NPT side
of the adapter.

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–Tech Tip courtesty of Aeromotive

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