Tech and Install > NPT AN and Hard Line Sizing

NPT AN and Hard Line Sizing

National Pipe Thread (NPT)

Many of our tanks, bolt in pickups and fuel pump assemblies come with female NPT fittings for fuel feed and return.  NPT is a U.S standard for tapered threads used on fittings and threaded pipes.  A taper thread starts loose and tightens as the fitting is threaded in.  Unlike a typical parallel or straight thread fitting, a taper thread will pull tight and make a fluid tight seal when torqued as the flanks of the threads compress against each other.  Because the threads themselves are what is sealing the fitting it is common to see threads showing on NPT fittings and adapters.   Teflon tape is also recommended to ensure a leak free seal.

Adapter fittings for NPT threads are prevalent and easy to find at most automotive or hardware stores making it easy to adapt from NPT to whatever type of line you choose.  We have adapters to go from NPT to AN here and NPT to hose barb here

The sizing of NPT fittings is often misunderstood.  This is because unlike hard lines which designate the O.D. of the line NPT sizing designates the I.D.  Therefore NPT sizing is larger than their size may seem to indicate.

AN Thread

AN thread specification was derived from the US military as a standard agreed upon by the Army and Navy (AN).  AN fittings are commonly used to connect flexible hoses.  AN sizing is designated by a dash (-) followed by a number.  That number stands for the number of 1/16 of an inch of the O.D. of a hard line.  For instance a -6 AN line is 6/16" or 3/8".

NPT, AN, and Hard Line Size Compatibility

In the chart below we have outlined the nearest equivalent size between NPT, AN and hard line.

1/4" NPT -6 AN 3/8" hard line
3/8" NPT -8 AN 1/2" hard line
1/2" NPT -10 AN 5/8" hard line


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Tank's Inc.
260 Welter Drive
at Hot Rod Lane
Monticello, IA 52310

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