NPT and Aircraft Fittings–Don’t Get That Size Wrong!

Plane and simple.

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National Pipe Thread‭ ‬is a U.S‭. ‬standard for‭ ‬tapered‭ ‬threads‭, ‬which we find on many aircraft fittings‭. ‬Contrast this with‭ ‬straight‭ ‬threads‭, ‬which are used on the nuts and bolts that hold our aircraft together‭. ‬Tapered threads are used‭ ‬for‭ ‬sealing connections carrying gas and‭ ‬fluids‭. ‬In our kit aircraft we use all sorts of fittings with tapered threads such as‭ ‬those found on fuel lines‭, ‬brake lines‭ ‬and even avionic instrument air lines‭. ‬Our household water pipes use the same NPT tapered thread standard‭.‬

Components with NPT threads are common in aircraft fuel systems. Note that the blue AN fittings have NPT threads on one end and a flared tube connection on the other.

There are two important points to‭ ‬consider when selecting a tapered fitting‭. ‬First‭, ‬the way sizes are assigned to these fittings is not intuitive‭. ‬This may cause you grief the first time you need to‭ ‬order one from your favorite aircraft‭ ‬supplier‭. ‬The two most common sizes‭ ‬we use in our aircraft are‭ ‬1/8‭ ‬and‭ ‬1/4‭ ‬inch‭. ‬

The nominal size of an NPT thread fitting is not the same as its actual dimension.

However‭, ‬if you measure one of these fittings‭, ‬the size will not match any actual dimension of the part‭! ‬This is somewhat‭ ‬similar to expecting a 2×4‭ ‬piece of lumber to measure either 2‭ ‬or 4‭ ‬inches‭. ‬It is only a nominal size description‭ (‬in name‭ ‬only‭). ‬Without understanding this‭ ‬principle‭, ‬you will find yourself returning and reordering fittings because the diameter of the threaded portion may not fit your application‭.‬

These are the popular NPT thread sizes used in homebuilt aircraft.

Using a chart‭, ‬you can determine the fitting’s nominal size by measuring the‭ ‬outer diameter of the threaded portion‭. ‬Fortunately‭, ‬there is an easy way to identify the two popular sizes‭. ‬If the outer diameter is less than‭ ‬½‭ ‬inch‭, ‬then you have a‭ ‬1/8‭-‬inch fitting‭; ‬if the diameter is a‭ ‬tad greater than‭ ‬½‭ ‬inch‭, ‬then you have a‭ ‬¼‭-‬inch fitting‭. ‬The larger NPT‭ ‬fittings are less frequently found on‭ ‬our small homebuilts‭, ‬but measuring and using the chart will provide their proper size identification‭.‬

Nominal Size Actual Diameter
1/8 inch .405 inch
1/4 inch .540 inch
3/8 inch .675 inch
1/2 inch .840 inch

A fitting’s nominal size is determined by measuring the outer diameter of the threaded portion. 

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The second important point to remember when using NPT fittings is that a‭ ‬sealant‭ ‬must‭ ‬be used on the threads if you are interested in a leak-free connection‭. ‬This requirement is due to the microscopic channel in the spiral thread design that allows a path for leaks to occur‭. ‬A number of good choices for sealant are‭ ‬available depending on the application‭. ‬For example‭, ‬Loctite 567‭ ‬or similar‭ ‬products from Permatex work well for a fuel‭ ‬system‭. ‬A good sealant manufacturer‭ ‬will describe the compatible materials‭ ‬to be used with their product‭. ‬Avoid‭ ‬using plumber’s Teflon tape as small‭ ‬bits of this material are easily cut by the threads and can enter the system‭. ‬Also‭, ‬never confuse an NPT thread connection with‭ ‬a flare nut connection‭. ‬Sealant is‭ ‬never‭ ‬used in a flare connection but always used for an NPT connection‭. ‬

With time and experience you will soon be able to identify the nominal sizes of NPT fittings by just looking at them‭. ‬You will also be surprised how‭ ‬many of these fittings your finished‭ ‬aircraft uses‭.

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