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AN Bolt

Bolts & Threads

When you built your own airplane you quickly find that non US / Canadian made parts, like the Rotax engine, use metric sized bolts and nuts. This requires you to have multiple spanners or wrenches sized in inches (or fractions thereof) and in millimeters for the metric system.

The standard bolts used in US or Canadian made aircraft is the AN or NAS type. These bolts have a tensile strength of at least 125000 to 160000 psi, much stronger than an average hardware store bolt and with much better corrosion resistance.

But vibrations can work a bolt loose. And as aircraft do not sit still all the time we need a secure way to keep the nuts from separating from the bolt and causing a problem by loosing parts in flight. Or as some call it, a rapid unscheduled disassembly.

UTS & Metric system

Although there are two systems in use (Unified Thread Standard and ISO Metric) the thread itself is basically the same (60°). You will find that the difference is in the major and minor diameters and pitch.

Thread Form

The organizations concerned with these standards are for UTS: ASME/ANSI and for the Metric system it is the International Organization for Standardization (ISO).


UTS type thread hardware is identified by its nominal diameter and by the pitch in tpi (threads per inch) and sometimes followed by the letter combination UNC, UNF or UNEF for the coarse, fine and extra fine pitch series.

ISO Metric hardware is coded the same way (diameter x pitch) but leaded by the letter M. For example: M5x10 and it has by default a coarse pitch.

Tolerance Class

The UTS system uses a classification for interchangeability of fabricated nuts and bolts. Class 1 is loose fit and 3 is close tolerance. Class 2 is most commonly used throughout. The class number is followed by an A for external or a B for internal threads.

Under load

When a bolt and nut are put under load they stretch a little length wise and the distribution of the applied load will not be equal on all threads. In fact, for an infinitely long bolt the first thread takes about 30%, the first three threads 75% and the first six threads take all the load.

So it is not necessary to have more than six threads on a nut, which saves weight and money in your construction.


Preloading or torquing a bolt and nut makes sure that the bolt does not fail from fatigue. Preloading is normally done by a torque wrench calibrated in inch/lbs or newton/meter but an experienced A&P or LAME can torque a bolt by feel. More info about torquing bolts and nuts on site.


For pipes and fittings mainly used in fuel and oil lines the National Pipe Thread, NPT, is used. With NPT, the thread (also 60°) is tapered under an angle of about 1.7899°.

By using a tapered thread a good seal is obtained where a straight thread is only able to hold the parts together. The taper makes sure that the flanks are also compressed together. For a leak free seal a teflon thread seal tape or sealant compound (Loctite®) is used, this also prevents corrosion and facilitates future removal should that be required.


Whitworth Thread

British Standard Whitworth (BSW) has a thread angle of 55°, so these won't fit ISO metric or UTS ASME/ANSI hardware. Used mainly in the United Kingdom and in Australia, but ISO Metric is becoming more popular than the old Imperial units. ISO 7 also still defines BSW for British Standard Pipe Thread.

Thread Locking & sealing

Locking a nut or a bolt is especially important in an aircraft as vibrations and continuous use will make sure that you can count on things to separate from the aircraft eventually. There are several ways of safetying a bolt or nut: the use of self locking nuts, safety wire through pre-drilled holes or the use of a chemical as Loctite® will make sure that every bolt or nut will stay in its place.


Sealing is important when threaded hoses or lines for fuel or oil lines are connected and pressurized to operating pressures. None of the fluids should leak from the mechanical connection. Thread sealing prevents this from happening.

For aircraft construction wire safetying, thread locking and sealing is very important, so make sure that you understand and master these techniques.

Written by EAI.

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