Using Aircraft Quality Hardware on Your Vehicle Part 2

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In our last issue, we took a look at why AN hardware is important. We also examined the basics, and gave a more in-depth breakdown, of the AN numbering system. There’s more however: In a typical light airplane, you'll seldom find a fastener larger than AN8 (1/2-inch diameter), at least in the airframe. However, there are some limited locations where larger fasteners are found (all the way up to -35). This arrangement happens to pretty much correspond to the bolts you’ll find on the chassis of a typical car (for example, you'll seldom find bolts larger than 1/2-inch diameter in a car, aside from maybe a few suspension members).

A useful note when considering bolts is that if the dash number that defines the length of the bolt is made up by two digits (as shown in the example in the last issue), the first digit is the length in whole inches while the second includes the length in additional 1/8-inch increments. That would mean that the length of something like an AN6-14 bolt would be 1-4/8-inches long (or 1-1/2-inches long).

How long should a bolt actually be? Aircraft construction practice tells us the length of the bolt must be sufficient to ensure no more than 1 thread is actually inside the bolthole. This figure is the grip length of the bolt, and is measured from below the head to the point at which the threads begin. The grip length should be equal to the thickness of the material being held by the bolt, or slightly longer. A washer may be used if the bolt is slightly longer. A piece of welding rod or safety wire can be used to measure the length of the hole.

The nuts found in aircraft applications typically lack identification, but they are manufactured from materials identical to aircraft bolts. In an airplane application, the most common method of securing fasteners is by way of self-locking nuts, castle nuts and, of course, plain nuts. In non-structural, non-critical areas, anchor nuts and wing nuts are sometimes used, but due to the vibration found in an aircraft application, the majority of nuts must have some form of a locking device to keep them in place. It should be no secret that similar vibrations can also trouble a racecar or a fast street car. That’s why a locking device of some sort (in order to retain the fastener) is a very good idea. You can use fiber insert (nylock) nuts, castle nuts for use with cotter pins, lock washers and safety-wire to keep a nut from working loose.

Nuts that self-lock do not require an additional locking device (such as a cotter pin or safety wire or a lock washer). Two types of self-locking nuts are currently acceptable for use on airframes: All-metal types or fiber (nylon) lock type nuts. The most common method of locking consists of a nylon or "fiber" insert. This insert has a smaller diameter than the nut. When the thread of a bolt enters the nut it physically taps into the fiber insert, which in turn produces a locking action. The FAA has some words of advice when it comes to fiber lock nuts, and their recommendations are pretty good practice on any application:

"After the nut has been tightened, make sure the rounded or chamfered end bolts, studs or screws extend at least the full round or chamfer through the nut. Flat end bolts, studs or screws should extend at least 1/32-inch through the nut. When self-locking nuts are reused, check the fiber carefully to make sure it has not lost its locking friction or has become brittle. Do not reuse locknuts if they can be run up finger tight. Bolts 5/16-inch diameter and over with cotter pin holes may be used with self-locking nuts, but only if free from burrs around the holes. Bolts with damaged threads and rough ends are not acceptable. Do not tap the fiber locking insert."

Nylon or fiber nuts are temperature limited to 250 degrees F. In locations where high temperatures may be experienced (for example, close to headers or the exhaust system), you should use an all-metal lock nut. Instead of a fiber insert, the metal locking nut threads narrow slightly at one end to provide more friction. It is also possible to find all-metal lock nuts with threads that are basically out of phase with the load carrying section. There are also all-metal lock nuts available with a saw-cut insert and a pinched-in thread in the locking section. The locking action of the all-metal lock nut depends upon the resiliency of the metal when the locking section and the load-carrying section are engaged by screw threads. This type of nut (all-metal) is capable of withstanding temperatures to 550 degrees F. And by the way, lock nuts are available in two heights -- standard and thin.

Recall the dash number system we discussed in our last segment? It holds true for lock nuts, and it defines the thread size (AN bolts and nuts are fine thread). Self-locking nuts are very popular (particularly in racing) and dirt simple to use. One disadvantage of self-locking nuts is that they should not be used on a part that subjects the nut or bolt to any sort of rotation.

Next issue, we’ll examine plain nuts, washers and other hardware. In the meantime, check out the photos for a closer look at various lock nuts.

Self-locking nuts simply work (if you follow the rules). Two different types are commonly found in an airplane – nylon (or fiber) types and metal types. The nylon style lock nut is the most commonly used.

All-metal lock nuts are used where temperatures exceed 250 degrees F. All-metal lock nuts are capable of withstanding temperatures to 550 degrees F.

Self-locking nylon nuts are available in standard or ½ height.