In Part 1 of Nut Case, we talked about the importance of using high-quality fasteners and covered the ins and outs of aircraft bolts. In Part 2, we’ll look at the hardware used to secure those bolts, starting with nuts.
The nuts in airplane applications usually have no identification on them, but they are manufactured from materials identical to aircraft bolts. In an aircraft application, the most-common method of securing fasteners is by way of self-locking nuts, castle nuts, and of course, plain nuts. In some non-structural, non-critical areas, anchor nuts and wing nuts are sometimes used. However, 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.
A lumpy-cammed hot rod can be plagued by similar vibrations, so we’d also recommend some sort of locking device for your modified car. You can use fiber insert (nylock) nuts, castle nuts for use with cotter pins, lock washers, or safety wire to keep a nut from working loose. You can also use self-locking nuts, which 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 bolt thread 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 it’s a 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 high-temperature spots (close to headers, for example), 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. An all-metal nut is capable of withstanding temperatures to 550 degrees F. Lock nuts are available in two heights—standard and thin.
Remember the dash number we mentioned in Nut Case (Part 1)? It’s also used on lock nuts, and defines the thread size (AN bolts and nuts are fine thread). Self-locking nuts are very popular (particularly in racing) and are simple to use. Avoid using self-locking nuts on parts that subject nuts or bolts to rotation.
The castle nut is another common airframe hardware item. In automotive applications they are most often used for tension loads. Castle nuts are used with drilled shank bolts, clevis bolts, and eye bolts. The slots in the nut accommodate a cotter key or pin (or lock wire) for safety purposes. Castle nuts are fabricated from steel and finished with cadmium plating. Corrosion-resistant castle nuts are also manufactured and have a letter “C” added to the AN number suffix. The “C” designates “stainless.” Two types of castle nuts are available: standard and thin. The dash number that follows the “AN310” or “AN320” prefix indicates the bolt size the nut fits.
What about plain nuts? They actually have limited use on aircraft structures and require an auxiliary locking device such as a check nut or lock washer. The half-height check nut used to hold a plain nut in place is an AN316. If a lock washer is used, then a plain washer must be placed under the lock washer in order to prevent damage to the surface. These nuts are manufactured with both right-hand and left-hand thread. Like the self-locking nuts, these fasteners are available in a wide range of thread sizes (sizing follows the AN description we talked about earlier).
When it comes to washers, the main job is to provide a shim, act as a smooth load-bearing surface, and to adjust the position of castle nuts in relation to the drilled hole in a bolt. Plain washers are used under a lock washer in order to prevent surface damage. The most common aircraft flat washers are “AN960″ examples. They are manufactured in two thicknesses—regular and one-half thickness. The dash number following the AN960 indicates the bolt size for which they are used. The numbering system is slightly different from the balance of aircraft hardware. For example, an AN960-616 washer is used with a 3/8-inch bolt (616 = 6/16 or 3/8”). There’s more too: If you encounter an “L” following the dash number that signifies a thin or “Light” washer. A “C” suffix indicates a stainless washer.
Another washer you might come across is an “AN970.” The dash number is completely different, simply because it is a large area flat washer typically used with wood or composite applications. The wider surface area is designed to protect the wood or composite.
There are other types of washers found in aviation that can be used in a high performance application. For example, the aviation industry uses lock washers with either a split ring, internal tooth lock washers, or external tooth lock washers (internal/external tooth lock washers are sometimes called “shake proof” washers). When considering lock washers, the aircraft industry notes they may be used with bolts or machine screws, but should only be used with a self-locking nut. In an airplane, they are not to be used to fasten primary or secondary structures. Additionally, they should not be used where they are subject to frequent removal or where corrosion is an issue. In these cases, a self-locking nut or a cotter key or safety wire is preferred over a lock washer.
When it comes to cotter keys, the AN380 and AN381 are most often used in light aviation. The AN380 versions are cadmium plated while the AN381 variants are manufactured from stainless steel. Cotter pins are designed for safety bolts, screws, and nuts, as well as other pins (they are normally used in conjunction with castle nuts). The dash number used for cotter pins indicates pin diameter and length.
You can typically find a huge selection of AN fasteners at aircraft houses; however, Summit Racing also carries a large variety for your hot rod or truck. If you search ARP products, you’ll find extreme quality bolts, nuts, and washers cataloged under “bulk” fasteners. Whereas aircraft houses may list their fasteners as AN6 or AN8, Summit Racing denotes AN fasteners as -8AN or -6AN.
In the end, the message is simple: Use high-quality hardware.
The costs are reasonable, and you will be safer. And we can all live with that.