When you’re dealing with complex, highly-engineered, and expensive aircraft, it is important that every single piece does its job. From the engine down to the smallest fastener, each part has an important role to play. Bolts, a small but critical fastener in the construction of any aircraft, are no different. The stress of flight means that non-aviation grade hardware simply doesn’t do the trick. It has neither the strength nor the corrosion resistance to work effectively in flight. This has required aviation authorities to develop strenuous standards for aircraft bolts, as well as an exhaustive description and coding system.
Bolts used for aviation purposes are manufactured to AN (Army/Navy) standards. AN bolts are used for either tensile or load-bearing applications. They are made from a variety of different materials including cadmium-plated nickel alloy steel, corrosion resistant steel, or aluminum alloy. Their comprehensive coding system seems daunting but, once you break it down, is easy to follow. For example, a typical bolt coding will look something like this:
“AN” is a common prefix simply standing for Army Navy, the two authorities from which this coding system came. The number 4 denotes that the bolt has a diameter of 4/16th of an inch. If the bolt was 3/16th of an inch it would be AN3. The dash following AN4 seems like nothing, but actually signifies that the bolt is constructed of cadmium-plated steel. (H) designates that the bolt has a drilled head bolt and is left off the code in the event that no drilling in the head is necessary. The fourth section, the number 10, indicates that the bolt is ten inches in length. Much like the diameter, this number changes with the length of the part. The final character, the letter A, is used to show that a cotter pin hole is not required.
As you would expect, the bolt codes can differ drastically based on a number of factors and specifications. This standardized coding system is a convenient identification tool and can save operators a lot of headaches during their search for proper bolts.