Standard Aircraft Bolts

A bolt is designed to hold two or more items together. Bolts that are typically used for airframe structural applications have hex heads and range in size from AN3 to AN20. Bolts are identified by their diameter and length. A diameter represents the shank diameter while the length represents the distance from the bottom of the head to the end of the bolt. A bolt's grip length is the length of the unthreaded portion. See figure 1 for example.

(Figure 1)

Aircraft bolts are available in cadmium-plated nickel steel, corrosion resistant steel and in 2024 aluminium alloy. Unless specified, a bolt is made of cadmium-plated nickel steel. A corrosion resistant bolt on the other hand is identified by letter "C" inserted between the diameter and the length designations. Aluminium alloy bolts are identified by letters "DD". For example, a bolt that is 1/4 inch in diameter, 3/4 inch long and made of cadmium-plated nickel steel is identified by the code AN4-6. However if the same bolt is made of corrosion resistant steel it carries the code AN4C6, whereas an aluminium alloy bolt would be AN4DD6.

In addition to the designation code, most aircraft bolts have a marking on their head identifying what the bolt is made of and in many cases, the manufacturer. The FAA forbids the use of aluminium alloy bolts and alloy steel bolts smaller than AN3 on structural components. Furthermore, since repeated tightening and loosening of aluminium alloy bolts eventually will ruin their threads, they are not used in areas where they must be removed and installed frequently. Aluminium alloy nuts can be used with cadmium-plated steel bolts loaded in shear but only on land aircraft. However since exposure to moist air increases the possibility of dissimilar metal corrosion, they cannot be used on seaplanes. See figure 2 for different types of bolt head.

(Figure 2)

Types of bolt head

Some AN bolts such as those used to fasten a propeller into flanged shaft must be safetied by passing safety wire through holes drilled through the bolts head. A bolt drilled for this type of safetying has the letter "H" following the number indicating its diameter. For example, the part number AN6H34A identifies a bolt that is 3/8 inch in diameter, made of nickel-steel has a drilled head, is 3 1/2 inches long and has an undrilled shank.