Aircraft bolts, screws, and nuts are threaded in the American National Coarse (NC) thread series, the American National Fine (NF) thread series, the American Standard Unified Coarse (UNC) thread series, or the American Standard Unified Fine (UNF) thread series. There is one difference between the American National series and the American Standard Unified series that should be pointed out. In the 1-inch diameter size, the NF thread specifies 14 threads per inch (1-14 NF), while the UNF thread specifies 12 threads per inch (1-12 UNF). Both types of threads are designated by the number of times the incline (threads) rotates around a 1-inch length of a given diameter bolt or screw. For example, a 1/4-28 thread indicates that a 1⁄4-inch (4⁄16 inch) diameter bolt has 28 threads in 1 inch of its threaded length.

Class of fit also designates threads. The Class of a thread indicates the tolerance allowed in manufacturing:

Aircraft bolts are almost always manufactured in the Class 3, medium fit.

A Class 4 fit requires a wrench to turn the nut onto a bolt, whereas a Class 1 fit can easily be turned by hand. Generally, aircraft screws are manufactured with a Class 2 thread fit for ease of assembly.

Bolts and nuts are also produced with right-hand and left-hand threads. A right-hand thread tightens when turned clockwise; a left-hand thread tightens when turned counterclockwise.

Aircraft bolts are fabricated from cadmium- or zinc-plated corrosion-resistant steel, un-plated corrosion-resistant steel, or anodized-aluminum alloys. Most bolts used in aircraft structures are either general purpose, AN bolts, NAS internal wrenching or close tolerance bolts, or MS bolts. In certain cases, aircraft manufacturers make bolts of different dimensions or greater strength than the standard types. Such bolts are made for a particular application, and it is of extreme importance to use like bolts in replacement. The letter “S” stamped on the head usually identifies special bolts.

AN bolts come in three head styles: hex head, Clevis, and eyebolt. [Figure 7-19] NAS bolts are available in hex head, internal wrenching, and countersunk head styles. MS bolts come in hex head and internal wrenching styles.

The hex head aircraft bolt (AN-3 through AN-20) is an all-purpose structural bolt used for general applications involving tension or shear loads where a light drive fit is permissible (0.006-inch clearance for a 5⁄8-inch hole and alloy bolts smaller than 1⁄4 inch in diameter are not used in primary structures. Aluminum-alloy bolts and nuts are not used where they are repeatedly removed for purposes of maintenance and inspection. Aluminum-alloy nuts may be used with cadmium-plated steel bolts loaded in shear on land airplanes, but are not used on seaplanes due to the increased possibility of dissimilar metal corrosion.

The AN-73 drilled head bolt is like the standard hex bolt, but has a deeper head, which is drilled to receive wire for safetying. The AN-3 and the AN-73 series bolts are interchangeable, for all practical purposes, from the standpoint of tension and shear strengths.

Close tolerance bolts are machined more accurately than the general-purpose bolt. Close tolerance bolts may be hex headed (AN-173 through AN-186) or have a 100° countersunk head (NAS-80 through NAS-86). They are used in applications where a tight drive fit is required. (The bolt moves into position only when struck with a 12- to 14-ounce hammer.)

Internal wrenching bolts, (MS-20004 through MS-20024 or NAS-495) are fabricated from high-strength steel and are suitable for use in both tension and shear applications. When they are used in steel parts, the bolt hole must be slightly countersunk to seat the large corner radius of the shank at the head. In Dural material, a special heat-treated washer must be used to provide an adequate bearing surface for the head. The head of the internal wrenching bolt is recessed to allow the insertion of an internal wrench when installing or removing the bolt. Special high-strength nuts are used on these bolts. Replace an internal wrenching bolt with another internal wrenching bolt. Standard AN hex head bolts and washers cannot be substituted for them, as they do not have the required strength.

Bolts are manufactured in many shapes and varieties. A clear-cut method of classification is difficult. The shape of the head, method of securing, material used in fabrication, or the expected usage can identify bolts.

AN-type aircraft bolts can be identified by the code markings on the bolt heads. The markings generally denote the bolt manufacturer, the material used to make the bolt, and whether the bolt is a standard AN-type or a special purpose bolt.