Aircraft Bolt

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.

Aircraft Bolt
Figure 1. Aircraft bolt identification

AN bolts come in three head styles: hex head, Clevis, and eyebolt. [Figure 1] NAS bolts are available in hex head, internal wrenching, and countersunk head styles. MS bolts come in hex head and internal wrenching styles.

General Purpose Bolts

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 other sizes in proportion).

Alloy-steel bolts smaller than No. 10-32 and aluminum-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

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

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.

Identification and Coding

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.
  • AN standard steel bolts are marked with either a raised dash or asterisk or a single raised dash.
  • AN aluminum-alloy bolts are marked with two raised dashes to indicate corrosion-resistant steel.
  • Additional information, such as bolt diameter, bolt length, and grip length, may be obtained from the bolt part number.

For example, in the bolt part number AN3DD5A,
  • The “AN” designates that it is an Air Force-Navy standard bolt.
  • The “3” indicates the diameter in sixteenths of an inch (3⁄16).
  • The “DD” indicates the material is 2024 aluminum alloy.
  • The letter “C” in place of the “DD” would indicate corrosion-resistant steel, and the absence of the letters would indicate cadmium-plated steel.
  • The “5” indicates the length in eighths of an inch (5⁄8).
  • the “A” indicates that the shank is undrilled. If the letter “H” preceded the “5” in addition to the “A” following it, the head would be drilled for safetying.

Close tolerance NAS bolts are marked with either a raised or recessed triangle. The material markings for NAS bolts are the same as for AN bolts, except that they may be either raised or recessed. Bolts inspected magnetically (Magnaflux) or by fluorescent means (Zyglo) are identified by means of colored lacquer or a head marking of a distinctive type.

Special-Purpose Bolts

Bolts designed for a particular application or use is classified as special-purpose bolts. Clevis bolts, eyebolts, Jo-bolts, and lockbolts are special-purpose bolts.

Clevis Bolts

The head of a Clevis bolt is round and is either slotted to receive a common screwdriver or recessed to receive a cross point screwdriver. This type of bolt is used only where shear loads occur and never in tension. It is often inserted as a mechanical pin in a control system.


The eyebolt is a special-purpose bolt used where external tension loads are to be applied. The eyebolt is designed for the attachment of devices, such as the fork of a turnbuckle, a Clevis, or a cable shackle. The threaded end may or may not be drilled for safetying.


Jo-bolt is a trade name for an internally threaded three-piece rivet. The Jo-bolt consists of three parts: a threaded steel-alloy bolt, a threaded steel nut, and an expandable stainless steel sleeve. The parts are factory preassembled. As the Jo-bolt is installed, the bolt is turned while the nut is held. This causes the sleeve to expand over the end of the nut, forming the blind head and clamping against the work. When driving is complete, a portion of the bolt breaks off. The high shear and tensile strength of the Jo-bolt makes it suitable for use in cases of high stresses where some of the other blind fasteners would not be practical. Jo-bolts are often a part of the permanent structure of late model aircraft. They are used in areas that are not often subjected to replacement or servicing. (Because it is a three-part fastener, it should not be used where any part, in becoming loose, could be drawn into the engine air intake.) Other advantages of using Jo-bolts are their excellent resistance to vibration, weight saving, and fast installation by one person.

Presently, Jo-bolts are available in four diameters:
  • 200 series, approximately 3⁄16 inch in diameter
  • 260 series, approximately 1⁄4 inch in diameter
  • 312 series, approximately 5⁄16 inch in diameter
  • 375 series, approximately 3⁄8 inch in diameter.

Jo-bolts are available in three head styles: F (flush), P (hex head), and FA (flush millable).


Lockbolts are used to attach two materials permanently. They are lightweight and are equal in strength to standard bolts. Lockbolts are manufactured by several companies and conform to Military Standards, which specify the size of a lockbolt’s head in relation to the shank diameter, plus the alloy used in its construction. The only drawback to lockbolt installations is that they are not easily removable compared to nuts and bolts.

The lockbolt combines the features of a high-strength bolt and rivet, but it has advantages over both. The lockbolt is generally used in wing splice fittings, landing gear fittings, fuel cell fittings, longerons, beams, skin splice plates, and other major structural attachments. It is more easily and quickly installed than the conventional rivets or bolts and eliminates the use of lock washers, cotter pins, and special nuts. Like the rivet, the lockbolt requires a pneumatic hammer or “pull gun” for installation. When installed, it is rigidly and permanently locked in place. Three types of lockbolts are commonly used: the pull type, the stump type, and the blind type. [Figure 2]

Aircraft Bolt
Figure 2. Lockbolt types

Pull Type

Pull-type lockbolts are used mainly in aircraft primary and secondary structures. They are installed very rapidly and have approximately one-half the weight of equivalent AN steel bolts and nuts. A special pneumatic “pull gun” is required to install this type of lockbolt. One person can accomplish installation since bucking is not required.

Stump Type

Stump-type lockbolts, although they do not have the extended stem with pull grooves, are companion fasteners to pull-type lockbolts. They are used primarily where clearance does not permit installation of the pull-type lockbolt. A standard pneumatic riveting hammer (with a hammer set attached for swaging the collar into the pin locking grooves) and a bucking bar are tools necessary for the installation of stump-type lockbolts.

Blind Type

Blind-type lockbolts come as complete units or assemblies. They have exceptional strength and sheet pull-together characteristics. Blind lockbolts are used where only one side of the work is accessible and, generally, where it is difficult to drive a conventional rivet. This type of lockbolt is installed in the same manner as the pull-type lockbolt.

Common Features

Common features of the three types of lockbolts are the annular locking grooves on the pin and the locking collar, which is swaged into the pin’s lock grooves to lock the pin in tension. The pins of the pull- and blind-type lockbolts are extended for pull installation. The extension is provided with pulling grooves and a tension breakoff groove.


The pins of pull- and stump-type lockbolts are made of heat-treated alloy steel or high-strength aluminum alloy. Companion collars are made of aluminum alloy or mild steel. The blind lockbolt consists of a heat-treated alloy steel pin, blind sleeve and filler sleeve, mild steel collar, and carbon steel washer.


Alloy-steel lockbolts may be used to replace steel high-shear rivets, solid steel rivets, or AN bolts of the same diameter and head type. Aluminum-alloy lockbolts may be used to replace solid aluminum-alloy rivets of the same diameter and head type. Steel and aluminum-alloy lockbolts may also be used to replace steel and 2024T aluminum-alloy bolts, respectively, of the same diameter. Blind lockbolts may be used to replace solid aluminum-alloy rivets, stainless steel rivets, or all blind rivets of the same diameter.

Numbering System

The numbering systems for the various types of lockbolts are explained by the break-outs in Figure 3.

Aircraft Bolt
Figure 3. Lockbolt numbering system

Grip Range

To determine the bolt grip range required for any application, measure the thickness of the material with a hook scale inserted through the hole. Once this measurement is determined, select the correct grip range by referring to the charts provided by the rivet manufacturer. Examples of grip range charts are shown in Figures 4 and 5.

Aircraft Bolt
Figure 4. Pull-and stump-type lockbolt grip ranges

Aircraft Bolt
Figure 5. Blind-type lockbolt grip ranges

When installed, the lockbolt collar should be swaged substantially throughout the complete length of the collar. The tolerance of the broken end of the pin relative to the top of the collar must be within the dimensions given in Figure 6.

Aircraft Bolt
Figure 6. Pin tolerance ranges

When removal of a lockbolt becomes necessary, remove the collar by splitting it axially with a sharp, cold chisel. Be careful not to break out or deform the hole. The use of a backup bar on the opposite side of the collar being split is recommended. The pin may then be driven out with a drift punch.

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