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Builders guide to safe aircraft materials

AN, MS hardware — rivets, bolts
and locking devices

Rev. 10a — page content was last expanded 22 January 2012
Page edited by RA-Aus member Dave Gardiner www.redlettuce.com.au January 2010
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In addition to the fittings discussed in module 11 the term 'aircraft hardware' also includes the fasteners used in aircraft assembly. In this and the next module of the Guide we look at rivets, threaded fasteners and locking devices.

Airframe fasteners are designed and manufactured to conform with long-established and proven standards for a system of fasteners. The standards most widely recognised are those originally defined by committees associated with the US military prior to and during World War 2 — the Army–Navy [AN] standards. The Military Standards [MS] followed — which superseded some AN standards and complemented others — and later came the National Aerospace Standards [NAS]; thus the common terms AN, MS and NAS. Those older US standards are in inches and fractions of inches only so there are few references to SI units on this page.

Some of the following material is noted as an extract from the FAA advisory circular AC 43.13-1B Chapter 7. The complete volume 'Acceptable methods, techniques, and practices — aircraft inspection and repair' (~ 650 pages and incorporating the 2001 changes) is available from the RA-Aus online shop for a reasonable price. It is bound together with the FAA advisory circular AC 43.13-2A 'Acceptable methods, techniques, and practices — aircraft alterations' (~ 100 pages).
12.1 Solid or driven rivets
The standard fastener for light aircraft constructed from sheet metal is the aluminium rivet; a recreational light aircraft built from aluminium sheet might use 6000 to 8000, maybe more. Rivets are used primarily to fasten aircraft skins to the substructure and to fabricate structural assemblies. Such joints are generally concerned with shear and tension loads. There are two rivet classes; the simple solid shank rivet, which is 'driven' using an air-operated rivet gun and a 'bucking bar' or 'dolly'; and the more complex 'blind' rivets requiring simpler (if the rivets are small diameter) installation tools to 'pull' them, but the tools may be unique to each pulled rivet manufacturer.

AN426 and AN470 rivets
There are quite a number of solid rivet types each made from a variety of materials — aluminium alloys, steel, corrosion-resistant steel, Inconel and Monel. However, apart from engine bay applications, there is generally only one solid rivet type of interest to light aircraft builders. That is the general structural use aluminium alloy 2117-T4 rivet generally with a 'universal' head (the upper rivet in the diagram) or possibly a 100° countersunk head (the lower rivet) — note the differing methods for measuring rivet length. The 2117-T4 material is galvanically compatible with the 6061 alloy and although such rivets have only about 80% of the strength of 2024-T3 rivets, the handling process is much simpler. Another type occasionally used in non-structural applications is the softer, low-strength 99% aluminium alloy 1100 rivet with the same head options.

10 degree countersinkThe 100° countersink is the aircraft standard and all AN, MS standard countersunk head rivets and screws will conform. However, some brand name pulled rivets may have 120° or other taper.

Dimpled holes for flush rivetCountersunk head rivets are used to attach skins to the substructure so that the heads are flush with the surface, thus reducing drag. However, the skin material in very light aircraft is so thin that metal removal to produce a countersunk hole is not possible; consequently 'dimpling', as shown at left, is commonly used. Skin thicknesses of 1mm (0.040") are on the border between countersinking and dimpling; greater than 1 mm the metal becomes difficult to dimple, below 1 mm countersinking will probably produce a weaker joint. When the skin is being riveted to thicker structures such as longerons then the drilled hole in that structure would be countersunk and only the thin skin dimpled – with a dimple die set. However, protruding head riveting is not a significant drag problem for low-speed aircraft and universal head riveted joints will be easier, probably stronger and certainly less time-consuming to use.

Hole clamps inserted at intervals and generally known as clecos — the trade name of the first such clamp — are used to hold the metal parts tightly together while riveting.

Identification code: airframe fasteners are generally specified and catalogued by their hardware identification code (or dash number), which consists of the AN/MS/NAS basic specification followed by a series of numbers/letters. Rivets manufactured in accordance with the AN/MS standards are identified by a four-part code:
  • firstly the AN or MS specification and head type, then
  • one or two letters that indicate the material, followed by
  • a number that indicates the shank diameter in 1/32nd inch increments, then
  • a dash followed by a number that indicates rivet length in 1/16th inch increments.
So a standard universal head solid rivet manufactured from 2117-T4 aluminium, 1/8 inch diameter and 5/16 inch in length would be coded either AN470AD4-5 or MS20470AD4-5. The code for the same rivet in the softer 1100 aluminium is AN470A4-5 or MS20470A4-5.
  • AN470 or MS20470 denotes the specification for universal head types
    (AN426 or MS20426 denote countersunk head types).
  • AD is the material code for 2117-T4 (A=1100, B=5056, C=copper, D=2017, DD=2024, F=stainless and M=Monel).
  • 4 = 4/32 or 1/8 inch diameter.
  • -5 = 5/16 inch length. Half sizes in length are indicated by '.5'; e.g. -5.5 would indicate a length of 11/32 inch.
Physical identification of the rivet alloy is aided by a marking in the head. AN/MS rivets made from 2024 have a raised double dash on the head while a recessed dimple in the head indicates 2117-T4. (That recessed dimple, acting as a drill centering guide, greatly assists when mis-riveted and drilling out, and replacement is the only option.) Solid aluminium rivets must not be less than 3/32 inch or greater than 1/2 inch diameter but shank diameters of 3/32, 1/8 and 5/32 or 3/16 inches are those most commonly used in light aircraft. The 3/32 diameter would generally be used for attaching the aircraft skin, the 1/8 for assembly of structural parts and the larger diameters for fabrication of wing spars.

Bucking bar Use: solid rivets are generally used in single-lap (two layers of material) or double-lap (three layers) joints where parts formed from sheet metal are clamped together by rivets, and the riveter is able to access both sides of the work, with the manufactured head on the exterior. The diameter of the drilled holes is made slightly greater than the rivet shank so that when the (roughly) 0.5 gram rivet is repetitively driven against the inertia of a 0.4–0.6 kg bucking bar (shown at left), or a dolly, the compressive strain causes the shank to expand to completely fill the hole. The required bucking bar mass varies with rivet diameter.

Driven rivet At the same time the shank end or tail is plastically deformed to produce the 'driven' head, which clamps the parts together tightly. The driven head should have a thickness around 0.5 times and a width around 1.5 times the diameter of the shank. The dimensions achieved primarily result from the choice of rivet length compared to the thickness of the materials; if the length of the chosen rivet exceeds the combined thickness of the parts being joined (the 'grip length') by about 1.5 × rivet diameter then a satisfactory driven head should result.

More information is contained in the US Military Specification MIL-R-47196A 'Rivets, buck type, preparation for and installation of'.

If correctly prepared and riveted, the material being joined is under a compressive load and the elastic reaction places a tensile load on the rivet; these loads ensure a good seal between the undersides of the rivet heads and the material, providing a waterproof and corrosion-resistant joint — particularly so if driven when wet with zinc chromate primer that fills the small chamfers formed in the barrel of the hole where the edges have been deburred. Also the expansion of the shank to completely fill the drilled hole precludes any relative movement between rivet and material. The repetitive cold-working of the rivet strain hardens the material so the driven T4 rivet gains strength equivalent to the T3 temper.

The military specification MIL-R-47196A states: "When the rivet material is dissimilar to the material being riveted [see galvanic corrosion], the rivet hole, countersink, and rivet shall be coated with zinc chromate primer in accordance with TT-P-1757 prior to installation. The rivet shall be installed while the primer is in the wet condition." There are other corrosion-inhibiting zinc chromate or barium chromate pastes (e.g. JC5A, Duralac) are also used under rivet/bolt heads but there appear to be conditions where such compounds break up when in high pressure contact with water; e.g. in the proximity of the landing gear.

Riveted joints in light aircraft are usually designed so that the rivets will give way before the structural parts fail; the premise being that it is easier to replace a few failed rivets than repair/replace torn metal.

Extract from AC43.13-1B: the following table has been extracted from the advisory circular; some additional comments have been added. The table indicates just a few types of solid rivets, the specified material and the identification mark.

If you are interested I have placed the complete Chapter 7 'Aircraft hardware, control cables, and turnbuckles' of AC 43.13-1B on this website for download in PDF format (2.8 MB).

Solid rivet ID table

"Standard solid-shank rivets and the universal head rivets (AN470) are used in aircraft construction in both interior and exterior locations. All protruding head rivets may be replaced by MS20470 (supersedes AN470) rivets. This has been adopted as the standard for protruding head rivets in the United States.

Countersunk head rivets MS20426 (supersedes AN426 100-degree) are used on the exterior surfaces of aircraft to provide a smooth aerodynamic surface, and in other applications where a smooth finish is desired. The 100-degree countersunk head has been adopted as the standard in the United States. Refer to MIL-HDBK-5 Metallic Materials and Elements for Flight Vehicle Structures, and U.S.A.F./Navy T./O. 1-1A-8, Structural Hardware.?

Material applications: Rivets made with 2117-T4 are the most commonly used rivets in aluminum alloy structures. The main advantage of 2117-T4 is that it may be used in the condition received without further treatment.

The 2017-T3, 2017-T31, and 2024-T4 rivets are used in aluminum alloy structures where strength higher than that of the 2117-T4 rivet is needed. See Metallic Materials and Elements for Flight Vehicle Structures (MIL-HDBK-5) for differences between the types of rivets specified here.

The 1100 rivets of pure aluminum are used for riveting nonstructural parts fabricated from the softer aluminum alloys, such as 1100, 3003, and 5052.

When riveting magnesium alloy structures, 5056 rivets are used exclusively due to their corrosion-resistant qualities in combination with the magnesium alloys.

Mild steel rivets are used primarily in riveting steel parts. Do not use galvanized rivets on steel parts subjected to high heat.

Corrosion-resistant steel rivets are used primarily in riveting corrosion-resistant steel parts such as firewalls, exhaust stack bracket attachments, and similar structures.

Monel rivets are used in special cases for riveting high-nickel steel alloys and nickel alloys. They may be used interchangeably with stainless steel rivets as they are more easily driven. However, it is preferable to use stainless steel rivets in stainless steel parts."

[Extract ends]

12.2 Blind or pulled rivets
Blind rivet Blind rivets are used in conditions where there is no physical access to one side of the work — when riveting a collar to a tube, for example. Thus blind rivets, particularly stainless steel, have particular applications in aluminium tube airframes. There are a number of manufacturers each with a range of types from relatively simple to quite complex and, of course, a subsequent range in prices. Solid rivets might cost around $15 per 1000; the least costly aluminium blind rivets probably 10–20 times that. The diagram at left shows a simple three-piece blind rivet; the aluminium rivet (green portion) has a hollow shank assembled with a solid steel stem and a small locking collar. The stem is partly pulled up through the shank during the rivet setting process to form the internal rivet head. The outer part of the stem is then broken off at a designed position so that the retained stem is locked: (a) flush with the outer head (a filled core or Q-type rivet); or (b) only a short stem is retained within the rivet tube (a semi-filled or N-type rivet). The Bulbed CherryLock described below is a filled core rivet.

A pulled rivet, being hollow, does not have the same tensile strength of a solid rivet of the same diameter, so stronger material or larger diameter rivets — or more of them — are necessary; and of course the retained steel stem makes them perhaps 20–50% heavier than a solid aluminium rivet performing the same function. However, a filled core improves the shear strength of the aluminium rivet.

The diameter of the manufactured head of a blind rivet is usually twice the shank diameter and will have a greater surface area than the blind side head. The joint should be designed and the rivet installed so the blind side head doesn't apply its more concentrated load to the thinner of the materials being joined.

Good quality blind riveting requires precise hole drilling. Generally the shanks of the less costly pulled rivets do not swell when installed and the joint should not be considered watertight. Although the steel stems of aluminium rivets are phosphated or otherwise coated, the bare metal exposed after the stem is broken off must be considered a corrosion potential and sealed in some way.

Some manufacturers of sheet metal kit aircraft recommend extensive use of particular brands of aluminium pulled rivets and, though more costly than solid rivets, they are certainly easier to install. There is no reason not to use pulled rivets instead of solid rivets in a homebuilt aircraft as long as the type and manufacturer are carefully selected.

The following extract from AC43.13-1B describes the Bulbed CherryLock blind rivet.

CherryLock rivet "A Bulbed CherryLock consists of three parts; a rivet shell, a puller, and a locking collar. The puller or stem has five features which are activated during installation; a header, shank expanding section, locking collar indent, weak or stem fracture point, and a serrated pulling stem. Carried on the pulling stem, near the manufactured head, is the stem locking collar. When the stem reaches its preset limit of travel, the upper stem breaks away (just above the locking collar) as the locking collar snaps into the recess on the locking stem. The rough end of the retained stem in the center on the manufactured head must never be filed smooth, because it will weaken the strength of the locking collar, and the center stem could fall out."

The figure illustrates the six insertion stages.

CherryLock mechanical lock rivet

[Extract from AC43.13-1B ends]

POP rivet The POP® rivet is the brand name of a range of blind rivets originally manufactured by the United Shoe Machinery Co. [USM]. These rivets have been used in home and industrial applications for over 80 years. Sometimes the generic pop rivet is used when referring to blind rivets or the manual installation tools for blind rivets are referred to as pop riveters but, just to make it clear, POP® rivets are blind rivets but most blind rivet types are not POP® rivets.

The common POP® rivets are not suitable for use in the primary structure of an aircraft though they might be used in other areas where there are no loads requiring structural grade riveting. However, if the engine/propeller installation is of the pusher type you might want to consider the consequences of part, or all, of a POP rivet being ingested into the engine or passing through the propeller disc.

12.3 Bolted joints
There are a number of names (bolts, screws, machine screws, set screws, cap screws) for headed, external screw-threaded fasteners designed either: (a) for use with an internally threaded nut to clamp two or more parts together, or (b) to clamp one or more parts to another internally threaded metal body. Generally it can be said that fasteners tensioned by turning a threaded nut are bolts while those tensioned by turning the head are screws. However, there is a class of bolt, often referred to as 'engine bolts', where the joint is tensioned by turning the bolt head to screw it into an internally threaded metal body. Machine/cap screws are threaded for their full length and are manufactured from carbon steels; structural screws have an unthreaded grip length and are made from alloy steels. In this section we will concentrate on bolt and nut applications in airframe structures.

There are two commonly used structural bolted joint designs, one type where the high tensile strength of the bolt shank is used to clamp members together and the joint functionality relies on the surface friction between the members rather than the bolt shank; the joint will hold as long as the friction force is greater than any shear force applied. The other joint type is where the joint relies primarily on the shear strength of the bolt shank — such as seen in aluminium tubular truss structures — and there is only sufficient tensile load applied to the bolt/nut to prevent movement after locking.

Bolted joint pre-loadsTorque. If a turning force or torque is applied with a wrench to the nut of a bolt and nut pair already 'snugged up' (i.e. holding all joint interfaces in intimate contact but with little or no tension in the bolt) the under-surface of the bolt head and the inner surface of the nut (or intermediate washers if fitted) will apply a compressive force to the members, clamping them together. Depending on the stiffness of the joint members, the periphery of that compressive effect extends to around 4–5 times the diameter of the bolt shank. The greater the torque applied to the nut, the greater the tension in the bolt and the greater the compression in the members (or the crushing force applied to the member(s) and any intermediate sealing gasket). 'Hard' joints may only require the nut to be rotated through a 30° angle from the snugged position to achieve the full torque. A 'soft' gasketed joint may require a rotation of two full turns from the snugged position.

Pre-loading. Referring to the stress-strain diagram in the module 'Properties of metals' it can be seen that as long as the tensile stress in the bolt is less than the yield strength, the resulting bolt stretch (the strain) will stay within the elastic region. While that tension continues, the bolt elasticity (the potential energy) will apply the clamping force holding the joint together. This clamping force is called the pre-load or pre-tension which, for a high-stress joint (such as a propeller hub/crankshaft flange joint), might be set at 70% or more of the bolt yield strength — the position indicated by the small green cross in that stress-strain diagram.

(Because bolt threads act as stress concentrators, permanent deformation will occur at loads a little below yield strength — maybe around the 95% level. This is termed the bolt proof strength, proof stress or proof load.)

The compressive force in the members is equal to the tensile force in the bolt(s) but if the members are stiffer than the bolts, the amount of compressive movement would be less than the amount of bolt elongation.

The stretch in a pre-tensioned bolt is probably less than 0.25% of its initial length. But of course a 0.25% strain in a bolt 100 mm long is 10 times the physical stretch of a 0.25% strain in a bolt 10 mm long.

Note on turning force: only about 10–15% of the torque applied increases bolt tension; i.e. stretches it. Perhaps 40–50% of the turning force is needed to overcome the friction between the male and female threads; the balance is needed to overcome the turning friction between the under-surface of the nut and the material being clamped. Thus if some form of thread lubricant is used, the torque required to produce the same pre-load is perhaps 25% less. The cadmium plating on the bolt and nut for corrosion protection also acts as a lubricant, so the torque required is reduced.

The turning force to be applied to a nut (or the angle through which it is to be turned from the snugged position) to achieve a particular pre-load will be specified in torque charts or by the designer. If any coating, corrosion inhibiting compound/paste or lubricant is used that is not specified by the designer, then there is a very good chance that applying the specified torque will stress the bolt beyond its yield point and lead to joint failure. Also, torque wrenches may have only a plus/minus 25% accuracy.

There is more information below.

Clamped joints. Having calculated the in-service loads that will be applied to a structural joint the aircraft designer will determine the number of bolts required and their spacing plus tensile strength, physical dimensions, thread type, thread pitch, corrosion protection and then the pre-load to be applied. Most of the resistance to shear within the joint comes from the friction between the clamped surfaces of the joint members — so of course there may be quite a number of bolts within the joint.

Bolted joint pre-loadsThe diagram at left shows the forces acting within a pre-loaded joint. When there is no external tension forces the compressive force [Fc] in the joint members equals the pre-load force [Fp] in the bolt. In flight, the joint will be loaded with external tension forces [Ft] and shear forces [Fs]. The external tension forces decrease the pre-load joint compression. However, such joints are designed so that the members are quite stiff and the bolts resilient. So, a quite high external load will cause a decrease in joint load, but not to the point of separation, and only a slight increase in the tensile load on the bolt(s). Designers will generally opt for a larger number of smaller diameter bolts in a joint, rather than a smaller number of larger diameter bolts; for example, the centre joint of the left and right main wing spars for a twin-engine Piper aircraft utilises fourteen 3/8 inch bolts to join the top spar caps — with a similar arrangement for the bottom spar caps — and sixteen 3/16 inch bolts for joining the webs: 44 bolts in one joint.

External forces acting on a structural joint are generally not pure tension or pure shear; the force vector will have a tension component and a shear component. As long as the external load is somewhat less than the pre-load, a joint clamping load exists, but this ceases if those tension forces exceed the pre-load force. Then the tensile stress on the bolts will increase, the bolts elongate (still elastically) and the mating parts begin to slip, thereby reducing joint functionality and imposing all the shear forces in the joint onto the bolt shanks. The tensile stress may take the bolts past their yield point, and the combination of shear and tension will cause the bolts to bend so that even if the external load is released the joint will no longer be functional.

Pre-load and metal fatigue. Pre-loading has the effect of reducing the dimension of the fatigue cycles to which the fastener is exposed. The forces applied to the bolt from in-flight loads are generally much less than the pre-load, so the increases in bolt tension are comparatively slight thus reducing the level of cyclic stress and keeping it inside the fatigue limit.

Embedding. After some exposure to flight loads, joint surfaces tend to embed into each other (the rougher the surfaces, the greater the embedding) which has the effect of relaxing the bolt pre-load.

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12.4 Aircraft bolt standards
AN general-purpose bolt identification code. General-purpose aircraft structural bolts manufactured in accordance with the AN3–20 standards are commonly high-strength 8740 alloy steel with a minimum tensile strength around 125 000 psi; but other steel alloys are included in the specification. The standard bolts have hexagonal heads, are centreless ground and roll threaded after heat treatment, then cadmium plated, and are used in shear or tension applications. The bolt head and/or shank may have holes drilled for safetying wire or cotter pins. Aluminium bolts are also included in the specification but such bolts are unlikely to be used in a structural role in a light aircraft.

AN3–20 bolts are identified by a multi-part code:
  • firstly the AN specification identity, then
  • one or two numbers that indicate the shank diameter in 1/16 inch increments starting at AN3 (3/16") and ending at AN20 (1 1/4"), which may be followed by
  • a dash indicating the material is the standard cadmium plated 8740 or 4037 alloy steel, otherwise one or two letters for the material (e.g. 'C' indicates CRES, 'DD' is 2024 aluminium), then
  • if the hexagonal head is drilled for safetying wire, the letter 'H'
  • one or two numbers, which indicates the length of the shank from under the head to the tip in 1/8 inch increments; if two numbers, the first indicates whole inches and the second indicates the 1/8 inch increments (e.g. 23 indicates a shank length of 2 3/8" — but there may be variations from this system), then
  • the letter 'A' indicating the bolt shank is not drilled and thus intended for use with a self-locking nut (which is the norm); the letter is absent if the shank is drilled for castle nut and cotter pin locking.
For example: AN6-H7A
  • AN6 denotes the specification for general-purpose hexagonal head bolts with a 3/8" (6/16") diameter shank
  • the dash indicates the material is the standard cadmium-plated alloy steel
  • H indicates the bolt head is drilled for safetying wire
  • 7 = 7/8 inch shank length and
  • A = indicates the shank is not drilled.
Bolt threads. The standard aircraft thread is the 'unified national' form either in the fine [UNF] series or the coarse [UNC] series. Both series are based on a 60° thread. That is, if the thread is viewed in cross-section each thread forms an equilateral triangle, but with the roots and crests of the threads rounded during the rolling process to avoid sharp corners and thus to minimise stress concentrations. The coarse series have fewer threads per inch [tpi] for the same bolt diameter.

The AN3–20 bolts use only the UNF threads; the AN3 bolt has 32 tpi, the AN4 is 28 tpi, AN5 and AN6 are 24 tpi, and AN7 and AN8 are 20 tpi.

Thread length and grip. The threaded length of AN bolts is about 3/8" for AN3, 7/16" for AN4, 1/2" for AN5 and 9/16" for AN6–AN8. The grip is the shank length minus the threaded length, which for the AN6-H7A bolt would be 7/8" shank length minus 9/16" thread length = 5/16" grip. Thus an AN3-4 bolt would have a grip of only 1/8" and might, at first glance, present the appearance of a fully threaded shank.

The threaded length should not be subject to shear loads. The specification allows shank lengths to be from 1/32 to 3/32 inches longer than the nominal length.

Extract from AC 43.13-1B [with added comments]:

7-35. BOLTS. Most bolts used in aircraft structures are either (a) general-purpose, (b) internal-wrenching or (c) close-tolerance AN, NAS, or MS bolts. Design specifications are available in MIL-HDBK-5 or USAF/Navy T.O. 1-1A-8/NAVAIR 01-1A-8. References should be made to military specifications and industry design standards such as NAS, the Society of Automotive Engineers (SAE), and Aerospace Material Standards (AMS).

AC43-13.1B Figure 7.1. Typical aircraft bolt  markings 7-36. IDENTIFICATION. Aircraft bolts may be identified by code markings on the bolt heads. These markings generally denote the material of which the bolt is made, whether the bolt is a standard AN-type or a special-purpose bolt, and sometimes include the manufacturer.

a. AN standard steel bolts are marked with either a raised cross or asterisk [most of those pictured], corrosion resistant steel is marked by a single dash [row 1, number 4], and AN aluminum-alloy bolts are marked with two raised dashes [row 3, number 5].

b. Special-purpose bolts include high-strength, low-strength, and close-tolerance types. These bolts are normally inspected by magnetic particle inspection methods. Typical markings include 'SPEC' (usually heat-treated for strength and durability) [row 2, number 5] , and an aircraft manufacturer's part number stamped on the head [row 3, number 1]. Bolts with no markings are low strength. Close-tolerance NAS bolts are marked with either a raised or recessed triangle [row 3, number 4]. The material markings for NAS bolts are the same as for AN bolts, except they may be either raised or recessed. Bolts requiring non-destructive inspection (NDI) by magnetic particle inspection are identified by means of colored lacquer, or head markings of a distinctive type.

Grip length modified with washers7-37. GRIP LENGTH. In general, bolt grip lengths of a fastener is the thickness of the material the fastener is designed to hold when two or more parts are being assembled. Bolts of slightly greater grip length may be used, provided washers are placed under the nut or bolthead. The maximum combined height of washers that should be used is 1/8 inch. This limits the use of washers necessary to compensate for grip, up to the next standard grip size. All bolt installations which involve self-locking or plain nuts should have at least one thread at the end of the bolt protruding through the nut.

(Comment: only the unthreaded portion of the shank – the grip – should carry shear loads, so a maximum of one or 1.5 inner end threads are acceptable within the grip length, though the nut should not be run down to the inner end of the threaded length.)

7-38. LOCKING OR SAFETYING OF BOLTS. Lock or safety all bolts and/or nuts, except self-locking nuts. Do not reuse cotter pins or safety wire.

7-39. BOLT FIT. Bolt holes, particularly those of primary connecting elements, have close tolerances. Generally, it is permissible to use the first-lettered drill size larger than the nominal bolt diameter, except when the AN hexagon bolts are used in light-drive fit (reamed) applications and where NAS close-tolerance bolts or AN clevis bolts are used. A light-drive fit can be defined as an interference of 0.0006 inch for a 5/8 inch bolt. Bolt holes should be flush to the surface, and free of debris to provide full bearing surface for the bolt head and nut. In the event of over-sized or elongated holes in structural members, reaming or drilling the hole to accept the next larger bolt size may be permissible. Care should be taken to ensure items, such as edge distance, clearance, and structural integrity are maintained.

7-40. TORQUES. The importance of correct torque application cannot be overemphasized. Undertorque can result in unnecessary wear of nuts and bolts, as well as the parts they secure. Overtorque can cause failure of a bolt or nut from overstressing the threaded areas. Uneven or additional loads that are applied to the assembly may result in wear or premature failure. The following are a few simple, but important procedures, that should be followed to ensure that correct torque is applied.

NOTE: Be sure that the torque applied is for the size of the bolt shank not the wrench size.

a. Calibrate the torque wrench at least once a year, or immediately after it has been abused or dropped, to ensure continued accuracy.

b. Be sure the bolt and nut threads are clean and dry, unless otherwise specified by the manufacturer.

c. Run the nut down to near contact with the washer or bearing surface and check the friction drag torque required to turn the nut. Whenever possible, apply the torque to the nut and not the bolt. This will reduce rotation of the bolt in the hole and reduce wear.

d. Add the friction drag torque to the desired torque. This is referred to as 'final torque', which should register on the indicator or setting for a snap-over type torque wrench.

e. Apply a smooth even pull when applying torque pressure. If chattering or a jerking motion occurs during final torque, back off the nut and retorque.

NOTE: Many applications of bolts in aircraft/engines require stretch checks prior to reuse. This requirement is due primarily to bolt stretching caused by overtorquing.

f. When installing a castle nut, start alignment with the cotter pin hole at the minimum recommended torque plus friction drag torque.

NOTE: Do not exceed the maximum torque plus the friction drag. If the hole and nut castellation do not align, change washer or nut and try again. Exceeding the maximum recommended torque is not recommended.

g. When torque is applied to bolt heads or capscrews, apply the recommended torque

h. If special adapters are used which will change the effective length of the torque wrench, the final torque indication or wrench setting must be adjusted accordingly. Determine the torque wrench indication or setting with adapter installed as shown in figure 7-2 [not shown in this extract].

i. Table 7-1 shows the recommended torque to be used when specific torque is not supplied by the manufacturer. The table includes standard nut and bolt combinations, currently used in aviation maintenance.

AN3 - 20, hexagon head, general purpose bolt7-41. STANDARD AIRCRAFT HEX HEAD BOLTS (AN3 THROUGH AN20). These are all-purpose structural bolts used for general applications that require tension or shear loads. Steel bolts smaller than No. 10-32, and aluminum alloy bolts smaller than 1/4 inch diameter, should not be used in primary structures. Do not use aluminum bolts or nuts in applications requiring frequent removal for inspection or maintenance.

(Comment: small diameter bolts and screws are numbered 1 through 12. A No. 5 has a nominal diameter of 0.125 inch and 40 threads per inch [tpi] if coarse thread and 44 tpi if fine thread; No. 8 is 0.164 inch diameter available as No. 8-32 [coarse] and No. 8-36 [fine]; No. 10 is 0.19 inch diameter available as No. 10-24 [coarse] and No. 10-32 [fine].)

MS20074, hexagon head, drilled head, coarse thread7-42. DRILLED HEAD BOLTS (AN73 THROUGH AN81). The AN drilled head bolt is similar to the standard hex bolt, but has a deeper head which is drilled to receive safety wire. The physical differences preventing direct interchangeability are the slightly greater head height, and longer thread length of the AN73 through AN81 series. The AN73 through AN81 drilled head bolts have been superseded by MS20073, for fine thread bolts and MS20074 for coarse thread bolts. AN73, AN74, MS20073, and MS20074 bolts of like thread and grip lengths are universally, functionally, and dimensionally interchangeable.

AN173,  hex head, drilled or undrilled head and shank, close tolerance7-44. CLOSE-TOLERANCE BOLTS. Close-tolerance, hex head, machine bolts (AN173 through AN186) ... are used in applications where two parts bolted together are subject to severe load reversals and vibration. Because of the interference fit, this type of bolt may require light tapping with a mallet to set the bolt shank into the bolt hole. The shanks of close tolerance bolts are re-ground after cadmium plating.

NAS144,  internal wrenching, drilled or undrilled head or shank 7-46. INTERNAL WRENCHING BOLTS (MS20004 THROUGH MS20024) AND SIX HOLE, DRILLED SOCKET HEAD BOLTS (AN148551 THROUGH AN149350). These are high strength bolts used primarily in tension applications. The NAS144 through NAS158 and NAS172 through NAS176 are interchangeable with MS20004 through MS20024 in the same thread configuration and grip lengths. The AN148551 through AN149350 have been superseded by MS9088 through MS9094 with the exception of AN149251 through 149350, which has no superseding MS standard.

MS9088,  12 point head drilled 7-47. TWELVE POINT, EXTERNAL WRENCHING BOLTS, (NAS624 THROUGH NAS644). These bolts are used primarily in high-tensile, high-fatigue strength applications. The twelve point head, heat-resistant machine bolts (MS9033 through MS9039), and drilled twelve point head machine bolts (MS9088 through MS9094), are similar to the (NAS624 through NAS644); but are made from different steel alloys, and their shanks have larger tolerances.

AN21, clevis bolt7-50. CLEVIS BOLTS (AN21 THROUGH AN36). These bolts are only used in applications subject to shear stress, and are often used as mechanical pins in control systems. A clevis is a U-shaped fitting similar to a shackle.

AN21, eye bolt7-51. EYEBOLTS (AN42 THROUGH AN49). These bolts are used in applications where external tension loads are to be applied. The head of this bolt is specially designed for the attachment of a turnbuckle, a clevis, or a cable shackle. The threaded shank may or may not be drilled for safetying.

[Extract ends]

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12.5 Nuts, locking devices and washers
Because of the vibrations associated with aircraft the fasteners used in structural joints must be locked after torquing to ensure that the bolt and/or the nut can't loosen. In fixed airframe joints the resistance to vibration loosening is generally accomplished using self-locking nuts and the standard AN3-20 bolts, without drilled head or shank. Self-locking nuts are, most commonly, of the elastic nylon or fibre insert type (AN365). However those types cannot be used in the engine compartment or anywhere else in the aircraft where exposed to in-flight temperatures exceeding 250 F (120 C) because the material starts to lose elasticity and thus its resistance to movement. In such applications a full-metal non-circular self-locking nut (AN363) or a castellated nut (AN310) plus locking cotter pin is required; in the latter case, a drilled shank bolt is necessary.

Some types of nylon insert self-lockers are claimed to seal the bolt thread against entry of fluids.

Self-lockers can't be used where the joint is subject to any rotational movement; bolts in such circumstances are locked using non-friction locking — castellated nuts (AN310) with cotter pins or safetying wire.

When a nut is fully torqued, about 33% of the total tensile load is placed on the first (most inward) thread and the mating bolt thread. The second thread takes a further 23% of the load. The third thread takes about 14% so that the stress on the first three engaged threads of both the nut and bolt is about 70% of the total, and the first six threads take about 99% of the tensile load. This indicates it is just added weight (and possible space constriction) for a nut to be longer than 6 to 8 threads; and the same for the bolt — plus an allowance for thread start and thread run-out. It also follows that when a bolt is primarily loaded in shear, a light nut with possibly only three threads is ample for the task of just keeping the bolt in position for the unthreaded shank to carry the shear load.

When using some types of self-locking nuts it should be borne in mind that the first three outward bolt threads have been slightly tapered to facilitate running on the nut and reducing the chance of cross-threading — more below.

Extract from AC 43.13-1B [with added comments]:

7-63. GENERAL. Aircraft nuts are available in a variety of shapes, sizes, and material strengths. The types of nuts used in aircraft structures include castle nuts, shear nuts, plain nuts, light hex nuts, checknuts, wingnuts, and sheet spring nuts. Many are available in either self-locking or nonself-locking style.

7-64. SELF-LOCKING NUTS. These nuts are acceptable for use on certificated aircraft subject to the aircraft manufacturer?s recommended practice sheets or specifications. Two types of self-locking nuts are currently in use, the all-metal type, and the fiber or nylon type.
  • DO NOT use self-locking nuts on parts subject to rotation.

  • Self-locking castellated nuts with cotter pins or lockwire may be used in any system.
    [Comment: there is a well-established view that only castellated nuts with cotter pins or lockwire should be used in all control systems; i.e. no self-locking nuts.]

  • Self-locking nuts should not be used with bolts, screws, or studs to attach access panels or doors, or to assemble any parts that are routinely disassembled before, or after each flight. They may be used with anti-friction bearings and control pulleys, provided the inner race of the bearing is secured to the supporting structure by the nut and bolt.

  • Metal locknuts are constructed with either the threads in the locking insert out-of-round with the load-carrying section [deformed-thread locknuts], or with a saw-cut insert with a pinched-in thread in the locking section. The locking action of the all-metal nut depends upon the resiliency of the metal when the locking section and load-carrying section are engaged by screw threads. Metal locknuts are primarily used in high temperature areas.

  • MS20364, thin shear only lock nut showing nylon insert Fiber or nylon locknuts are constructed with an unthreaded fiber or nylon locking insert held securely in place. The fiber or nylon insert provides the locking action because it has a smaller diameter than the nut. Fiber or nylon self-locking nuts are not installed in areas where temperatures exceed 250 F [120 C]. After the nut has been tightened, make sure the bolt or stud has at least one* thread showing past the nut.

    [*Comment: because the outward three threads of a bolt have been tapered it is considered that at least two threads should show past the nut because if less the nylon locking insert may not distort [and thus lock] as much as it should.]

    DO NOT reuse a fiber or nylon locknut, if the nut cannot meet the minimum prevailing torque values.

    Comment: the identification code for AN365 locknuts consists of the AN specification identity [AN365 or MS20365], then a dash followed by three or four numbers. The last two numbers are the threads per inch and the first number is the mating bolt size e.g. AN365-428 indicates it is used with the AN4 28 tpi bolt. However the mating nut for an AN3 32 tpi bolt differs from the others by using the 3/16 inch number '10' screw designation so the part number is AN365-1032. The same coding system applies to the thin shear-only locknut version AN364 or MS20364 and to the AN363 all-metal locknuts.

  • Self-locking nut plates are produced in a variety of forms and materials for riveting or welding to aircraft structures or parts. Certain applications require the installation of self-locking nuts in channel arrangement permitting the attachment of many nuts in a row with only a few rivets.

7-65. NUT IDENTIFICATION FINISHES. Several types of finishes are used on self-locking nuts. The particular type of finish is dependent on the application and temperature requirement. The most commonly used finishes are described briefly as follows.

AN365 - MS20365, lock nut, cadmium plated with dichromate conversion coating  for added corrosion protectiona. Cadmium Plating. This is an electrolytically deposited silver-gray plating which provides exceptionally good protection against corrosion, particularly in salty atmosphere, but is not recommended in applications where the temperature exceeds 450 F / 230 C. Cadmium melts at 610 F / 320 C but when used in temperatures in excess of 450 F, the cadmium coating will diffuse into the grain boundaries of the base metal causing it to become very brittle and subject to early failure. This is known as liquid metal embrittlement.

[Comment — hydrogen embrittlement: the cadmium plating process (like most electrolytic coating processes) generates free hydrogen atoms, which can be absorbed into the surface metal. The hydrogen atom is the smallest atom and can readily diffuse through the base metal, reacting with the carbon and tending to concentrate in high stress locations. This can lead to development of minute fractures, and considerable reduction in metal ductility and strength sufficient that a nut could readily crack when torquing and even weeks after torquing. This is a form of stress corrosion cracking. To remove the hydrogen the AN and MS fastener standards specify an extended period of baking at 375 F following plating.]

[Comment: plated cadmium and zinc, being more anodic than steel, form a sacrificial coating on bolts, nuts and other fasteners. Any corrosion will decompose the plating not the base metal. When black spots or other stains appear on the cadmium or zinc it is evidence of the coating fulfilling its role and such marks should not be removed because of the likelihood of also removing more of the protective layer.]

The following additional finishes or refinements to the basic cadmium can be applied.
  • Chromic Clear Dip. Cadmium surfaces are passivated, and cyanide from the plating solution is neutralized. The protective film formed gives a bright, shiny appearance, and resists staining and finger marks.

  • Olive Drab Dichromate. Cadmium-plated work is dipped in a solution of chromic acid, nitric acid, acetic acid, and a dye which produces additional corrosion resistance.

  • Iridescent Dichromate. Cadmium-plated work is dipped in a solution of sodium dichromate and takes on a surface film of basic chromium chromate which resists corrosion. Finish is yellow to brown in color.

b. Silver plating. Silver plating is applied to locknuts for use at higher temperatures. Important advantages are its resistance to extreme heat (1,400 F) and its excellent lubricating characteristics. Silver resists galling and seizing of mating parts when subjected to heat or heavy pressure.

c. Anodizing for aluminum. An inorganic oxide coating is formed on the metal by connecting the metals and anodes in a suitable electrolyte. The coating offers excellent corrosion resistance and can be dyed in a number of colors.

d. Solid Lubricant Coating. Locknuts are also furnished with molybdenum disulfide for lubrication purposes. It provides a clean, dry, permanently-bonded coating to prevent seizing and galling of threads. Molybdenum disulfide is applied to both cadmium and silver-plated parts. Other types of finishes are available, but the finishes described in this chapter are the most widely used.

AN310, castle nut7-66. CASTLE NUT (AN310). The castle nut is used with drilled shank hex head bolts, clevis bolts, drilled head bolts, or studs that are subjected to tension loads. The nut has slots or castellations cut to accommodate a cotter pin or safety wire as a means of safetying.

[Comment: the dash number of the AN310 nut indicates its mating bolt, i.e. AN310-4 is mated with an AN4 bolt, AN310-6 with an AN6 bolt. The same system applies with the light shear only nut AN320.]

AN320, castellated shear nut7-67. CASTELLATED SHEAR NUT (AN320). The castellated shear nut is designed for use with hardware subjected to shear stress only. It has fewer threads and is weaker than the AN310 nut.

AN335, plain nut7-68. PLAIN HEX NUT (AN315 [fine thread] AND AN335 [coarse thread]). The plain nut is capable of withstanding large tension loads; however, it requires an auxiliary locking device, such as a checknut, lockwasher or safety wire. Use of this type on aircraft structures is limited.

AN345, plain nut7-69. LIGHT HEX NUTS (AN340 [fine thread] AND AN345 [coarse thread]). These nuts are used in nonstructural applications requiring light tension. Like the AN315 and AN335, they require a locking device to secure them.

AN316, check nut7-70. CHECKNUT (AN316). The double chamfered checknut [or jam nut] is used as a locking device for plain nuts, screws, threaded rod ends, and other devices.

AN350, wing nut7-71. WINGNUTS (AN350). The wingnut is used where the desired torque is obtained by use of the fingers or handtools and where the object is frequently removed. Wingnuts are normally drilled to allow safetying with safety wire.


7-85. GENERAL. The type of washers used in aircraft structure are plain washers and special washers.

7-86. PLAIN WASHERS (AN960 AND AN970). Plain washers are widely used with hex nuts to provide a smooth bearing surface, act as a shim to obtain the proper grip length, and to position castellated nuts in relation to drilled cotter pin holes in bolts. Use plain washers under lock washers to prevent damage to bearing surfaces. Cadmium-plated steel washers are recommended for use under boltheads and nuts used on aluminum alloy or magnesium structures to prevent corrosion. The AN970 steel washer provides a larger bearing surface than the plain type, and is often used in wooden structures under bolt heads and nuts to prevent local crushing of the surface.

Comment: the identification code for AN960 washers consists of the AN specification identity [AN960], then a dash followed by three digits giving the hole diameter in 1/16 inches. e.g. AN960-516 indicates a hole diameter to fit a 5/16 inch diameter AN5 bolt. However the mating washer for an AN3 bolt differs from the others by using the number '10' screw designation so the part number is AN960-10. There is a half standard thickness version of the AN960 series washers which is used only for bolt grip adjustment purposes and usually with castle nuts. The same coding system applies to the thin washer versions but is suffixed with the letter L presumably for 'light', e.g. AN960-516L.

The identification code for the large diameter AN970 washers consists of the AN specification identity [AN970], then a dash followed by one or two digits indicating the AN bolt the washer is associated with. e.g. AN970-5 indicates a hole diameter to fit an AN5 bolt, the washer for an AN3 bolt is AN970-3 and that for an AN10 bolt is AN970-10.

7-87. LOCKWASHERS (AN935 AND AN936). Lock washers may be used with machine screws or bolts whenever the self-locking or castellated type nut is not applicable. Do not use lock washers where frequent removal is required, in areas subject to corrosion, or in areas exposed to airflow. Use a plain washer between the lock washer and material to prevent gouging the surface of the metal.

CAUTION: Lock washers are not to be used on primary structures, secondary structures, or accessories where failure might result in damage or danger to aircraft or personnel.

7-88. BALL SOCKET AND SEAT WASHERS (AN950 AND AN955). Ball socket and seat washers are used in special applications where the bolt is installed at an angle to the surface or when perfect alignment with the surface is required. These washers are used together as a pair.

7-89. TAPER PIN WASHERS (AN975). Taper pin washers are used with the threaded taper pin. NAS143 and MS20002 washers are used with NAS internal wrenching bolts and internal wrenching nuts. They may be plain or countersunk. The countersunk washer (designated as NAS143C and MS20002C) is used to seat the bolthead shank radius, and the plain washer is used under the nut.

(Extract ends)

In some circumstances where a number of bolts are aligned to form a joint it is expedient to use an additional rigid bearing plate to the joint — rather than a number of washers — which acts as a doubler and distributes the compressive forces more evenly.

12.6 Cable terminals
Generally hardware such as fork terminals (AN667), eye terminals (AN668) and thimble eye splices at cable ends or splicing sleeves in the bight of a cable are permanently attached to the cable by swaging. The swaging technique used for cable fittings is a cold plastic deformation, by mechanical pressure, of steel tubes or perhaps copper sleeves so that the outside and inside diameters are reduced and the length increased. During deformation the cable is held in place within the fitting and mechanically bonded to it; standard swaged terminals develop the full cable strength. (Flaring tube ends for fluid connectors is another form of swaging).

There is quite a lot of information on swaged fittings in section 8 of Chapter 7 'Aircraft hardware, control cables, and turnbuckles' of AC 43.13-1B on this website for download in PDF format (2.8 MB). Section 8 commences at page 27.

12.7 Mechanical joints in wood structures
The following section is an extract as is from chapter 4 of 'ANC–18 Design of wood aircraft structures' second edition issued June 1951 by the Subcommittee on Air Force–Navy–Civil Aircraft Design Criteria of the Munitions Board Aircraft Committee (US).

4.70. GENERAL. Mechanical joints in wood are usually limited to types employing aircraft bolts. Since bolts in wood can carry a much higher load parallel to the grain of the wood than across the grain, it is generally advantageous to design a fitting and its mating wood parts so that the loads on the bolts are parallel to the grain. The use of a pair of bolts on the same grain line, carrying loads perpendicular to the grain and oppositely directed, is likely to increase the tendency to split. When a long row of bolts is used to join two parts of a structure, consideration should be given to the relative deformation of the parts, as explained in section 4.82.

4.71. USE OF BUSHINGS. Bushings are often used in wood to provide additional bearing area and to prevent crushing of the wood when bolts are tightened. See figure 4-36. When bolts of large length/diameter ratio are used, or when bolts are used through a member having high density plates on the faces, plug bushings may be used to advantage. Figure 4.36 Types of bushings
4.72. USE OF HIGH DENSITY MATERIAL. Wherever highly concentrated loads are introduced, greater bearing strength can be obtained by scarfing-in high-density material (section 4.63). Some high density materials are quite sensitive to stress concentrations and the possibility of the serious effects of such stress concentrations should be considered when large loads must be carried through the high-density material.

Wherever metal fittings are attached to wood members, it is generally advisable to reinforce the wood against crushing by the use of high-density bearing plates (figure 4-37) and to use a coat of bitumastic or similar material between the wood and metal to guard against corrosion. Cross banding of these plates will help to prevent splitting of the solid wood member. Figure 4.37 Typical wing beam attachment

4.73. MECHANICAL ATTACHMENT OF RIBS. When ribs carry heavy or concentrated loads it is sometimes desirable to insure their attachment by use of mechanical fastenings (see figure 4-39). Figure 4.39 Mechanical attachment of ribs

4.74. ATTACHMENT OF VARIOUS TYPES OF FITTINGS. Fittings should always have wide base plates to prevent crushing at edges. Wood washers have a tendency to cone under tightening loads. Where possible, it is desirable to use washer plates for bolt groups, as illustrated in figure 4-40, but if washers are used, a special type for wood, AN-970 or equivalent, are necessary to provide sufficient bearing area. Figure 4.40 Hinge fittings

Figure 4.41 Installation of clamp fittings Clamps around wood members should be constructed so that they can be tightened symmetrically (figure 4-41).

4.75. USE OF WOOD SCREWS, RIVETS, NAILS, AND SELF-LOCKING NUTS. Wood screws and rivets are sometimes used for the attachment of secondary structure but should not be used in connecting primary members. Wood screws have been successfully used to prevent cleavage of plywood skin from stringers in some skin-stringer applications. Nails should never be used in aircraft to carry structural loads.

Self-locking nuts of approved types designed for use with wood and plywood structures are preferable to plate or anchor nuts. When the latter type is used, however, attachment may be made to the structure with wood screws or rivets provided that care is taken not to reduce the strength of load-carrying members. Rivetting through wood is always questionable because of the danger of crushing the rivet heads and the possibility of bending the shank while bucking the rivet. Also, there is no way of tightening the joint when dimensional changes from shrinkage occur.

[Extract from ANC-18 ends]

The next module in this metals and hardware group is fastener safetying.

Builders guide to aircraft materials – metals and hardware modules

| Guide contents | Properties of metals | Metal corrosion | Hardware fittings in aircraft structures |

| [AN, MS hardware — rivets, bolts and locking devices] | Safetying |

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