Screws and Bolts

Screws and bolts are commonly used as structural and non-structural fastening materials in aerospace, military and a wide range of industrial applications. Although quite often these terms are used interchangeably, screws differ from bolts as they are generally made of lower strength materials.Screws can be installed with a loose fitting thread, and the head shapes are made to engage a screwdriver or wrench. Some screws have a clearly defined grip (unthreaded portion) while others are threaded along their entire length (fully threaded). Bolts are generally used in high strength demanding applications. Where this strength is not required, the use of screws remains an option.

Like other fasteners, screws and bolts comform to different specifications/standards (aerospace/military or industrial) and are ususally structured in families (series) based on their conforming standard. Respective letter and number designators are used in their part numbers to denote their conforming standard and series, their material & finish and other characteristics.

For screws and bolts used in military and aerospace applications common standards include those of Army & Navy (AN), superseded by Military Standards (Military Specs, MS), established by the United States Department of Defense (DoD), which in turn have been converted to National Aerospace Standards (NAS) by tthe U.S. Aerospace Industries Association (AIA). Despite the conversions, quite often MS and AN part numbers of these fasteners are still used for their procurement. They are structured in different series which determine specifications of these fasteners. Depending on the series, NAS screws can be furnished in different materials and finishes. 

Other international & national Standards, such as ISO, European Standards (EN), the American National Standards Institute, the American Society for Testing and Materials (ASTM), German DIN, British BSI, or those adopted by major airframe builders (such as Boeing (BAC) or Airbus (ASNA) are also used as part of standardisation efforts of these fasteners from governments and the industry worldwide.

An extensive collection of screws, bolts, and other fasteners complying with the above standards is already listed in our Web Platform for Supplies. Sourced from quality manufacturers, these fasteners are offered in different materials, finishes, and sizes in Imperial (inch) or Metric (mm) measurement systems. They can be furnished either as OEM parts of their respective manufacturers or as parts of our own labelled (AVIATIONEU NEW ERA) product lines. Usually sold in pieces but also in packs for convenience and economy, where the number of fasteners per pack is indicated after the letters "PK" in the product SKU. Appearing prices are always for the used unit of measurement each time. As these fasteners come in countless combinations of conforming standards, types, materials, finishes, and inch or metric sizes, some parts may be missing. In this case please contact us as these may be also available through our network of co-operating suppliers. 

An overview of screws and bolts is provided below with emphasis on those used in Aerospace conforming to the U.S. AN, MS and NAS Standards. It is intended as a short guide, helping a better understanding of their potential buyers on their characteristics and their suitability of of different screws and bolts for their planned applications.

Screws

Screws are classified in three categories outlined below. A fourth category, drive screws, are considered to be nails rather than screws as they are driven into metal parts with a mallet or hammer and their heads are not slotted or recessed.
(1) Structural screws, made of alloy steel, which have the same strength as equal size bolts; 
(2) Machine screws, general purpose screws offered in low carbon steel, brass, corrosion resistant steel, and aluminum alloys. They can be either coarse-threaded or fine-threaded, and are usually stronger than many other screw types, and 
(3) Self-tapping screws, used for attaching lighter parts.

Structural Screws

Aerospace structural screws are made of alloy steel, are properly heat treated, and can be used as structural bolts. They have a definite grip and the same shear strength as a bolt of the same size. Shank tolerances are similar to AN hex head bolts, and their threads are National Fine. Structural screws are available with round, brazier, or countersunk heads. The recessed head screws are driven by either a Phillips or a Reed and Prince screwdriver.

These screws are found in the NAS 204 through NAS 235 and AN 509 and AN 525 series. The AN509 (100°) flat head screws are used in countersunk holes where a flush surface is necessary. The AN 525 washer head structural screws are selected where raised heads are not objectionable. The washer head screws provide a large contact area.

Machine Screws

Machine screws are general purpose screws offered in different types, head styles, materials and finishes, and sizes (nominal diameters, thread sizes, grip lengths) in Inch or Metric systems. They are typically used for fastening metal parts and panels securely together in a range of construction and other applications including aerospace, machinery, vehicles, engines, tool assemblies, electronic devices, and large-scale industrial equipment. 

In terms of their characteristics, machine screws:

• are usually stronger than many other screw types. They are manufactured to high standards using refined techniques, providing better all-round quality, precision, and thread uniformity,
• typically feature finer, more accurate threads than alternative fastener types and the majority of them are fully threaded,
• tend to be somewhat smaller (both in length and diameter) than many other fasteners,
• almost always are blunt-ended (flat-tipped),
• offered in different matrials including steel, CRES (stainless steel), and brass alloys.

Although their most common head styles are flat head (countersunk), roundhead, or washer head, they also come in other head styles to serve intended application requirements. 

Hex head machine screws  (sometimes called set screws) have a solid, six-sided hexagon head and they are usually the type most closely resembling traditional bolts. Installed with the use of a standard wrench or spanner for extra torque. They may also have a recessed drive socket in the head for use with a typical screw driving tool.
Flat head machine screws, selected for applications where the installed fastener needs to be flush with the surface it is being driven into. Countersunk screws allow the head of the screw to sit flush with or below the surface of the surrounding material. A flattened top and countersunk underside profile enables them to offer a neat and flush surface finish on joined panels and components.They may also be used to remove the burr left from a drilling or tapping operation thereby improving the finish of the product and removing any hazardous sharp edges.
Oval head machine screws consist a middle way solution between the traditional, industrial pan head (round head) machine screws, and their flat head versions. They are less noticeable due to a curved underside creating a slightly countersunk profile but not the one provided by their  flat head counterparts.featuring a single straight socket running across the head of the machine screw, for fastening with a flathead screwdriver.
Cheese head machine screws are similar to traditional round head screws when viewed from above, but in profile, the flat-topped head is cylindrical and has a notable amount of depth to it. This type is often chosen for extra strength and durability.
Machine screw drive types (socket types) such as Slot, Cross (Phillips), Hex or Torx. Slot screws feature a single straight socket running across the head of the machine screw, for fastening with a flathead screwdriver. Cross (Phillips) drive machine screws have an X-shaped socket in the head, providing slightly more potential torque than a standard slot drive. Hex socket screws with hexagonal recessed drive sockets in the head of the machine screw usually indicate that they are to be driven with a hex driver or Allen wrench. In hexalobular recess (often called Torx or star drive), the socket takes the shape of a six-pointed star, and can only effectively be driven with a matching star drive or Torx tool.

Machine screws used in Aerospace applications are usually of the flathead (countersunk), roundhead, or washer head styles. They are general purpose screws available in low carbon steel, brass, corrosion resistant steel, and aluminum alloys. Common screws of this category include NAS machine screws included in different series, such as NAS  220, 229, 333,  514,  517, 560, 583, 584, 600, 623, 662, 860, 870, 1101, 1102, 1189, 1190, 1191, 1581, 1630, 1635, 1801, and 1802. 

AN Standard machine screws are also used and listed below.

Round head screws, coarse threaded AN 515 and AN 520 fine threaded machine screws with either slotted or recessed heads. 

Countersunk machine screws are listed as AN505 and AN510 for 82°, and AN507 for 100°. The AN505 and AN510 correspond to the AN515 and AN520 roundhead in material and usage.

Fillister head screws included in AN 500 through AN 503 series, used as a capscrew in light mechanisms, including attachments of cast aluminum parts such as gearbox cover plates.Their AN 502 and AN 503 counterparts are made of heat treated alloy steel, have a small grip, and are available in fine and coarse threads. These screws are used as capscrews where great strength is required. The coarse threaded screws are commonly used as capscrews in tapped aluminum alloy and magnesium castings because of the softness of the metal.

AN 500 coarse threaded and AN 501 fine threaded series screws offered in low carbon steel, corrosion resistant steel, and brass alloys. Screws above No. 6 size have a hole drilled through the head for safetying purposes.

Self-tapping Screws

Aerospace machine self-tapping screws are listed as AN 504 and AN 506 parts. The AN504 screw has a roundhead, and the AN506 a 82° countersunk head. These screws are used for attaching removable parts, such as nameplates, to castings and parts in which the screw cuts its own threads.

AN 530 and AN 531 self-tapping sheet metal screws are blunt on the end and used in the temporary attachment of metal for riveting, and in the permanent assembly of non-structural assemblies. Self-tapping screws should not be used to replace standard screws, nuts, bolts, or rivets.

AN 535 drive screws are plain head self-tapping screws used as capscrews for attaching nameplates in castings and for sealing drain holes in corrosion proofing tubular structures. They are not intended to be removed after installation.

Bolts

Bolts are fasteners used in securing materials across various engineering applications, from large-scale construction to small mechanical assemblies. They come in different head styles:
Hex Head: the most widely used bolt head, offering a large surface for applying torque. This head style provides solid clamping force, making it suitable for applications requiring high-strength connections.
Countersunk (or Flat) Head: designed to sit flush with the material, often used in countersunk holes. Countersunk heads distribute clamping forces evenly across the material’s surface, reducing stress concentrations at the point of contact. This is particularly beneficial for applications where smooth surfaces and secure fastening are crucial, such as in structural or aesthetic installations.
Round Head: A dome-shaped head often used for aesthetic purposes or in applications where a low-profile finish is not needed. While round heads are less effective at distributing clamping forces compared to hex or countersunk heads, they provide sufficient strength for low to moderate stress applications.

Different drive types (tool interface on the bolt head) are used for bolt tightening or loosening. Hex Drive is the most common drive type, typically used with a wrench or socket, Phillips Drive, a cross-shaped drive used with a Phillips screwdriver, or Torx Drive, a star-shaped drive that allows for better torque transfer without stripping the bolt. The choice of drive type and head style affects both the ease of installation and the final appearance of the assembly.

Aerospace Bolts

Aerospace bolts are used in applications where high strength is needed. Where this strength is not necessary respective screws can be used. Aircraft quality bolts are made from different materials such as steel, corrosion resistant (stainless) steel, aluminum or titanium alloys, with the first two being the most common in aerospace applications. Aircraft bolts have a marking on their head varying according to the manufacturer. 

Each bolt typically has a hexagon shaped head and a shank that fits into the hole. Different terms are used to designate the portions (parts) of a bolt. The threaded portion on the end of the bolt is termed as shank and the unthreaded portion of the shank as grip. The diameter of a bolt is the width of the grip. Bolts are offered with drilled shank for cotter pin or undrilled shank for stop nut application, and with or without drilled head for safety wire. 

AN / NAS Bolts

Both NAS (National Aerospace Standard) and AN (Army-Navy Standard) bolts are widely used in aerospace applications but they exhibit worth considering differences. The first (NAS) are high-strength, precision-engineered fasteners typically rated around 160,000 psi (tensile), designed for critical load-bearing applications in hgh-technology, high-load, and critical airframe structures. The second (AN) are general-purpose, high-strength aerospace bolts rated around 125,000 psi, designed for general-purpose lower-stress applications. NAS bolts often feature a "cupped" or recessed head, while AN bolts typically have a hexagon head with a raised dash or "X" identification marking. Both NAS & AN bolts use specific head markings to identify materials (e.g., steel, aluminum, corrosion-resistant). In terms of interchangeability, some NAS bolts can replace AN bolts, but the reverse is not often possible, especially in close-tolerance applications. 

MS (Military Standard) bolts often fill the gap between AN and NAS in terms of, tolerances and strength, and are also common in modern aerospace applications.

The most significant AN / NAS bolt differences are outlined below:
Strength: NAS bolts are generally stronger, often offering superior shear and tensile strength compared to AN bolts.
Precision/Tolerance: NAS bolts have much closer manufacturing tolerances than AN bolts. Close tolerance bolts are machined more accurately than general purpose bolts and they are used in applications requiring a very tight fit. Close tolerance bolts can be either AN or NAS and typically have a head marking consisting of a raised or recessed triangle.
Cost: NAS bolts are generally more expensive due to their high-precision manufacturing.

AN (Army-Navy Standard) hex head machine bolts commonly used in aircraft construction are manufactured to the US Army-Navy (AN) Standards, predecessor to Mil-Spec (MS), which is superseded by the AIA National Aerospace Standards (NAS) and come in different series. These bolts conform to Military Specification AN3 through AN20 (Rev. 12) which is canceled and superseded by NASM 3 through 20. This specification covers aircraft machine bolts part numbers, AN 3, AN 4, AN 5, AN 6, AN 7, AN 8, AN 9, AN 10, AN 14, AN 16, AN 17, AN 18, and AN 20 now superseded by their NASM 3 through NASM 20 counterparts.The Part Identification Number (PIN) for AN 3 through AN 20 Rev. 12 and the part number for NASM 3 through NASM 20 are identical. The part numbers have not been changed.

AN / NASM hex bolts are made from high-strength type 4037 or 8740 alloy steel or 431 Stainless Steel (CRES) and cadmium plated per specification QQ-P-416A, Type II, Class 3 for superior corrosion resistance compared to commercial grade zinc plated bolts. They can be furnished with shank drilled for cotter pin or undrilled for stop nut application, and with a drilled head for safety wire or without. Minimum Tensile Strength (Steel and CRES) - 125,000psi. The length of AN aircraft bolts is measured from under the head to the end of the shank. The "grip" is the unthreaded portion of the shank. Threads are pressed into the steel, not cut, thus maintaining strength throughout the entire shank. The surface under the head of AN bolts is designed to be a bearing surface to reduce non-uniform stress as the bolt is torqued thus reducing premature fatigue. NAS bolts have a higher tensile strength (usually about 160,000 psi) and can be identified by a cupped out head.

The material, size, and other characteristics of a bolt are identified by its AN part number. A breakdown of a typical AN bolt part number is provided below for AN4-8A.

AN: AN Standard designator
4:  identifies the diameter of the bolt shank in 1/16" increments
8: identifies the length of the shank (*) in 1/8" increments
A: the shank of the bolt is undrilled (no letter here means a drilled shank) (**)
(*) The length of AN aircraft bolts is measured from under the head to the end of the shank. 
(**) Bolts with undrilled shank are specified by letter "A" after the dash number. 
"C" letter code is used for bolts made of 431 Stainless Steel (CRES)
"H" after the AN number indicates bolts with drilled head (eg. AN4H-15)

Clevis bolts are special-purpose bolts with a domed head, often slotted or recessed for easy handling. These bolts have been designed for applications requiring shear loads only. By design they feature a short threaded portion only and a notch for increased shear strength. Their diameter is specified in 1/16 inch increments, while their length is critical and also measured in 1/16 inch increments with a dash number. A longer grip length increases shear strength and allows for freer rotation in the hole. Commonly used with forked-end cable terminals and secured with a shear self-locking nut. Alternatively, clevis bolts are safetied with a cotter pin if they include a drilled hole. AN (Army-Navy Standard) Clevis bolts are manufactured from nickel steel alloy, SAE 2330 and cadmium plated in same manner as AN3 - AN8 aircraft bolts.They are special-purpose bolts with a a slotted brazier type head and used where a large shearing stress occurs (never in tension). Minimum tensile strength 125,000 PSI. They are available with drilled or undrilled shank (designated by the letter "A" after the dash number in its part cod for undrilled shank). For the AN12-AN23 bolts the second digit denotes the fine thread machine screw size (e.g. for AN21 the size is 6-40, for AN22 8-36 and for AN23 10-32). For the AN24-AN36 bolts the second digit denotes the diameter in 1/16 inch increments (AN25 has a diameter of 5/16 inch).

NAS (National Aerospace Standard) bolts are high-strength, close-tolerance fasteners typically used in aerospace but also in high-performance racing and other applications. These bolts are characterized by their thin, dimpled heads,  fine-threads, and their alloy steel or stainless steel construction. Their uses include among others, critical structural joints, such as suspension and chassis fixtures, or other applications where high tensile and shear loads require precision, high-strength hardware. Some of their key features are outlined below:

Key Specifications:
Tensile Strength: 160,000 psi (160 ksi).
Shear Strength: 95,000 psi (95 ksi).
Material: Usually Alloy Steel (per AMS 5731/5737) or A-286 stainless steel.
Finish: Cadmium plated (QQ-P-416, Type II, Class 2) or passivated.
Threads: Fine-threaded (UNF).
Grip Lengths: Available in 1/16” increments.
Head Type: Hex head, usually with a thin, dimpled design. 

Common series of these bolts include NAS 6600 and 6200 Series superseding their respective NAS 1300 & NAS 1100 counterparts.NAS 1300/6600 Series are high-tensile, cadmium-plated hexagonal bolts while NAS 1100/6200 Series are bolts with similar strength to 1300 series but with shorter thread lengths. Typical diameter ranges include 3/16” (NAS6203) to 3/8” (NAS6206), though larger sizes are available. Their diameter coding is usually through the last two digits of the NAS number which indicate the diameter in 16ths of an inch (e.g. 04 corresponding to 4/16" or 1/4" diameter). 

NAS family of bolts (NAS 6603 through NAS 6610 series) includes hex head, close tolerance, alloy steel, long thread, self-locking & non-locking bolts. Their shearing feature is used to minimize damage to more critical components within the application. They are made from alloy steel with cadmium or chromium finishes and offered in different grips and lengths in inches with undrilled or drilled head and/or shank. Used in applications where a premium-quality, high strength bolt is necessary. The following letters after the basic part numbers specify the material, finish and drilling condition of these bolts.
No Finish Code for cadmium plated bolts
"C" for chromium plated bolts
"D" for drilled shank bolts
"L" for self-locking bolts
"P" for self-locking bolt with patch type locking element
"H" for bolts with drilled head
More than one code letters may be used in sequence.

NAS family of bolts (NAS 6203 through NAS 6220 series, superseding their NAS 11(xx) counterparts) are tension hexagon head high strength close tolerance short-thread alloy steel bolts with a minimum tensile strength of 160,000 psi and a minimum shear strength of 95,000 psi.They are made from different steel alloys such as 4140 (UNS G41400) or  4340 (UNS G43406) with either Cadmium Plate  Type II, Class 2 or Chromium Plate on shank only with all other surfaces cadmium plated. Depending on the series they come in different nominal diameters in inches (e.g. NAS 6203 0.1875" (3/16"), NAS 6204 0.25" (1/4"), NAS 6205  0.3125" (5/16"), NAS 6206 0.375" (3/8"), NAS 6208 0.500" 1/2") or NAS 6210 0.625" (5/8") grips (in 1/16” increments beginning at 1/16”), thread and overall fastener lengths. They can be supplied undrilled or with drilled head and/or thread and with or without locking elements (nylon or equivalent).The following letters after the basic part numbers specify the material, finish, drilling condition, and self-locking capability of these bolts.
No Finish Code for Cadmium II plated bolts
"C" for Chromium plated bolts (shank only)
"L" for Self-locking bolts
"P" for Self-locking bolts with patch type locking element only
Grip Dash Number indicating grip in .0625 increments
"D" after the grip dash number for drilled thread bolts
"H" after the grip dash number for bolts with drilled head
More than one code letters may be used in sequence.

PN Examples:
NAS6204-10 - NAS 6204 Series Bolt, .2500-28 Thread, .625 Grip, Undrilled, Non-locking, Alloy Steel Cadmium II Plated.
NAS6204-10D - Same bolt as above with Drilled Thread.

For additional information please refer to the respective product presentations at our Web Platform for Supplies (shop.aviationeu.supplies).