Every bolt, nut, and screw installed on a certificated aircraft must meet exact dimensional and material standards. The world of aircraft hardware identification is built on thread systems, specification codes, and marking conventions that allow a technician to select, inspect, and replace fasteners with complete confidence. A misidentified fastener — even one that looks right — can introduce a fatigue crack, strip a threaded hole, or fail under load. Understanding thread systems and standards is therefore not just a knowledge-test topic; it is a fundamental airworthiness skill.
This article walks through the major thread systems used in aviation, how to read a bolt specification, the grip-length and dash-number system, identification markings on fastener heads, and the most commonly tested facts from the FAA Aviation Maintenance Technician Handbook — General (FAA-H-8083-30).
Thread Systems Used in Aviation
Two primary thread systems appear on aircraft: the Unified National Thread System (the American standard, abbreviated UN) and the National Aerospace Standard (NAS) system, along with older Military Standards (MS) that are still widely found on legacy aircraft. Both systems measure thread pitch differently depending on whether the fastener uses coarse or fine threads.
Unified National Coarse and Fine
The Unified National Thread system has two main series relevant to aircraft. UNC (Unified National Coarse) has fewer threads per inch and is used where quick assembly and resistance to stripping in softer materials is needed. UNF (Unified National Fine) has more threads per inch, offering greater strength and better resistance to loosening under vibration — which is why fine threads are overwhelmingly preferred in airframe applications. A third series, UNEF (Unified National Extra Fine), appears in very thin-walled components and precision parts.
Thread pitch in the Unified system is expressed as the number of threads per inch (TPI). For example, a 1/4-28 UNF fastener has a nominal diameter of 1/4 inch and 28 threads per inch. In contrast, a 1/4-20 UNC fastener has the same diameter but only 20 threads per inch, making the two incompatible — a critical point for the test and for the shop floor.
Metric Threads (ISO)
Metric threads appear on aircraft manufactured outside the United States and on many modern domestic aircraft. The ISO metric system expresses pitch as the distance between threads in millimeters rather than as threads per inch. An M8 × 1.25 bolt, for instance, has an 8 mm nominal diameter and a 1.25 mm thread pitch. Metric and inch-system fasteners must never be mixed, since the thread form angles and pitches are incompatible even when a metric fastener may physically start to engage an inch-system hole.
Bolt Identification and the AN/MS/NAS System
Most aircraft structural bolts are specified under the Air Force-Navy (AN), Military Standard (MS), or National Aerospace Standard (NAS) numbering systems. The part number encodes the fastener's type, diameter, length, and material in a compact code.
Reading an AN Bolt Part Number
Consider the part number AN3DD5A. Breaking it down:
- AN — Air Force-Navy standard specification.
- 3 — Nominal diameter in 1/16-inch increments; here, 3/16 inch.
- DD — Material code: 2024-T4 aluminum alloy. A single D indicates 2017-T aluminum; no letter indicates cadmium-plated alloy steel; C indicates corrosion-resistant steel (CRES/stainless); B indicates bronze.
- 5 — Grip length in 1/8-inch increments; here, 5/8 inch.
- A — A suffix A means the bolt is undrilled (no cotter-pin hole). The absence of this suffix indicates the bolt is drilled for a cotter pin.
Grip length is the unthreaded shank portion — the part that actually bears the shear load inside a structural joint. Selecting the correct grip length is as important as selecting the correct diameter; a grip that is too short leaves threads inside the joint, creating a stress concentration and a weaker connection.
NAS and MS Fasteners
NAS fasteners are generally close-tolerance, high-strength bolts machined to tighter dimensional limits than standard AN bolts. They are used in highly stressed or close-fit holes where AN bolts would have too much play. MS fasteners cover a broader family including bolts, screws, nuts, and pins, often with additional specifications for material coating and heat treatment. When a maintenance manual calls for a specific NAS or MS part number, no substitution is permitted without engineering approval.
Thread Classes — Fit and Tolerance
Within the Unified system, threads are assigned a class number that defines the tolerance and allowance (clearance) between mating threads. Classes 1, 2, and 3 apply to external (bolt) threads; classes 1B, 2B, and 3B apply to internal (nut) threads.
- Class 1A/1B — Loose fit; easiest to assemble, but more play. Used where rapid assembly is more important than precision.
- Class 2A/2B — General-purpose fit; the most common in commercial and industrial applications.
- Class 3A/3B — Tight fit; closest tolerance, used in high-precision or high-strength aircraft applications where no slop is acceptable.
For the knowledge test, remember that Class 3 is the tightest and most precise aircraft-quality thread, while Class 1 is the loosest.
Identifying Bolts by Head Markings
The head of an aircraft bolt is stamped with markings that identify its specification and material. These markings allow a technician to verify a fastener visually without referring to a parts list.
- Plain head (no marks) — Unrated or commercial bolt; not acceptable for structural use on certificated aircraft.
- Single raised dash — AN standard bolt, cadmium-plated alloy steel, meets specification.
- Two raised dashes or cross — Indicates a corrosion-resistant steel (stainless) AN bolt.
- Triangle or other raised mark — NAS or MS designations; specifics vary by standard, so always cross-reference the part number with the applicable standard.
- Raised letters or numerals — May indicate manufacturer identification or material grade on certain MS/NAS fasteners.
Aluminum alloy bolts are typically identified by the absence of a head marking combined with the aluminum appearance and the DD or D in the part number. Because aluminum and steel bolts can look similar after painting or plating, always verify by part number.
Nuts, Washers, and Internal Threads
Self-locking nuts are standard on aircraft because vibration will back off a plain nut. The two main families are all-metal locking nuts (which use a distorted thread or crimped top to create friction) and fiber-insert (elastic stop) nuts, such as the AN365, which use a nylon collar. Fiber-insert nuts lose their locking ability if exposed to temperatures above approximately 250 °F (121 °C) and must never be reused once removed if they no longer provide resistance. All-metal locking nuts can tolerate higher temperatures and may be reused if they still provide the required prevailing torque.
Plain washers (AN960) distribute load and protect the surface. Lock washers are not approved for use on aircraft structures — only flat and shakeproof washers in specific applications. Shakeproof (star) washers may be used in certain electrical bonding applications but never as a substitute for a self-locking nut on a structural joint.
Key Numbers and Rules
- AN bolt diameter is encoded in 1/16-inch increments (e.g., AN4 = 4/16 = 1/4 inch diameter).
- AN bolt grip length is encoded in 1/8-inch increments (e.g., dash-5 = 5/8 inch grip).
- UNF (fine thread) is preferred in aircraft for vibration resistance.
- Class 3 thread fit is the tightest tolerance; Class 1 is the loosest.
- Metric and unified inch threads are never interchangeable.
- Fiber-insert self-locking nuts are limited to approximately 250 °F service temperatures.
- Grip length must span the full material thickness — threads must not be in the shear plane of a structural joint.
- A plain, unmarked head bolt is not approved for structural aircraft use.
Common Test Traps
- Confusing TPI with pitch: Unified threads are measured in threads per inch (higher number = finer thread), while metric threads use pitch in millimeters (smaller number = finer thread). The test may present both and ask you to identify which is finer.
- Grip length vs. overall length: The dash number on an AN bolt specifies grip length, not total bolt length. Students often confuse these, leading to a bolt whose threads enter the shear plane.
- Material codes: DD means 2024-T4 aluminum; D means 2017-T aluminum; no letter means cadmium-plated steel. The test frequently asks you to identify the material from the part number alone.
- Reuse of self-locking nuts: Fiber-insert nuts that spin freely are worn out and must be replaced. The test may ask whether a nut that offers no resistance can be reused — the answer is no.
- Plain head bolts: A bolt with no head markings is a commercial, unrated fastener. It is never acceptable for structural installation on a certificated aircraft, regardless of apparent size or strength.