AN4-5A – Hexagon Head Undrilled Aerospace Bolt (1/4-28)
Available now through the AirParts Aero marketplace.
Overview Section
The AN4-5A is a standard-strength Hexagon Head Aerospace Bolt manufactured to the Army-Navy (AN) specification.1 This versatile fastener is a cornerstone of aircraft structural and mechanical assembly, designed for use in applications where high tensile and shear strength are required but where safety-wiring the head is unnecessary.
The "AN4" designation indicates a 1/4-inch diameter (1/4-28 fine thread), and the "-5" specifies a length of 21/32 inches.2 The "A" suffix is critical, as it denotes that the shank of the bolt is undrilled (it does not have a hole for a cotter pin), making it ideal for use with self-locking nuts (such as the MS21042 or MS21044 series).
Key Features Section
High-Strength Alloy Steel: Manufactured from heat-treated carbon steel (minimum tensile strength of 125,000 PSI) to handle the rigorous structural loads of flight.
Cadmium Plating: Coated with a protective layer of yellow or silver cadmium (per QQ-P-416) to provide exceptional corrosion resistance and a consistent torque-tension relationship.
Precision-Cut Threads: Features Class 3A fine threads (1/4-28) for maximum thread engagement and resistance to loosening under vibration.
Undrilled Shank ("A" Suffix): Designed for use with locking nuts, offering a cleaner installation and higher fatigue resistance than drilled bolts in non-rotational joints.
Standardized Hex Head: Features a precision-machined 7/16-inch hex head, allowing for high-torque application with standard aerospace tools.
Traceable Manufacturing: Each bolt is marked with the "AN" symbol or a specific manufacturer's ID on the head to ensure airworthiness and material traceability.
Technical Specifications Section
| Specification | Detail |
| Part Number | AN4-5A |
| Diameter | 1/4 inch (0.250") |
| Thread Pitch | 1/4-28 UNF-3A (Fine) |
| Total Length | 21/32 inches (0.656") |
| Grip Length | 1/8 inch (0.125") |
| Thread Length | 17/32 inches (0.531") |
| Material | Alloy Steel (Heat Treated) |
| Finish | Cadmium Plated (Type II, Class 3) |
| Head Marking | Raised "AN" or Manufacturer ID |
Aircraft Compatibility Section
The AN4-5A is a "universal" fastener found in nearly every airframe in the AirParts Aero inventory:
General Aviation: Standard hardware for engine baffles, fairing attachments, and secondary structural brackets on Cessna and Piper aircraft.
Beechcraft King Air: Utilized in interior cabin framing, floorboard installations, and avionics rack mounting.
Experimental Aircraft: The primary choice for builders of Vans RV and other kit planes for a wide variety of structural and non-structural joints.
Control Systems: Often used to secure pulleys, brackets, and bellcrank supports in non-primary-flight control areas.
Technical Applications & Safety
In aerospace, the "Grip" (the unthreaded portion of the shank) is designed to take the shear load. Because the AN4-5A has a specific 1/8" grip, it is essential to ensure that no threads are "in bearing" (inside the hole of the structure being fastened).
Safety Warning: Never substitute an AN bolt with a Grade 5 or Grade 8 hardware store bolt. While they may look similar, AN bolts have specific radius fillets under the head to prevent fatigue cracking and are manufactured from specific alloys that do not become brittle in extreme cold at high altitudes.
Technical Engineering Insight
The engineering value of the AN4-5A lies in its Thread-to-Shank Concentricity. In high-performance aircraft, fasteners must sit perfectly flush to avoid creating "point loads" on the airframe. The AN4-5A is manufactured with a cold-rolled thread process, which is significantly stronger than cut threads because the grain of the steel follows the contour of the thread rather than being severed.
Additionally, the Cadmium Type II plating acts as a lubricant.5 When used with an MS21044N4 (Nyloc) or MS21042-4 (All-metal) nut, the cadmium prevents "galling" and ensures the nut can be torqued to its specific value without the threads seizing. Choosing genuine AN hardware through AirParts Aero ensures that the bolt's metallurgical integrity will withstand the vibration cycles of an engine-mount or landing-gear environment.
Installation & Maintenance Addendum
Washer Usage: Always use at least one AN960-416 (or NAS1149) washer under the nut to prevent the nut from "bottoming out" on the threads.
Torque Values: Standard torque for an AN4 bolt is typically 50–70 inch-pounds, but always consult the specific Aircraft Maintenance Manual (AMM) for the application.
Visual Inspection: During annual inspections, look for "smoking" rivets or fasteners (black streaks), which indicate a loose AN4-5A that is vibrating in its hole.
Applicable Textron Aircraft
- Beechjet 400/400A
- Caravan or Grand Caravan EX
- Citation CJ1 or CJ1+
- Citation CJ2 or CJ2+
- Citation CJ3, CJ3+, CJ3+ Gen 2, CJ3 Gen 3
- Citation CJ4, CJ4 Gen 2, CJ4 Gen 3
- Citation Excel, XLS, XLS+, XLS Gen 2
- Citation III, VI or VII
- Citation Latitude
- Citation Longitude
- Citation M2, M2 Gen 2, M2 Gen 3
- Citation Mustang
- Citation Sovereign or Sovereign+
- Citation V, Ultra, Encore or Encore+
- Citation X
- Citation X+
- King Air 100
- King Air 350 Fusion
- King Air 350C FM-36 & On
- King Air 350C Fusion
- King Air 350i
- King Air 360
- King Air 360C
- King Air B200GT
- King Air B200GT Fusion or B200CGT Fusion
- King Air C90GTi
- King Air C90GTx
- King Air C90GTx Fusion
- SkyCourier
- Skyhawk
- Skylane
- Stationair
- Super King Air 350
- Super King Air 350C FM-1 to FM-35
Other Aircraft Models
- PA-60-600
- PA-31T3
- 737-200
- 767-200SF
- 777-200
- 206-B
- 777-300
- 767-200EM
- 787-8
- MD90
- PA-32-R301T
- 787-9
- 737-100
- 737-700C
- PA-60-601
- PA-34-200
- PA-46-350P
- PA-36-375
- 727-200
- 787-10
- 747
- 206-L
- 707
- 767-200EREM
- 757
- 182
- 757-200
- 206-L1
- DC10
- PA-31-T1
- PA-23-235
- 767-300BCF
- 737NG-800MMA
- 717
- 206-A
- PA-60-601P
- 767-200
- 747-400ERF
- 737-300
- 777-300ER
- PA-34-200T
- 777-200LR
- PA-36-285
- PA-39
- PA-32-300
- 767-300
- 767-300F
- PA-60-602P
- PA-36-300
- BBJ 737-800
- PA-31-T2
- 737-900
- 206-L3
- PA-23-150
- 787
- PA-46R-350T
- 737-800
- PA-32-R301
- 206-B3
- PA-28-181
- PA-32-RT300
- 767-300ER
- 747-400PC
- 172
- 767-200ERSF
- PA-28-RT201T
- PA-24-250
- PA-28-RT201
- PA-31-T
- PA-23-160
- 777-200ER
- PA-28-161
- 737-700
- 737-500
- 737-600
- 767-200ERT
- PA-24-260
- 777-200F
- 737-300QC
- PA-23-250
- KC-10
- 208
- PA-32-R300
- PA-44-180
- DC9
- 767
- 737-400
- PA-24-400
- 767-200ER
- SR22
- 767-300EREM
- PA-60-700P
- PA-32-RT300T
- 737-300SF
- Citation Bravo
- PA-28-151
- PA-30
- MD11
- PA-32-260
- PA-24-180
- 747-8
- PA-18-150
- PA-28-R201
- 747-400F
- 777
- 727-100
- PA-46-310P
- 747-400
- BBJ 737-700
- PA-42-1000
- PA-28-236
- PA-44-180T
- 737-400SF
- MD80
- 206-L4
- PA-34-220T