4.3 Special Structural Fasteners, Blind Rivets & Precision Hardware

4.3 Special Structural Fasteners, Blind Rivets & Precision Hardware

FAA Airframe Exam Focus: Modern high-performance aircraft structures rely on specialized structural fasteners, pin-and-collar systems, mechanically locked blind rivets, close-tolerance bolts, and prevailing-torque locknuts. An AMT must understand fastener identification markings, grip length sizing formulas, hole preparation tolerances, and critical operational temperature and rotation limitations under FAA-H-8083-31A and AC 43.13-1B.

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1. High-Strength Pin-and-Collar Fasteners (Hi-Lok, Hi-Tique, Hi-Lite & Lockbolts)

Pin-and-collar fasteners combine the high shear strength of an alloy steel bolt with the rapid installation and consistent clamping preload of a solid rivet.

HI-LOK PIN AND COLLAR ASSEMBLY & INSTALLATION MECHANISM:

         Hi-Lok Pin Shank               Threaded Collar with Shear Hex
    ┌──────────────────────────┐          ┌──────┬───────────┐
    │  Smooth Precision Shank  │══════════╡░░░░░░│ █ █ █ █ █ │ ◄── Driving Hex Head
    └──────────────────────────┘          └──────┴───────────┘
                 ▲                          ▲          ▲
                 │                          │          └── Torques off at calibrated limit!
                 │                          └───────────── Retained Threaded Collar Body
                 └──────────────────────────────────────── Grip Length (Equals Material Stack)
    
    INTERNAL ALLEN HEX IN PIN TIP:
       Holds pin stationary while socket driver rotates the collar.

A. Hi-Lok Fasteners

Hi-Lok fasteners are the most prevalent high-strength permanent structural fasteners in commercial and military airframes:

1. Two-Piece Component Architecture:
   • Pin: Precision-ground shank made of alloy steel (AISI 4140/8740), titanium (6Al-4V), or corrosion-resistant stainless steel (A-286). The pin tip features a reverse internal hexagonal Allen drive recess.
   • Threaded Collar: An aluminum alloy (2024-T6), stainless steel, or titanium collar featuring an internal thread, a smooth base flange, a calibrated shear groove, and an outer driving hex head.
2. Self-Torquing Mechanical Principle:
   • During installation, an Allen hex key is inserted into the pin tip to prevent the pin from rotating.
   • A motorized or manual socket tool rotates the threaded collar onto the pin threads, drawing the structural sheets tightly together.
   • When the clamping preload reaches the exact engineered design limit, the shear hex head twists off cleanly at the shear groove, leaving a vibration-proof, perfectly torqued fastener.
   • Advantage: Eliminates the need for operator torque wrenches and guarantees consistent clamping tension across thousands of fasteners.
3. Inspection & Grip Rules:
   • The smooth pin shank must extend completely through the material stack-up. No threads are permitted to bear against the hole wall in shear.
   • The collar must be firmly seated against the sheet surface.
   • If the pin grip is slightly long, a maximum of two washers (total thickness $\le 1/8\text{ in}$) may be installed beneath the collar.

B. Hi-Tique & Hi-Lite Fasteners

• Hi-Tique: A specialized variant of the Hi-Lok pin that features a slight bead or gentle swell on the shank. As the pin is drawn into an interference-fit hole, this bead cold-works the hole bore, imparting compressive residual stresses that significantly enhance joint fatigue life in high-cycle wing structures.
• Hi-Lite: An optimized lightweight version of the Hi-Lok where pin thread runout is shortened and the collar wall is thinned, providing a $15%\text{ to }30%$ weight reduction across transport-category airframes.

C. Lockbolts (Huck Bolts & CherryLOCK)

Lockbolts are heavy-duty, permanent structural fasteners that replace high-strength bolts in heavy primary joints (landing gear bulkheads, wing spar attachments):

• Pull-Type Lockbolt: Features annular locking grooves (not threads) and a break-off pintail. A pneumatic pulling gun grips the pintail, pulls the sheets together under hydraulic tension, swages an unthreaded locking collar into the annular grooves, and snaps the pintail off at a calibrated breakneck groove.
• Stump-Type Lockbolt: Driven in tight-clearance bays using a standard pneumatic rivet gun and bucking bar to swage the collar.

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2. Mechanically Locked Blind Fasteners & Blind Rivets

Blind fasteners are designed for structural installations where the technician has physical access to only one side of the structural assembly (e.g., closed wing leading edges, control surfaces, box spars, and tubular frames).

+-----------------------------------------------------------------------------------------+
|                        AIRCRAFT BLIND RIVET CLASSIFICATION                              |
+----------------------------+------------------------------------------------------------+
| Fastener Category          | Mechanical Locking Method & Structural Suitability         |
+----------------------------+------------------------------------------------------------+
| Friction-Lock Blind Rivet  | Stem held solely by friction; PROHIBITED in primary struct.|
| Mechanical-Lock (CherryMAX)| Visible mechanical locking ring; APPROVED structural repl. |
| Structural Blind Bolt      | Threaded expander sleeve (Jo-Bolt); High shear/tensile use |
+----------------------------+------------------------------------------------------------+

CHERRYMAX (NAS 9300 SERIES) 4-PIECE ASSEMBLY & INSTALLATION:

      Manufactured Head       Locking Collar       Driving Anvil        Pulling Stem
          ┌───┐                  ┌──┐                  ┌──┐                  │
          │   │══════════════════╡  ╞══════════════════╡  ╞══════════════════╡═══════► (PULL)
          └───┘                  └──┘                  └──┘                  │
            │                      │                     │                   │
    Rivet Sleeve Core       Expands Blind Head     Drives Lock Ring    Breaks at Notch

A. Friction-Lock vs. Mechanical-Lock Blind Rivets

1. Friction-Lock Rivets (Non-Structural): The center pulling stem is retained inside the hollow rivet shank solely by friction. Under engine vibration and acoustic buffet, the stem vibrates loose and falls out, leaving a hollow tube with only $30%\text{ to }40%$ of solid rivet shear strength. Friction-lock rivets are strictly prohibited in primary aircraft structures.
2. Mechanically Locked Stem Rivets (CherryMAX / NAS 9300):
   • Four-Part Assembly: Consists of a tubular rivet sleeve, a blind-head forming anvil, a pulling stem with a breaknotch, and a mechanical locking ring (collar).
   • Installation Mechanics:
     1. The pneumatic pulling tool pulls the stem while holding the sleeve against the outer sheet.
     2. The stem head expands the blind end of the sleeve into a large, flat, fully formed shop head.
     3. As pulling continues, the driving anvil forces the mechanical locking collar into a precision chamfer in the rivet head, permanently locking the stem to the sleeve.
     4. The stem fractures cleanly at the engineered breaknotch flush with the manufactured head.
   • Structural Equivalence: Because the locked stem cannot fall out, CherryMAX rivets provide shear and tensile strengths equivalent to standard solid MS20470AD (2117-T4) rivets and are approved as direct structural replacements on a diameter-for-diameter basis.
   • Visual Quality Inspection: A correctly installed CherryMAX rivet must have its locking collar completely seated flush in the head recess and the stem broken flush (within $\pm 0.010\text{ in}$) with the head surface.

B. Structural Blind Bolts (Jo-Bolt & Visu-Lok)

Jo-Bolts and Visu-Loks are ultra-high-strength three-piece blind structural fasteners consisting of an alloy steel or titanium outer threaded sleeve, a driving nut, and an inner bolt core:

• As the bolt core is driven, it pulls the tapered nut into the sleeve, expanding the sleeve against the blind sheet surface and torquing the threaded joint to extreme preload.
• Application: Used in heavy-gauge wing skins, landing gear fittings, and heavily loaded structural joints where solid bolts cannot be bucked.

3. Hole Preparation, Precision Reaming & Interference Fits

High-strength structural fasteners require precise hole geometry to ensure equal load distribution and prevent fatigue failure.

HOLE FIT CLASSIFICATIONS IN AIRCRAFT STRUCTURES:

1. CLEARANCE FIT (Standard Rivets & General Bolts)
   Hole Diameter > Fastener Shank Diameter (+0.002" to +0.004" Clearance)
   • Fastener slides through hole by hand with zero resistance.

2. INTERFERENCE FIT (Hi-Lok & Taper-Lok in High-Fatigue Wing Skins)
   Hole Diameter < Fastener Shank Diameter (-0.0010" to -0.0025" Interference)
   • Pin driven with light mallet or hydraulic puller; pre-stresses hole wall in compression,
     preventing fatigue crack initiation under cyclic flight tension.

A. Drilling, Reaming & Deburring Standards

1. Pilot Drilling & Step Reaming: Holes for close-tolerance fasteners must never be drilled directly to final size. The hole is pilot-drilled undersize (e.g., $1/64\text{ to }1/32\text{ in}$ under) and brought to final dimensions using a high-speed spiral-flute precision reamer.
2. Surface Finish (Roughness): Reamed holes for NAS bolts and Hi-Loks must achieve a surface finish of $32\text{ to }63\text{ micro-inches}$ ($Ra$). Rough drill scores act as stress risers that initiate fatigue cracks.
3. Deburring: Both hole exits must be deburred using a dedicated zero-flute or rotating deburring blade. However, never create a deep countersink or chamfer during deburring—removing excess edge metal weakens the bearing surface and causes fastener head knife-edging.

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4. Aviation Bolt Identification & Standard Hardware (AN, MS, NAS)

Aircraft structural bolts are manufactured to rigid military and aerospace standards (AN = Air Force-Navy, MS = Military Standard, NAS = National Aerospace Standard).

+-----------------------------------------------------------------------------------------+
|                        AIRCRAFT BOLT HEAD MARKING IDENTIFICATION                        |
+--------------------------+------------------------------+-------------------------------+
| Bolt Specification       | Head Physical Marking        | Material & Tensile Strength   |
+--------------------------+------------------------------+-------------------------------+
| Standard Steel (AN3-AN20)| Raised Cross (+) or Asterisk | SAE 4037/8740 Steel (125 ksi) |
| Close-Tolerance (NAS)    | Raised Triangle (Δ)          | Ground Alloy Steel (160 ksi)  |
| Corrosion-Resistant (CRES)| Single Raised Dash (-)       | Stainless Steel (A-286/304)   |
| Aluminum Alloy (AN)      | Two Raised Dashes (--)       | 2024-T4 Aluminum (62 ksi)     |
| Magnetically Inspected   | Stamped 'M' or Orange Paint  | Certified MPI Tested          |
+--------------------------+------------------------------+-------------------------------+

  BOLT HEAD MARKING GUIDE:
  
    Standard Steel    Close-Tolerance    Stainless (CRES)     Aluminum Alloy     Special Magnetic
        ┌───┐              ┌───┐              ┌───┐               ┌───┐               ┌───┐
        │ + │              │ Δ │              │ ─ │               │ ═ │               │ M │
        └───┘              └───┘              └───┘               └───┘               └───┘
     125 ksi Alloy     Ground Shank       A-286 CRES          2024-T4 Al          MPI Certified

A. Decrypting the Standard AN Bolt Part Number System

Standard aircraft hex head bolts are designated by the AN prefix followed by diameter and length codes: $\mathbf{AN[Diameter]-[Length][Suffix]}$

1. Diameter Code (in $1/16\text{-inch}$ increments):
   • AN3 = $3/16\text{ in}$ ($0.1875\text{ in}$)
   • AN4 = $4/16\text{ in} = 1/4\text{ in}$ ($0.2500\text{ in}$)
   • AN5 = $5/16\text{ in}$ ($0.3125\text{ in}$)
   • AN6 = $6/16\text{ in} = 3/8\text{ in}$ ($0.3750\text{ in}$)
   • AN8 = $8/16\text{ in} = 1/2\text{ in}$ ($0.5000\text{ in}$)
   • AN20 = $20/16\text{ in} = 1-1/4\text{ in}$ ($1.2500\text{ in}$)
2. Length Code (in $1/8\text{-inch}$ increments):
   • Lengths under 1 inch: Single digit representing eighths of an inch (e.g., AN4-7 = $1/4\text{ in}$ diameter, $7/8\text{ in}$ long).
   • Lengths of 1 inch and greater: First digit represents whole inches, and second digit represents additional eighths of an inch:
     • AN4-12 = $1\text{ in} + 2/8\text{ in} = 1-1/4\text{ in}$ total length
     • AN6-24 = $2\text{ in} + 4/8\text{ in} = 2-1/2\text{ in}$ total length
3. Hole Drilling Suffix Codes:
   • Standard A Suffix (e.g., AN4-12A): Designates an undrilled bolt shank (used with self-locking nuts).
   • Absence of A (e.g., AN4-12): Designates a drilled shank with a cotter pin hole through the threads (used with castle nuts).
   • H Prefix (e.g., AN4H-12A): Designates a drilled head with a safety wire hole through the hex head.
   • C Code (e.g., AN4C-12A): Designates corrosion-resistant stainless steel.
   • DD Code (e.g., AN4DD-12A): Designates 2024-T4 aluminum alloy.

B. Close-Tolerance, Clevis & Internal Wrenching Bolts

• Close-Tolerance Bolts (NAS / MS20004 to MS20024): Ground to ultra-precise shank diameters ($\pm 0.0005\text{ in}$). Identified by an embossed triangle ($\Delta$) on the head. Mandatory for structural joints subject to severe alternating shear and dynamic reversal loads (wing spar root fittings, flight control hinges).
• Clevis Bolts (AN21 to AN36): Feature a shallow, rounded head with a screwdriver or cross-recess slot and a short thread length. Designed strictly for pure shear loads (e.g., control cable fork ends); prohibited in tension applications.
• Internal Wrenching Bolts (NAS144 to NAS158 / MS2114): High-strength steel alloy bolts ($160\text{--}180\text{ ksi}$) featuring an internal Allen hex drive. Used in high-tension applications (engine mounts, landing gear trunnions). Must always be installed with dedicated heat-treated MS chamfered washers under the head to seat against the large head-to-shank fillet radius.

5. Self-Locking Nuts, Castle Nuts & Installation Restrictions

Aviation nuts must remain locked securely under extreme flight vibration and thermal expansion cycles.

+-----------------------------------------------------------------------------------------+
|                        AIRCRAFT LOCKNUT CLASSIFICATION & LIMITS                         |
+--------------------------+-----------------------+--------------------------------------+
| Nut Type                 | Temperature Limit     | Core Operating Limitation            |
+--------------------------+-----------------------+--------------------------------------+
| Nylon Insert (MS20365)   | 250°F (121°C) Max     | NO high temp / NO continuous rotation|
| All-Metal (MS21042)      | 450°F to 800°F        | NO continuous relative rotation      |
| Castle Nut (AN310)       | Temperature rated     | REQUIRES cotter pin or safety wire   |
+--------------------------+-----------------------+--------------------------------------+

SELF-LOCKING NUT INTERNAL LOCKING MECHANISMS:

1. ELASTIC STOP NUT (Nylon / Fiber Insert - MS20365)
   ┌───────────────┐
   │   █ █ █ █ █   │ ◄── Unthreaded Resilient Nylon Collar (Max 250°F)
   ├───┬───────┬───┤
   │ ░ │       │ ░ │ ◄── Standard Threaded Steel Body
   └───┴───────┴───┘
   
2. ALL-METAL PREVAILING TORQUE NUT (MS21042 / Beam Type)
   ┌───\───────/───┐
   │ ░  \     /  ░ │ ◄── Out-of-Round Crimped Crown / Slotted Locking Beams
   ├───┬───────┬───┤     (Withstands 450°F to 800°F on Engine Firewalls)
   │ ░ │       │ ░ │
   └───┴───────┴───┘

A. Elastic Stop Nuts (Nylon Insert / MS20365 / NASM20365)

Elastic Stop Nuts incorporate an unthreaded fiber or nylon locking insert ring inside the crown of the nut:

1. As the bolt threads penetrate the nylon ring, the nylon deforms elastically around the bolt threads, exerting continuous compressive frictional pressure (prevailing torque).
2. Operating Temperature Limit: The nylon insert degrades, softens, and loses elasticity at elevated temperatures. Elastic stop nuts are strictly limited to locations where operating temperatures do not exceed $250^\circ\text{F}$ ($121^\circ\text{C}$). They are prohibited on engine firewalls, exhaust manifolds, turbocharger mounts, or wheel brake assemblies.

B. All-Metal Self-Locking Nuts (MS21042 / NAS1291)

All-metal locknuts achieve prevailing locking torque by deforming the metal threads in the top crown (elliptical crimping or slotted flexible beams):

• Because they contain no polymer inserts, cadmium-plated carbon steel all-metal locknuts operate safely up to $450^\circ\text{F}$ ($232^\circ\text{C}$), while corrosion-resistant A-286 / Inconel locknuts operate up to $800^\circ\text{F}\text{--}1,200^\circ\text{F}$ on engine exhaust systems.

C. Critical Installation Rules & Prohibitions for Locknuts

1. The Continuous Rotation Prohibition: Self-locking nuts must NEVER be installed on bolts, pins, or joints subject to continuous relative rotation (such as control surface hinges, landing gear retract pivots, or throttle bellcranks). Frictional rotation against the nut face will back the nut off the bolt threads, causing catastrophic joint disassembly. Castle nuts (AN310) with cotter pins are mandatory for all pivoting/rotating joints.
2. Thread Protrusion Standard: When installed, a minimum of two full threads of the bolt shank must extend completely through the locking crown of any self-locking nut or castle nut to ensure 100% engagement with the locking element.
3. Reusability & Breakaway Torque Testing: Self-locking nuts may be reused provided the locking element retains adequate prevailing torque. If an AMT can thread the nut onto the bolt by hand fingers alone past the locking element, the nut has lost its frictional locking grip and must be discarded and replaced immediately.