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NAS6604-19 – Hex Head Close Tolerance Aerospace Bolt
Available now on the Airparts.aero Marketplace.
The NAS6604-19 is a high-strength, precision-engineered aerospace bolt manufactured to the National Aerospace Standard (NAS). Designed for structural applications where "close-tolerance" fitment is mandatory, this bolt is a critical fastener in airframe assemblies, engine mounts, and flight control surfaces. Unlike standard AN-series bolts, the NAS6604 series features a ground-shank diameter with a extremely tight tolerance, ensuring a "push-fit" or "interference-fit" that eliminates mechanical "play" or vibration-induced hole elongation in high-stress joints.
Constructed from Ams6322 or Ams6300 alloy steel (Heat treated to 160,000–180,000 PSI tensile strength), the NAS6604-19 is finished with a Type II Cadmium plating for superior corrosion resistance and controlled lubricity during torquing. The "-19" designation specifies a precise grip length, ensuring that the unthreaded portion of the shank (the "grip") extends through the structural material, placing only the strongest part of the bolt in the "shear plane" of the joint.
Key Features
Close-Tolerance Ground Shank: The shank diameter is precision-ground to a tolerance of +.0000 / -.0005 inches, preventing the bolt from "rattling" or shifting within the fastener hole under alternating flight loads.
High-Tensile Alloy Steel: Heat-treated to a minimum of 160 KSI, providing the structural integrity required for primary airframe load paths and high-vibration environments.
Reduced Hex Head Design: Features a compact hex head that saves weight and allows for installation in confined spaces where standard bolt heads might interfere with adjacent structures.
Large Fillet Radius: The transition between the bolt head and the shank features a cold-worked fillet radius to significantly increase fatigue life by reducing stress concentrations at the most vulnerable point of the fastener.
Cadmium Type II Plating: Provides a sacrificial barrier against corrosion and acts as a solid-film lubricant to ensure consistent preload when the bolt is torqued to its final specification.
Technical Specifications
Part Number: NAS6604-19
Standard: NAS6604 (160 KSI Close Tolerance)
Thread Size: 1/4-28 UNJF-3A (Fine Thread)
Shank Diameter: 0.2485" to 0.2490" (Close Tolerance)
Grip Length (-19): 1.188 Inches (Total length of unthreaded shank)
Overall Length: Approx. 1.623 Inches
Material: Alloy Steel (Heat Treated)
Finish: Cadmium Plated per QQ-P-416, Type II, Class 2
Head Style: Hexagon, Reduced Height
Drive: External Hex (7/16" Wrench Size)
Aircraft Compatibility
General Aviation (GA): Used in wing attachment bolts and engine mount-to-firewall interfaces for Beechcraft, Cessna, and Piper.
Business & Commercial Jets: A staple in Boeing and Gulfstream structural repair manuals (SRM) for primary skin-to-spar fastening.
High-Performance Aerobatics: Utilized in Extra and Pitts airframes where G-load reversals require zero-tolerance mechanical joints.
Note: Never substitute an NAS close-tolerance bolt with a standard AN bolt; the smaller diameter of an AN bolt will lead to structural "fretting" and eventual joint failure.
Applications
The NAS6604-19 is utilized for Primary Structural Fastening. Its primary roles include:
Shear-Critical Joints: Connecting wing spars to fuselage carry-through structures.
Engine Mounts: Securing heavy engine vibration isolators to the airframe structure.
Landing Gear Support: Fastening landing gear trunnion blocks where zero mechanical "slop" is permitted.
Why Choose NAS6604-19?
Choosing a genuine NAS6604-19 ensures the Load Transfer across the joint is uniform. In a multi-bolt pattern, if one bolt is slightly undersized (as with standard hardware), the remaining bolts must carry a disproportionate share of the load. The precision-ground shank of the NAS6604-19 ensures that every bolt in the assembly participates equally in the load path. Furthermore, the UNJF-3A threads feature a rounded root that resists fatigue-cracking better than standard industrial threads, ensuring the bolt survives thousands of pressurization cycles.
The NAS6604-19 operates on the principle of Interference and Clamping Force. The "Close Tolerance" refers to the relationship between the bolt shank and the hole. In aerospace engineering, a Reamed Hole is typically prepared to a diameter just .0005" larger than the bolt shank. This ensures that the load is transferred via bearing (direct contact) rather than just friction. The "J" in the 1/4-28 UNJF thread indicates a specialized aerospace thread form with a mandatory large root radius ($R = 0.15011P$ to $0.18042P$). This physics-based design choice eliminates the "sharp V" at the bottom of the threads where cracks usually begin, making the NAS6604-19 far superior to commercial fasteners in high-vibration structural environments.
Note: This is a flight-critical fastener. Always check for "grip-length" compatibility; there should be no more than two threads inside the hole, and at least one full thread must extend through the nut. Installation must be performed by a certified A&P/IA mechanic. Use a calibrated torque wrench and refer to the specific torque values in the Aircraft Maintenance Manual (AMM) or FAA AC 43.13-1B.
