NAS517-3-3 – Military Standard Drilled Fillister Head Machine Screw
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The NAS517-3-3 is a high-strength, structural machine screw manufactured in strict accordance with National Aerospace Standard (NAS) 517. This fastener is defined by its drilled fillister head, #10-32 UNF thread, and 3/16-inch length. It is engineered for applications requiring high driving torque and a positive, mechanical lock. The fillister head's cylindrical profile and deep, straight slot allow for high installation torque, while the multiple holes drilled through the head are specifically designed for the installation of safety wire (e.g., MS20995), providing a secure, vibration-proof fastening.
Fabricated from heat-treated alloy steel and finished with a protective cadmium plating, this screw is a flight-critical component. It is utilized in countless airframe, engine, accessory, and instrument applications where fastener integrity cannot be compromised. The NAS517 standard ensures that this component meets the highest levels of material strength, fatigue resistance, and dimensional precision required for certified aircraft.
Key Features
Drilled Head for Positive Locking: The defining feature of the NAS517 standard, the drilled head, allows for the use of safety wire, providing a positive mechanical lock that physically prevents the screw from loosening under vibration.
Fillister Head Design: The tall, cylindrical head with a deep slot is engineered to allow for high driving torque during installation.
Heat-Treated Alloy Steel Construction: Manufactured from high-grade alloy steel (e.g., SAE 4140 or 8740) and heat-treated to a minimum tensile strength of 125,000 PSI, providing excellent strength in a compact fastener.
Rolled Threads for Superior Fatigue Resistance: Features UNF-3A threads that are cold-rolled, not cut, a process that induces compressive residual stresses at the thread root, dramatically increasing resistance to fatigue failure.
Cadmium Plated for Corrosion Protection: Coated with a sacrificial layer of cadmium per QQ-P-416, providing excellent, long-lasting corrosion resistance in the harsh aviation environment.
Technical Specifications
Part Number: NAS517-3-3
Component Type: Drilled Fillister Head Machine Screw
Governing Specification: NAS517 (National Aerospace Standard)
Material: Alloy Steel (e.g., SAE 4140 / 8740)
Finish: Cadmium Plated per QQ-P-416, Type II, Class 3
Dimensions:
Thread: 10-32 UNF-3A
Length (Nominal, under head to end): 3/16 inch (0.1875 in)
Head Diameter (Nominal): 0.332 inches
Head Height (Nominal): 0.222 inches
Head Style: Fillister (Drilled)
Drive Type: Slotted
Minimum Tensile Strength: 125,000 PSI
Aircraft Compatibility
The NAS517-3-3 is a standard hardware component used universally across all categories of certified aircraft from all major manufacturers, including Textron (Cessna, Beechcraft), Piper, Boeing, Airbus, and others. It is specified in any application requiring a heat-treated, #10-32 drilled fillister head screw of this length.
Crucial Note: The correct part number for a specific application must be verified against the aircraft's Illustrated Parts Catalog (IPC) or maintenance manual.
Applications
The NAS517-3-3 is designed for any joint subjected to vibration where a positive lock is required. Common applications include:
Securing instruments and avionic components to the instrument panel.
Fastening brackets, supports, and clips within the airframe, engine compartment, and on accessories.
Attaching component covers and access panels.
Used in any joint where the maintenance manual explicitly calls for the fastener to be secured with safety wire.
Why Choose NAS517-3-3?
Choosing a genuine, traceable NAS517-3-3 is a flight-critical decision. Commercial or "hardware store" screws lack the certified material traceability, the mandatory heat treatment, and the superior fatigue life of rolled threads. The failure of a fastener due to vibration or improper strength can lead to component detachment, system failure, or inaccurate instrument readings. The drilled head of the NAS517 is not optional; it is a design feature for positive locking, a fundamental principle of aviation safety. The NAS standard is the only guarantee of this quality and safety.
The engineering principle behind the NAS517-3-3's drilled head is positive mechanical locking. This stands in contrast to friction-based locking mechanisms (e.g., lock washers, prevailing torque nuts) which merely increase the resistance to loosening. Positive locking provides a physical obstruction that makes rotation impossible. By threading safety wire (e.g., MS20995) through the head and anchoring it to another fastener or a fixed point, a mechanically redundant load path is created. This wire physically prevents the screw from rotating counter-clockwise (loosening) due to high-frequency vibration or thermal cycling. This is a non-negotiable safety requirement in many airframe and firewall-forward applications where a loosened fastener could lead to catastrophic failure. The fastener's high fatigue life is further enhanced by rolled threads, a cold-working process that induces compressive residual stresses at the thread root, making it highly resistant to the initiation of fatigue cracks.
Note: This is a flight-critical structural fastener. It is designed to be secured using safety wire. It MUST be replaced with a part of the same specification. Substitution with an undrilled screw (e.g., AN502) or a commercial-grade fastener is not permissible in this application. Torquing and safety wire procedures must be performed by a certified A&P mechanic in strict accordance with standard maintenance practices (e.g., AC 43.13-1B) and the applicable maintenance manual.
Documentation
The definitive technical requirements for this component are detailed in National Aerospace Standard NAS517. These proprietary documents must be sourced directly from the relevant standards organizations for complete engineering specifications.