NAS7403-2 - National Aerospace Standards Close Tolerance Philips Head Screw Bolt

NAS7403-2 – National Aerospace Standards Close Tolerance Bolt

Available now through the AirParts Aero marketplace.

Overview Section

The NAS7403-2 is a high-strength, Close Tolerance Hex Head Bolt engineered for critical structural joints in aerospace applications. Part of the NAS7400 through NAS7416 series, these bolts are designed for "interference fit" or "precision fit" applications where the bolt must serve as a shear pin with zero allowable "slop" or movement within the hole.

The NAS7403 designation identifies the bolt as a 10-32 (0.190") diameter fastener made from high-strength alloy steel. The -2 suffix specifies the grip length, which is critical for ensuring the unthreaded portion (the shank) extends exactly through the material stack-up. These bolts are heat-treated to a tensile strength of 160,000 to 180,000 PSI, making them significantly stronger than standard AN-series hardware.

Key Features Section

• Close Tolerance Shank: The shank diameter is precision-ground to a tolerance of +0.0000 / -0.0005 inches, ensuring a "tap-in" fit that prevents structural shifting under load.

• High-Strength Alloy Steel: Manufactured from 4140 or 8740 alloy steel, heat-treated to provide extreme resistance to shear and tension fatigue.1

  
  

  
  

• Cold-Rolled Threads: Threads are rolled after heat treatment to increase fatigue life by creating compressive stresses at the thread roots.

• Reduced Head Profile: Features a smaller hex head than standard bolts to save weight and allow for installation in recessed areas of the airframe.

• Cadmium Plated: Finished with Type II, Class 2 cadmium plating with a yellow chromate dip for maximum corrosion protection and consistent torque characteristics.

• Large Fillet Radius: The transition from the head to the shank features a precision-controlled radius to eliminate "stress risers" that cause head failure.

Technical Specifications Section

Aircraft Compatibility Section

The NAS7403-2 is found in high-load areas of nearly all modern aircraft:

• Wing-to-Fuselage Attach Points: Used in secondary structural junctions where shear loads are high.

• Engine Mount Assemblies: Found in the vibration-isolated brackets of Cessna, Beechcraft, and Piper airframes.

• Landing Gear Linkages: Specifically used in torque links and scissor assemblies where precise fitment prevents "shimmy."

• Flight Control Bellcranks: Ideal for pivot points in control systems where any "play" in the bolt would result in sloppy control feel.

Technical Applications & Safety

Close tolerance bolts are intended to be "mechanically interference-fit." This means the hole should be reamed, not just drilled.

Safety Warning: Do not force-thread these bolts. The NAS7403-2 has a UNJF thread profile, which has a larger root radius for fatigue resistance. These must be used with matching UNJF-3B nuts. Additionally, because the shank is so precisely sized, you must use a Reamer to prepare the hole (typically to a #12 or 0.190" size depending on the specific fit required). If you use a standard drill bit, the hole will likely be slightly "out of round," defeating the purpose of the close-tolerance fastener and leading to structural fatigue cracking.

Technical Engineering Insight

The engineering value of the NAS7403-2 lies in its Shear Load Distribution. In a standard AN3 bolt installation, there is often a small gap (clearance) between the bolt and the hole. Under load, the airframe must shift slightly before it "seats" against the bolt. Over thousands of flight hours, this shifting causes the hole to become "elongated" (oval). The NAS7403-2, by being ground to within five ten-thousandths of an inch, ensures that the bolt and the airframe act as a single unit, distributing the load across the entire surface of the hole and virtually eliminating hole-wear issues.

Installation & Maintenance Addendum

1. Hole Preparation: Use a high-quality chucking reamer to finish the hole. Lubricate the reamer with cutting oil to ensure a mirror-smooth finish.

2. Installation Force: An NAS7403-2 should typically require a light tap with a plastic mallet to seat. If it drops in freely, the hole is too large; if it requires a heavy hammer, the hole is too small and you risk galling the cadmium plating.

3. Washer Selection: Always use a NAS1149 (AN960) precision washer under the head to protect the radius fillet from being crushed against the edge of the hole.

Description: Close Tolerance Bolt 
Part Number: NAS7403-2
Alternate Part Numbers: No known alternate

Product Overview

An externally threaded fastener whose unthreaded portion is of a specified grip length plated or un-plated and is machined to a tolerance of one thousandth (0.001) of an inch (0.025 mm) or less of a specific diameter for items one inch (25 mm) or less in diameter. Items over one inch (25 mm) in diameter shall have a tolerance of fifteen ten-thousandths (0.0015) of an inch (0.038 mm) or less. The nominal major diameter of the threads shall be at least one thousandth (0.001) of an inch (0.025 mm) below the minimum shank diameter, but not below the minimum major diameter for applicable class of fit, as shown in the screw thread standards, fed std h28 and other nationally recognized standards. The head is not designed to be held or driven with an inserted driver, nor is it designed for rotation by the thumb and fingers. A locking feature may be incorporated in the design of the head or threads. For items with close tolerance and fabricated from a material with 160,000 pounds per square inch (1100 megapascals) minimum tensile strength, see bolt, shear. For items fabricated from a material with less than 160,000 pounds per square inch (1100 megapascals) minimum tensile strength and not a close tolerance, see bolt, machine.

Item Specification