AN960C10 - Stainless Steel Flat Aerospace Washer

AN960C10 – Military Standard Flat Washer, CRES (#10 Screw Size)

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The AN960C10 is a flight-critical flat washer manufactured in strict accordance with Army-Navy (AN) aerospace standards. This component, commonly known as a NASM960C10 under the current specification system, is designed to provide a smooth, flat bearing surface under the head of a fastener (bolt or screw) or under a nut. Its primary functions are to distribute the clamping load over a wider area, thereby reducing stress concentrations on the clamped material, and to prevent galling or marring of the airframe surface during fastener tightening.

This specific washer ("C" designation) is fabricated from high-quality Corrosion-Resistant Steel (CRES), typically an austenitic 300-series stainless steel alloy.1 This material is selected for its excellent resistance to corrosion from atmospheric conditions and common aviation fluids, making it ideal for general airframe use where high strength is not the primary driver (unlike heat-treated alloy steel washers). The "-10" size designation indicates it is designed for use with a No. 10 size screw or bolt (nominal 3/16 inch diameter).

Key Features

• Army-Navy (AN) / National Aerospace Standard (NASM) Certified: Guarantees adherence to stringent material, dimensional, tolerance, and quality requirements for certified aircraft.

• Corrosion-Resistant Steel (CRES) Construction: Manufactured from 300-series stainless steel, offering excellent protection against rust and corrosion in typical aircraft operating environments.

• Distributes Fastener Load: Increases the effective bearing area under the fastener head or nut, reducing surface stress and preventing damage to softer materials like aluminum skins.

• Provides Smooth Bearing Surface: Ensures even load distribution and prevents the fastener head or nut from digging into the clamped material during torquing.2

• Prevents Galling: Acts as a barrier between fasteners and the structure, reducing the potential for galling, especially in CRES-to-CRES fastener applications.

Technical Specifications

• Part Number: AN960C10 (May be referenced as NASM960C10)

• Component Type: Flat Washer

• Governing Specification: AN960 / NASM960

• Material: Corrosion-Resistant Steel (CRES), likely 300 Series (e.g., Type 302, 304, 316 per standard).3

• Finish: Passivated per AMS 2700 or similar standard to enhance corrosion resistance.4 (Note: 'C' type washers are NOT cadmium plated).

• Size: For No. 10 Screw/Bolt.

• Dimensions (Nominal per NASM960):

  • Inside Diameter (ID): 0.203 inches5

  • Outside Diameter (OD): 0.438 inches6

  • Thickness: 0.032 inches

Aircraft Compatibility

The AN960C10 is a standard hardware component used universally across all categories of certified aircraft from all major manufacturers (e.g., Textron, Piper, Boeing, Airbus, etc.). It is specified in countless structural and non-structural bolted joints.

Crucial Note: The correct washer size and material type (e.g., 'C' for CRES, plain steel for AN960, Aluminum for AN960L) for a specific application must be verified against the aircraft's Illustrated Parts Catalog (IPC) or maintenance manual.

Applications

The AN960C10 is used in a vast array of general-purpose fastening applications throughout the aircraft:

• Under Bolt Heads and Nuts: To distribute load and prevent surface damage, especially when fastening against aluminum or composite materials.7

  
  

  
  

• Shimming: Can be used for minor adjustments in stack-up height in non-critical applications (consult engineering documentation).

• Component Mounting: Securing instruments, brackets, fairings, and inspection panels.

Why Choose AN960C10?

Choosing a genuine AN/NASM specification AN960C10 washer is essential for ensuring the integrity and longevity of bolted joints. The primary benefit of the 'C' type is its inherent corrosion resistance, eliminating the risks associated with damaged plating on standard steel washers. Using non-certified hardware store washers introduces significant risks: unknown material composition (potential for corrosion or inadequate strength), incorrect dimensions (leading to poor load distribution or interference), and lack of traceability. AN/NASM standards guarantee the quality, material, and dimensions required for safe and reliable performance in the demanding aerospace environment.8

The fundamental purpose of the AN960C10 washer is stress reduction through load distribution.10 When a fastener is torqued, it creates a high clamping force ($P$). This force is concentrated over the area directly under the fastener head or nut. The bearing stress ($\sigma_{bearing}$) on the clamped material is calculated as $\sigma_{bearing} = P / A_{bearing}$, where $A_{bearing}$ is the contact area. Without a washer, especially when fastening against softer materials like aluminum, this high bearing stress can exceed the material's compressive yield strength, causing localized crushing, yielding, or "dishing" of the surface. The AN960C10 washer significantly increases the bearing area ($A_{bearing}$), thereby reducing the bearing stress to well below the material's yield point. The passivation finish on the CRES washer is a crucial chemical treatment (typically using nitric acid) that removes free iron and other contaminants from the surface and promotes the formation of a thicker, more uniform, and highly protective chromium oxide (11$Cr_2O_3$) passive layer.12 This layer is what gives stainless steel its excellent corrosion resistance.

Note: While seemingly simple, this is a flight-critical hardware component. Ensure the correct size and material type ('C' for CRES) are used per the maintenance documentation. Washers should be inspected for flatness, burrs, or damage before installation. In critical structural applications, replacement with a new washer upon reassembly is often recommended practice. Always use the washer under the element being turned (usually the nut) unless otherwise specified.

Documentation

The definitive technical requirements for this component are detailed in National Aerospace Standard NASM960 (which supersedes AN960). These proprietary documents must be sourced directly from the relevant standards organizations for complete engineering specifications.