AS3209-021 - Packing Preformed Fluorocarbon O-Ring

AS3209-021 – Fluorocarbon (Viton®) Boss Seal O-Ring, 75 Durometer

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Overview

The AS3209-021 is a high-performance, preformed O-ring packing seal manufactured to the precise Aerospace Standard (AS3209). This O-ring is fabricated from a Fluorocarbon (FKM) elastomer, a material widely known by its DuPont trade name, Viton®, and is specifically engineered for creating a static seal on straight thread tube fittings in high-temperature system bosses. Its 75 durometer hardness provides an optimal balance of sealing force and material compliance for standard pressure applications.

The defining characteristic of this O-ring is its exceptional thermal stability, allowing it to maintain a reliable seal in continuous service temperatures up to 400°F (204°C). The AS568-021 size is designed for use with large -12 (3/4 inch) tube fittings. This makes it the mandated sealing solution for major fluid connections in and around the engine compartment, firewall, and other high-temperature zones where a standard Nitrile (NBR) O-ring would rapidly degrade and fail.

Key Features

• High-Temperature Fluorocarbon (FKM): Fabricated from a Viton® or equivalent FKM elastomer, providing a continuous service temperature rating of up to 400°F (204°C).

• Designed for -12 Boss Seals: Specifically sized (AS568-021) for static axial sealing of -12 size straight thread tube fittings in their corresponding ports (bosses).

• Excellent Fluid Resistance: Offers superior chemical resistance to a wide range of petroleum-based and synthetic lubricating oils, hydraulic fluids, and jet fuels.

• 75 Durometer Hardness: The 75 Shore A hardness provides an excellent balance of sealing force and resistance to deformation for standard pressure applications.

• Aerospace Standard AS3209: Meets all dimensional, material, and quality requirements of AS3209, ensuring airworthiness and interchangeability.

• Material Identification: Typically brown in color to visually distinguish it from black Nitrile (NBR) O-rings, preventing incorrect installation.

Technical Specifications

• Part Number: AS3209-021

• Component Type: Preformed O-Ring Packing, Boss Seal

• Governing Specification: AS3209

• Material: Fluorocarbon Elastomer (FKM / Viton®)

• Hardness: 75 ± 5 Durometer Shore A

• Color (Typical): Brown

• Dimensions (AS568-021):

  • Inside Diameter (ID): 0.862 inches

  • Cross-Section (CS): 0.087 inches

• Typical Temperature Range: -15°F to +400°F (-26°C to +204°C)

Aircraft Compatibility

The AS3209-021 is a standard hardware component used across the entire aerospace industry in high-temperature fluid systems. It is not specific to one aircraft but is a fundamental component found on:

• Military Fighter, Transport, and Rotorcraft

• Commercial and Transport Category Airliners (Boeing, Airbus)

• Business and Corporate Jets (Textron Aviation, Bombardier, Gulfstream)

• High-Performance General Aviation Aircraft

Note: Verification of applicability is the responsibility of the installer. Always consult the current version of the aircraft or engine's Illustrated Parts Catalog (IPC) to confirm part number eligibility.

Applications

• Sealing of large -12 size (1-1/16-12 thread) straight thread tube fittings on firewall-forward components.

• Connections on main hydraulic pump pressure and return lines.

• Large-diameter fuel and oil line connections on engine accessories.

• Any major fluid system application where operating temperatures exceed the ~250°F limit of standard Nitrile (NBR) O-rings.

Why Choose AS3209-021?

The selection of the correct elastomer for the operating environment is a flight-critical decision. Substituting a standard black Nitrile (NBR) O-ring in a location specified for a brown Fluorocarbon (FKM) O-ring will result in the seal hardening, cracking, and failing from thermal degradation. This will cause a leak of hot, flammable fluid in a critical area of the aircraft. Using a certified AS3209-021 O-ring is the only way to guarantee you are installing the correct, high-temperature material required for a safe and airworthy repair.

The 75 durometer Shore A hardness specified for the AS3209-021 is a deliberate engineering choice that balances sealing effectiveness with material properties. Durometer is a measure of an elastomer's resistance to indentation, or its hardness. A lower durometer value (e.g., 60) indicates a softer, more pliable material that can easily conform to minor surface imperfections, creating an effective seal with low compressive force. A higher durometer value (e.g., 90) indicates a harder, stiffer material that offers superior resistance to being physically forced, or "extruded," into the clearance gap in a high-pressure application. The 75 durometer specified for this standard represents an optimal midpoint for general-purpose, high-temperature aerospace boss seals. It is soft enough to provide a reliable seal on standard machined surfaces, yet hard enough to resist extrusion in typical hydraulic system operating pressures (up to 3,000 PSI) when properly installed.

Note: This component has a finite shelf life defined by standards like ARP5316. The cure date must be verified prior to installation. It is a single-use part; a new O-ring must be used every time a fitting is reinstalled. The port and fitting sealing surfaces must be impeccably clean and free of scratches or damage. The O-ring must be lubricated with clean system fluid before installation.

Documentation

The base standard for this part is the Aerospace Standard AS3209. This is a controlled document managed by SAE International. OEM-specific application and installation data is contained within the aircraft's maintenance and parts manuals.