AS3208-08 - Fluorocarbon Rubber O-Ring

AS3208-08 – 70 Durometer Nitrile (Buna-N) O-Ring

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The AS3208-08 is a general-purpose, preformed O-ring packing manufactured in strict accordance with SAE Aerospace Standard AS3208. It is critical for technicians and procurement specialists to note that the AS3208 standard mandates a 70 durometer Nitrile (Buna-N) rubber compound, not Fluorocarbon. This material is specifically chosen for its excellent compatibility and reliable sealing performance with petroleum-based fluids in low-to-moderate pressure static applications. This O-ring is a workhorse component used in countless systems throughout an aircraft.

The AS3208 specification guarantees that the O-ring meets the stringent requirements for material composition, dimensional tolerance, and quality control demanded for certified aircraft. The 70 durometer hardness provides excellent flexibility, allowing the O-ring to conform easily to sealing surfaces, making it ideal for general-purpose fluid containment where pressures do not exceed 1500 PSI.

Key Features

• SAE Aerospace Standard (AS) Certified: Manufactured to the exact material, hardness, and dimensional tolerances of the AS3208 specification, ensuring reliable sealing performance.

• 70 Durometer Nitrile (Buna-N) Material: The specified elastomer provides excellent resistance to petroleum-based hydraulic fluids (e.g., MIL-PRF-5606), engine oils, greases, and aviation gasoline.

• Flexible for a Reliable Low-Pressure Seal: The 70 durometer hardness allows the O-ring to conform well to gland surfaces with minimal compressive force, creating an effective seal in standard static applications.

• General-Purpose Aerospace Seal: A versatile and widely used O-ring for countless fittings, housings, and component seals throughout the aircraft.

• Wide Operating Temperature Range: Maintains sealing integrity and physical properties across a broad temperature spectrum suitable for most airframe and engine applications.

Technical Specifications

• Part Number: AS3208-08

• Component Type: Preformed Packing, O-Ring

• Governing Specification: AS3208

• Material: Nitrile Rubber (Buna-N / NBR). (This part is not Fluorocarbon).

• Hardness: 70 ± 5 Durometer Shore A.

• Size (AS568A Dash Number): -008

• Dimensions:

  • Inside Diameter: 0.176 inches (4.47 mm)

  • Cross-Section: 0.070 inches (1.78 mm)

Aircraft Compatibility

The AS3208-08 is a standard hardware component used in countless applications across all categories of certified aircraft. It is not specific to any one airframe but is called for by part number in the maintenance and parts manuals for numerous systems and components.

Crucial Note: The correct part number for a specific application must be verified against the Illustrated Parts Catalog (IPC) or Component Maintenance Manual (CMM) for the component or aircraft being serviced.

Applications

The AS3208-08 is a versatile O-ring for static sealing in systems that use petroleum-based fluids and operate at pressures generally not exceeding 1500 PSI (unless used with a backup ring). Common applications include:

• Hydraulic System Components (using MIL-PRF-5606 fluid)

• Engine Lubrication System fittings and accessories

• Fuel System Components (sealing housings and fittings)

• Landing Gear and Brake System Components

Why Choose AS3208-08?

Choosing a genuine AS3208-08 O-ring is a critical decision for ensuring system integrity. The selection of an O-ring's material must be matched to the system's fluid. The AS standard guarantees the correct Nitrile material and quality control (free of nicks, cuts, or molding flaws) required for a reliable seal. Using an O-ring of a different, unspecified material (like Fluorocarbon) can lead to improper swell characteristics or incompatibility with the application's intended design, while using an uncertified part risks material of unknown quality, leading to premature failure.

The material specified for the AS3208 standard, Nitrile rubber (Acrylonitrile Butadiene Rubber - NBR), is an engineered copolymer whose properties are tailored for its application. Its exceptional resistance to non-polar fluids like petroleum oil is determined by its acrylonitrile (ACN) content. The ACN monomer introduces polar nitrile groups (-C≡N) into the polymer chain. These polar groups are not chemically attracted to the non-polar hydrocarbon molecules of the fluid. This principle of chemical dissimilarity prevents the fluid's molecules from permeating and swelling the polymer matrix, a process that would cause the O-ring to lose its hardness, reduce its sealing force, and ultimately fail. The AS3208 standard mandates a specific ACN content range to ensure this fluid resistance is consistently achieved, providing a reliable seal in the presence of petroleum-based products where an incompatible elastomer like Fluorocarbon may not be the specified material.

Note: O-rings are typically considered single-use components in aerospace applications and must be replaced whenever a fitting is disconnected or a component is disassembled. These components have a finite shelf life (cure date) which must be strictly observed. CRITICAL: The AS3208 standard specifies Nitrile (Buna-N) rubber. Do not use this O-ring in systems requiring a Fluorocarbon (FKM) seal (e.g., Skydrol systems, high-temperature applications). Installation must be performed by a certified A&P mechanic in accordance with the applicable maintenance manual.

Documentation

The definitive technical requirements for this component are detailed in SAE Aerospace Standard AS3208. These proprietary documents must be sourced directly from the relevant standards organizations for complete engineering specifications.