AS3208-08 - Textron Aviation OEM Preformed Packing

AS3208-08 – Textron Aviation 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. This component, specified extensively in Textron Aviation parts catalogs, is a workhorse seal for a vast range of low-to-moderate pressure static applications. It is fabricated from a 70 durometer Nitrile (Buna-N) synthetic rubber compound, the industry-standard material for sealing petroleum-based fluids. Its primary function is to prevent leakage in countless hydraulic, fuel, and lubrication systems found throughout the airframe and on the engine.

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

• OEM Applicability: Textron Aviation (Cessna, Beechcraft), and all other manufacturers utilizing AS-standard components.

• Material: Nitrile Rubber (Buna-N / NBR).

• 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 and hardness must be matched to the system's fluid and operating pressure. 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 uncertified part or an incorrect material can lead to fluid leaks, while using this softer 70 durometer seal in a high-pressure application that demands a 90 durometer seal (like AS3209) will result in extrusion and certain 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 polar nitrile groups (-C≡N) in the polymer chain are chemically dissimilar to the non-polar hydrocarbon molecules of the fluid, preventing the fluid from permeating and swelling the polymer matrix. The 70 durometer hardness of this standard offers greater pliability compared to harder 90 durometer seals. This lower modulus of elasticity allows the O-ring to conform more easily to surfaces with less-than-perfect finishes and requires less compressive force ("squeeze") to achieve an effective seal. This makes it the ideal choice for standard housings and fittings in systems operating below 1500 PSI, where high extrusion resistance is not the primary design driver.

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: Ensure the correct durometer is used for the application. Do NOT substitute this 70 durometer O-ring in a high-pressure location that specifies a 90 durometer seal (e.g., AS3209, MS9387). 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.