AS3209-218 - Packing Preformed O-Ring

AS3209-218 – Fluorocarbon (FKM) 90 Durometer O-Ring for AS5202 Boss Fittings

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The AS3209-218 is a high-pressure, preformed O-ring packing manufactured in strict accordance with SAE Aerospace Standard AS3209. This component is specifically engineered to provide a robust static seal in straight thread tube fitting bosses (per AS5202 / MS33649) within demanding aerospace hydraulic, fuel, and lubrication systems. Its function is absolutely critical, preventing fluid leakage at the interface between fittings and system components such as pumps, actuators, valves, and manifolds, even under extreme operational pressures and temperatures.

This O-ring is fabricated from a specialized Fluorocarbon (FKM) elastomer compound with a 90 Durometer Shore A hardness. This specific material and hardness combination is mandated for its superior resistance to extrusion under high pressure and its excellent chemical compatibility with a wide range of aerospace fluids, including mineral oil-based (MIL-PRF-5606) and synthetic hydrocarbon-based (MIL-PRF-83282) hydraulic fluids. The AS3209-218 is not a generic seal; it is a precision-engineered component whose material properties and dimensional tolerances are tightly controlled to ensure the integrity and safety of flight-critical fluid power systems.

Key Features

• Manufactured to SAE AS3209: Guarantees material composition, physical properties, and quality control meet the stringent requirements for aerospace applications.

• 90 Durometer Fluorocarbon (FKM): The hard, high-modulus FKM material provides exceptional resistance to extrusion in high-pressure systems (3000+ psi), preventing seal failure.

• Broad Chemical Resistance: Offers superior compatibility with petroleum oils, synthetic lubricants, hydrocarbon fuels, and most hydraulic fluids, preventing material degradation such as swelling or shrinking.

• Wide Operating Temperature Range: Maintains sealing integrity and physical properties across a broad thermal spectrum, from low-temperature takeoffs to high-temperature operational environments.

• Standard AS568 Sizing: The "-218" designator corresponds to a universal size standard (AS568), ensuring dimensional interchangeability and proper gland fill for specified fittings.

• Superior Compression Set Resistance: Resists taking a permanent set after being compressed for long periods at elevated temperatures, ensuring a reliable seal throughout its service life.

Technical Specifications

• Part Number / Specification: AS3209-218

• Component Type: Packing, Preformed O-Ring

• Governing Specification: SAE AS3209

• Material: Fluorocarbon (FKM) Elastomer, also known by trade names such as Viton®.

• Hardness: 90 ± 5 Durometer Shore A

• Color: Black (Typical for this specification)

• Dimensions (per AS568-218):

  • Inside Diameter (ID): 1.237 inches (31.42 mm)

  • Width / Cross-Section (CS): 0.103 inches (2.62 mm)

  • Outside Diameter (OD): 1.443 inches (36.65 mm)

• Typical Operating Temperature Range: -15°F to 400°F (-26°C to 204°C) in continuous service.

Aircraft Compatibility

The AS3209-218 is a standard hardware component, not specific to a single aircraft model. It is used across the entire spectrum of aviation, including commercial transport, military, and general aviation platforms, wherever AS5202 / MS33649 straight thread boss ports are utilized for fluid connections.

Crucial Note: While this is a standard part, verification against the specific aircraft or component's current Illustrated Parts Catalog (IPC) and maintenance manual is mandatory to ensure correct part number applicability for the specific location.

Applications

The primary and intended application for the AS3209-218 is to serve as a static face seal for straight thread tube fittings and plugs installed into hydraulic and fuel system components. Common applications include sealing connections on:

• Hydraulic Pumps and Power Transfer Units (PTUs)

• Flight Control Actuators (aileron, elevator, rudder)

• Landing Gear Actuators and Down-locks

• Brake System Valves and Manifolds

• Fuel Control Units (FCUs) and Fuel Pumps

• Engine Lubrication System Components

Why Choose AS3209-218?

The integrity of a multi-million dollar aircraft's hydraulic system can depend on a component costing only a few dollars. Choosing a certified AS3209-218 O-ring is the only way to guarantee the required performance. Using a commercial-grade or incorrect material O-ring introduces severe risks, including chemical incompatibility leading to seal degradation, insufficient hardness causing high-pressure extrusion, or poor compression set resulting in leaks after thermal cycling. A leaking fitting in a high-pressure hydraulic or fuel line constitutes a significant safety-of-flight risk. The AS3209 specification ensures the traceability, quality, and material science required to prevent such failures.

The specification of a 90 Durometer Fluorocarbon (FKM) elastomer for the AS3209-218 is a direct countermeasure to the failure mode of extrusion. FKM is a synthetic fluoroelastomer terpolymer whose molecular structure, characterized by extremely strong carbon-fluorine bonds, provides exceptional thermal stability and resistance to chemical attack from aggressive hydrocarbon fluids. Extrusion is the physical process where, under high system pressure (e.g., 3000 psi), the O-ring is forced into the microscopic clearance gap between the two mating metal surfaces of the gland. A softer elastomer with a low modulus of elasticity deforms and flows easily into this gap, leading to "nibbling" and eventual failure of the seal. The 90 Durometer Shore A hardness of the AS3209 standard provides a high modulus, significantly increasing the material's resistance to deformation and flow under pressure. This property makes it the requisite choice for high-pressure static face seals, ensuring the O-ring maintains its geometry and sealing force within the gland without being physically forced out.

Note: O-rings are typically considered single-use components in aerospace applications and must be replaced whenever the fitting is disconnected. These components have a finite shelf life determined by their cure date, which must be strictly observed. During installation, ensure the O-ring and gland are free of debris, and lightly lubricate the O-ring with clean system fluid to prevent abrasion or cutting. Installation must be performed by a certified mechanic in accordance with the applicable maintenance manual.

Documentation

The full text of SAE Aerospace Standard AS3209 and AS568 are proprietary documents available for purchase directly from SAE International. These documents contain the definitive technical requirements for this component.