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AS3209-028 – Aerospace Standard Fluorocarbon (FKM) O-Ring
Available now through the AirParts Aero marketplace.
Overview Section
The AS3209-028 is a high-performance Aerospace Standard (AS) O-ring designed for critical sealing in aviation fluid systems. Manufactured from Fluorocarbon (FKM), commonly known by the brand name Viton™, this O-ring is engineered to maintain structural integrity under extreme thermal stress and chemical exposure.1
The AS3209 series is a modern aerospace specification (often cross-referenced with the military M83248/1 standard). The -028 dash size provides a relatively large inside diameter with a slim 0.07-inch cross-section, making it an ideal "packing" for large-diameter low-profile fittings, such as fuel caps, oil reservoir plates, and accessory mounting flanges.
Key Features Section
Exceptional Thermal Range: Designed to operate reliably from -15°F to +400°F ($-26°C$ to $+204°C$), covering the requirements of engine nacelles and hot-section accessories.
Superior Fluid Compatibility: Specifically resistant to Jet-A, 100LL Avgas, Skydrol, and synthetic turbine oils, preventing the "swelling" common in standard rubber.
Low Compression Set: Engineered to retain its "spring-back" or sealing memory over long service intervals, reducing the risk of leaks during thermal contraction.
Ozone & UV Resistance: Highly resistant to environmental aging, making it suitable for airframe components exposed to atmospheric conditions at high altitudes.
Precision AS568 Sizing: Manufactured to strict aerospace tolerances for a perfect fit in standard glands, ensuring predictable "squeeze" and volume fill.
75 Durometer (Shore A): Provides sufficient hardness to resist extrusion under pressure while maintaining the flexibility needed for a gas-tight seal.2
Technical Specifications Section
| Specification | Detail |
| Standard | AS3209 (Cross-ref M83248/1-028) |
| Dash Size | -028 |
| Material | Fluorocarbon Rubber (FKM / Viton™) |
| Inside Diameter (ID) | 1.364 in (34.65 mm) |
| Outside Diameter (OD) | 1.504 in (38.20 mm) |
| Cross-Section (W) | 0.070 in (1.78 mm) |
| Hardness | 75 Shore A |
| Color | Black |
Aircraft Compatibility Section
The AS3209-028 is found in a variety of airframes and engine systems:
Cessna Citation & Beechcraft King Air: Often used in fuel system access panels and engine-driven pump mounting interfaces.
Turbine Engines (PT6A, TFE731): Utilized for sealing external oil scavenge lines and large-diameter pneumatic fittings.
Fuel Caps: Frequently used as the primary seal for high-speed "flush-mount" fuel filler caps on various business jets.
Hydraulic Manifolds: Used in low-profile port seals where space constraints require a thin cross-section.
Technical Applications & Safety
In aerospace applications, the O-ring creates a seal by occupying the space in a "gland" and being slightly compressed. Because of the slim 0.07-inch thickness of the -028 size, the surface finish of the metal gland is critical.
Safety Warning: Avoid material mismatch. While an AS3209-028 (Fluorocarbon) and a standard Nitrile O-ring look identical, Nitrile will rapidly degrade if exposed to synthetic turbine oils or high heat. Always verify the Cure Date on the vacuum-sealed packaging. While Fluorocarbon has an excellent shelf life, any seal showing signs of "flat-spotting" or surface bloom should be discarded.
Technical Engineering Insight
The engineering value of the AS3209-028 lies in its Chemical Inertness. Modern aviation fuels and oils contain aggressive additives (such as anti-corrosion and anti-icing agents) that can "leach" plasticizers from standard elastomers. AS3209 Fluorocarbon is molecularly stable, ensuring the O-ring does not shrink or become brittle. Because the -028 dash size has a thin cross-section ($0.07$ in), its Volume Fill within the gland is critical. If the gland is too deep, the thin O-ring will not provide enough "interference" to seal; if too shallow, the O-ring can be crushed. Always verify the gland dimensions against AS568 standards to ensure a 10%–15% compression.
Installation & Maintenance Addendum
Gland Inspection: Inspect the seating surface for "spiral" scratches. On thin O-rings like the -028, a microscopic scratch can create a bypass leak path.
Lubrication: Use a small amount of Parker Super O-Lube or clean system fluid to prevent the seal from "rolling" (spiral failure) during the assembly of large-diameter parts.
Non-Marring Tools: Always use brass or plastic O-ring picks for removal. Steel tools will gouge the aluminum housing, leading to permanent leaks that a new O-ring cannot fix.
