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MS28775-037 – Mil-Spec Nitrile Hydraulic O-Ring
Available now on the Airparts.aero Marketplace.
Overview
The MS28775-037 is a military-specification, preformed packing (O-ring) manufactured in strict accordance with the MS28775 standard. This is the industry-standard, general-purpose seal for static applications in petroleum-based fluid systems. The MS28775 specification mandates a Nitrile (Buna-N) rubber compound with a 75-durometer hardness, a material specifically chosen for its excellent resistance to swelling and degradation when in contact with common red aviation hydraulic fluids (like MIL-H-5606), as well as engine oils and greases.
The "-037" size code corresponds to a standard AS568 o-ring with a nominal 2-3/8 inch inside diameter and a 1/16 inch cross-section. It is typically used to create a static face seal on hydraulic components such as filter housings, actuator end caps, and reservoir covers. As a fully certified mil-spec component, the MS28775-037 is the proven, reliable, and specified seal for countless general-purpose hydraulic sealing applications across all categories of aircraft.
Key Features
Mil-Spec Certified: Manufactured and tested in strict accordance with MS28775, guaranteeing material composition, fluid compatibility, and dimensional accuracy.
Nitrile (Buna-N) Material: Made from a high-quality Nitrile (NBR) compound, providing excellent, low-swell resistance to petroleum-based hydraulic fluids, oils, and fuels.
General-Purpose Hydraulic Seal: The workhorse o-ring for static sealing applications in standard MIL-H-5606 hydraulic systems.
75 Durometer Hardness: A medium hardness (Shore A) that provides an excellent balance of sealing force and conformability for a wide range of static gland applications.
Cost-Effective Reliability: Offers proven, reliable performance in its specified applications at an excellent value.
Technical Specifications
Part Number / Specification: MS28775-037
Component Type: O-Ring / Preformed Packing (Static Seal)
OEM Manufacturer: Certified Mil-Spec Manufacturer
Material: Nitrile Rubber (Buna-N / NBR), 75 Durometer (Shore A)
Size Standard: AS568 Dash Number -037 (Nominal Dimensions: 2.375" ID, 0.070" [1/16"] Cross-Section).
Governing Specification: MS28775.
Typical Operating Temperature Range: -65°F to 275°F (-54°C to 135°C).
Aircraft Compatibility
Universal Aerospace Application: This is a standard mil-spec o-ring used on virtually any aircraft that utilizes petroleum-based hydraulic systems. It is found on aircraft manufactured by:
Textron Aviation (Cessna, Beechcraft), Piper, Boeing, Lockheed Martin, Bombardier, and many others.
Note: The correct o-ring material and part number must be verified in the specific aircraft or component maintenance manual. Material compatibility is flight-critical.
Applications
Hydraulic Filter Assemblies: Used as the main housing or bowl seal on hydraulic system filters.
Hydraulic Actuators: Provides static end cap and port sealing on landing gear and flight control actuators.
Reservoirs and Power Packs: Sealing access covers and line fittings on hydraulic reservoirs and power pack assemblies.
General Static Sealing: Any static face or radial seal application involving petroleum-based fluids within the material's temperature limits.
Forbidden Applications: Must not be used in phosphate-ester (Skydrol) hydraulic systems, automotive brake fluid systems, or in applications with significant ozone or weathering exposure.
Why Choose MS28775-037?
Elastomer compatibility is a critical aspect of aviation safety. The MS28775-037 is guaranteed to be made from the correct Nitrile compound specified for petroleum-based systems. Using the wrong o-ring material, such as an EPDM seal from a Skydrol system, in a MIL-H-5606 hydraulic system would cause the seal to rapidly swell, soften, and extrude, leading to a massive hydraulic leak and system failure. Conversely, using a Nitrile o-ring in a Skydrol system would cause it to become brittle and crack. The certified MS28775-037 is a low-cost guarantee that you are installing the correct, safe, and airworthy seal for the application.
The engineering principle that defines the performance of the MS28775-037 is the optimized acrylonitrile (ACN) content in its Nitrile (NBR) copolymer. NBR is a synthetic rubber created by polymerizing butadiene with acrylonitrile. The butadiene component provides the rubber-like flexibility and elasticity, while the acrylonitrile component, being a polar molecule, provides the resistance to non-polar petroleum oils and fuels. The MS28775 standard requires an NBR compound with a carefully optimized ACN content (typically a "medium-high" grade of 30-45%). This provides an excellent balance, maximizing fluid resistance while still maintaining good low-temperature performance (down to -65°F) and physical properties like tensile strength and abrasion resistance, making it the ideal, cost-effective choice for general-purpose aerospace hydraulic and oil systems.
Note: The MS28775-037 is a flight-critical seal. Never reuse O-rings. Always install a new, certified O-ring whenever a component is serviced. Inspect the sealing grooves and mating surfaces for any nicks, scratches, or corrosion before installation. Lightly lubricate the new O-ring with clean system fluid (e.g., MIL-H-5606) to prevent damage during installation. Ensure the O-ring is not twisted in the gland.
Documentation
MS28775: The military standard sheet defines all dimensional, material, and performance requirements for this series of o-rings and is publicly available through various online mil-spec libraries such as the Defense Logistics Agency ASSIST database.
