MS28778-2 – Textron Aviation OEM Bonded Boss Seal Gasket
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Overview
The MS28778-2 is a high-reliability, bonded boss seal gasket, supplied with the full certification and traceability of Textron Aviation as an Original Equipment Manufacturer (OEM) component. This is not a standard O-ring; it is an engineered sealing solution manufactured to Military Standard MS28778, designed for superior performance in straight thread boss connections within aircraft hydraulic, fuel, and lubrication systems using petroleum-based fluids. It consists of a precisely formed Nitrile (NBR) rubber sealing element chemically bonded to the inside diameter of a rigid, cadmium-plated steel retainer.
This composite construction offers significant advantages over a traditional O-ring. The steel retainer provides a positive mechanical stop against the face of the boss, preventing over-compression of the elastomer during installation. It also ensures the seal remains perfectly centered and provides zero clearance, making it exceptionally resistant to extrusion under high pressure. The -2 size is specifically designed for use with -2 (1/8 inch) tube fittings (5/16-24 thread). As a genuine Textron OEM part, it is the factory-specified component for numerous critical connections on Cessna, Beechcraft, and Hawker aircraft.
Key Features
Genuine Textron Aviation OEM Part: Guarantees the part is the factory-specified and fully certified component, ensuring correct performance and airworthiness.
Military Specification Compliance: Manufactured and tested in strict accordance with MS28778, ensuring material properties, dimensional accuracy, and performance.
Bonded Seal Construction: Integrates a durable Nitrile (NBR) elastomer seal within a structural steel retainer for superior sealing and reliability.
Prevents Over-Compression: The metal retainer acts as a positive mechanical stop, ensuring the correct amount of elastomer squeeze without risk of damage from over-torquing.
Superior Extrusion Resistance: The captured design provides zero clearance, making the seal exceptionally resistant to extrusion under high pressure and pressure impulses.
Self-Centering Design: The retainer accurately pilots the seal within the port, ensuring proper alignment and consistent sealing.
Petroleum Fluid Compatible: The Nitrile (NBR) sealing element provides excellent resistance to MIL-H-5606 hydraulic fluid, engine oils, and aviation gasoline.
Technical Specifications
Part Number: MS28778-2
Component Type: Bonded Seal Gasket / Packing with Retainer
OEM Authority: Textron Aviation
Governing Specification: MS28778
Seal Material: Nitrile Butadiene Rubber (NBR / Buna-N)
Retainer Material: Carbon Steel
Finish: Cadmium Plated per QQ-P-416
Size: Designed for -2 (1/8 inch) tube fittings with a 5/16-24 UNF straight thread.
Compatible Fluids: Petroleum-based hydraulic fluids (e.g., MIL-H-5606), lubricating oils, greases, and fuels.
Typical Temperature Range: -65°F to +275°F (-54°C to +135°C) (Limited by NBR element).
Aircraft Compatibility
As a Textron Aviation OEM part conforming to a military standard, this bonded seal is specified for numerous hydraulic and fuel system connections across their product lines and the wider aerospace industry.
Primary Textron Aviation Platforms:
Cessna Citation Series Business Jets
Beechcraft King Air Series
Hawker Business Jets
Cessna and Beechcraft Piston Aircraft
General Aerospace Usage: Used in any aerospace application requiring a high-reliability MS28778-2 seal for petroleum-based fluid systems.
Note: Verification of applicability is the responsibility of the installer. Always consult the current version of the aircraft's Illustrated Parts Catalog (IPC) and maintenance manual to confirm part number eligibility for a specific fitting location.
Applications
Sole Application: Creating a high-reliability static seal for straight thread fittings where they connect to the ports (bosses) of hydraulic, fuel, and lubrication system components operating with petroleum-based fluids. Often specified in high-pressure or high-vibration areas where an O-ring alone may be insufficient.
Why Choose this MS28778-2?
This component provides the dual assurance of meeting the public MS aerospace standard and the specific endorsement of Textron Aviation. The MS28778 bonded seal offers a superior sealing solution compared to a standard O-ring in many demanding applications. Its integrated retainer eliminates the possibility of installation damage from over-tightening and provides unparalleled resistance to extrusion failure. Using the genuine Textron OEM part guarantees the correct materials and dimensions, ensuring the integrity and safety of critical fluid systems.
The MS28778-2 bonded seal operates on the principle of constrained compression with a positive mechanical stop. Unlike a standard O-ring which relies solely on the calculated "squeeze" percentage determined by gland dimensions, this design incorporates a rigid steel retainer ring. When the mating fitting is tightened into the boss, the face of the retainer makes metal-to-metal contact with the machined surface of the boss. This contact serves two crucial functions: 1) It precisely limits the axial compression applied to the internal Nitrile (NBR) sealing element, guaranteeing the optimal squeeze without crushing or damaging the elastomer. 2) It fully confines the elastomer within the annular space between the retainer's ID and the fitting shank, providing zero extrusion gap and making the seal highly resistant to being forced out under high pressure or pressure spikes. This engineered approach provides a more robust and foolproof seal compared to a simple O-ring in demanding applications.
Note: This component has a finite shelf life due to its elastomeric element, governed by standards like ARP5316. The cure date must be verified prior to installation. This is a single-use part and must be replaced whenever the fitting is removed. The sealing surfaces on both the fitting and the boss must be perfectly clean, flat, and free of scratches, nicks, or damage for the seal to function correctly.
Documentation
The military standard governing this part is publicly available through the Department of Defense ASSIST database. OEM-specific application and installation data is contained within the Textron Aviation maintenance and parts manuals.
ASSIST Document Summary Page for MS28778: http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=45930