MS28778-2 - Military Standard Packing Ring

MS28778-2 – Military Standard Bonded Boss Seal Gasket

Available now on the Airparts.aero Marketplace.

Overview

The MS28778-2 is a high-reliability, bonded boss seal gasket manufactured to a precise Military Standard. This is not a standard O-ring; it is an engineered sealing solution 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 component built to a military standard, it guarantees the material properties and dimensional tolerances required for a safe, leak-free, and airworthy seal.

Key Features

• 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

• 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

The MS28778-2 is a standard hardware component used across all sectors of the aerospace industry where a high-reliability seal for petroleum-based fluid systems is required for -2 size fittings. This includes:

• Military Aircraft

• Commercial and Transport Category Aircraft

• Business and Corporate Jets (e.g., Textron Aviation, Bombardier, Gulfstream)

• General Aviation and Rotorcraft

Note: Verification of applicability is the responsibility of the installer. Always consult the current version of the applicable Illustrated Parts Catalog (IPC) or maintenance manual to confirm part number eligibility for a specific 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 MS28778-2?

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, a common issue with O-rings in high-pressure or large-clearance glands. Using a certified MS-spec part guarantees the correct materials, dimensions, and quality control required for a safe, airworthy, and leak-free repair.

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.

• ASSIST Document Summary Page for MS28778: http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=45930