MS29512-12 - Acrylonitrile Butadiene Rubber O-Ring 3/4 in ID 0.924 In OD 0.116 In Thick

MS29512-12 – Acrylonitrile Butadiene (Nitrile) Fuel & Oil O-Ring

Available now through the AirParts Aero marketplace.

Overview Section

The MS29513-12 (often cross-referenced under the MS29512 series for straight-thread tube fitting bosses) is a high-performance Nitrile (Buna-N) O-ring designed specifically for aircraft fuel and oil systems. Engineered to meet rigorous military specifications, this O-ring is the standard for sealing straight-thread fluid fittings in engines, airframes, and accessory components.

The Acrylonitrile Butadiene compound used in the MS29512-12 is formulated for high-nitrile content, providing maximum resistance to petroleum-based fluids. It is designed to maintain a leak-free seal in "boss" style fittings where the O-ring is compressed into a chamfered groove as the fitting is torqued, ensuring the integrity of the aircraft's fluid distribution lines.1

Key Features Section

• Petroleum Optimization: Specifically engineered to resist swelling, softening, and degradation when exposed to 100LL Avgas, Jet-A, and mineral-based engine oils.

• Excellent Compression Set: Designed to "rebound" and maintain sealing pressure even after years of exposure to thermal cycling and vibration.

• Low-Temperature Flexibility: Maintains a reliable seal in cold-soak conditions down to -40°F, essential for high-altitude flight safety.

• Precision Boss Sizing: Sized specifically for -12 (3/4") tube fittings, providing the correct "squeeze" ratio for standard aerospace ports.

• Vibration Resistance: The high-elasticity Nitrile compound absorbs high-frequency engine harmonics that can cause mechanical metal-to-metal seals to weep.

• MS-Certified Quality: Fully traceable to military-standard manufacturing lots, ensuring consistency in material density and shore hardness.

Technical Specifications Section

Aircraft Compatibility Section

The MS29512-12 is a critical seal found in almost every certified aircraft fluid system:

• Lycoming & Continental Engines: Used on oil cooler return lines, fuel pump inlets, and large-diameter crankcase breather fittings.

• Cessna, Beechcraft, & Piper Airframes: Found in hydraulic reservoir ports, fuel selector valve manifolds, and landing gear strut recharge valves.

• Turbine Powerplants: Utilized for secondary oil scavenge lines and external accessory drive port plugs.

• Agricultural Aircraft (Air Tractor / Thrush): Essential for high-flow fuel and chemical hopper valve fittings.

Technical Applications & Safety

The MS29512-12 is primarily used in Static Boss Seals. In this application, the O-ring is not moving but is held under high compression. Because it is often used in fuel and high-pressure oil lines, the integrity of the elastomer is a primary flight safety factor.

Safety Warning: Do not substitute with standard industrial O-rings. While a hardware-grade Nitrile O-ring may share the dimensions, it lacks the specific "high-nitrile" content required to resist the aromatic hydrocarbons in 100LL Avgas. Standard rubber will swell and lose structural integrity, leading to a "blowout" under pressure. Always check the Cure Date (shelf life) on the package; Nitrile seals should generally be replaced if they exceed 15 years in storage.

Technical Engineering Insight

The engineering value of the MS29512-12 lies in its Modulus of Elasticity specifically tuned for aerospace thread-boss geometry. In a straight-thread port, the O-ring is subjected to "triaxial compression." The MS29512 compound is designed to flow into the microscopic imperfections of the aluminum boss without "extruding" into the thread paths. This specific behavior prevents "shredding" during installation—a common failure in lower-grade O-rings that leads to internal system contamination and eventual seal bypass.

Installation & Maintenance Addendum

1. Thread Protection: When sliding the MS29512-12 over fitting threads, use an O-ring bullet or a thin layer of tape to prevent the threads from "nicking" the inside diameter of the seal.

2. Lubrication: Always apply a film of the system fluid (e.g., clean MIL-PRF-5606 or engine oil) to the O-ring and the boss chamfer to ensure the seal "seats" without twisting.

3. Torque Accuracy: Ensure the fitting is torqued to the values specified in the Standard Aircraft Handbook (AC 43.13-1B) to prevent over-compression, which can "crush" the O-ring and cause premature failure.