MS20813-1 – High-Pressure Valve Cap
Available now through the AirParts Aero marketplace.
Overview Section
The MS20813-1 is a specialized, High-Pressure Pneumatic Valve Cap designed for use on high-pressure air and nitrogen charging valves (specifically MS28889 and AN6287 styles). These valves are ubiquitous in aviation, serving as the primary charging points for landing gear struts (oleo struts), hydraulic accumulators, and emergency air bottles.
Unlike a standard automotive tire valve cap, the MS28913-1 is a pressure-holding component. It is engineered to provide a secondary, fail-safe seal capable of withstanding pressures up to 5,000 PSI.1 Its rugged, knurled-body construction and integrated high-pressure gasket make it a safety-critical item for maintaining the structural integrity of the aircraft's pressurized systems.
Key Features Section
High-Pressure Sealing: Specifically rated for aircraft systems with operational pressures ranging from 0 to 5,000 PSI.
Integrated Copper Gasket: Features a soft copper or specialized elastomer gasket inside the crown that deforms slightly when torqued, creating a hermetic seal against the valve stem.
Corrosion-Resistant Material: Typically manufactured from cadmium-plated brass or stainless steel to prevent seizing and galvanic corrosion on the valve stem.
Knurled Grip: Features a deeply knurled outer surface to allow for secure hand-tightening, even when the technician is wearing gloves or the cap is covered in hydraulic fluid.
Safety Wire Hole: Many variants include a small drilled hole to allow for safety-wiring the cap to the valve body, preventing loss due to vibration.
Universal Fitment: Designed with 0.305-32 threads (standard "large" aircraft valve thread), ensuring compatibility with all MS and AN-standard high-pressure valves.
Technical Specifications Section
| Specification | Detail |
| Part Number | MS20813-1 |
| Material | Brass (Cadmium Plated) or Stainless Steel |
| Max Working Pressure | 5,000 PSI |
| Thread Size | 0.305-32 UNEF-2B |
| Gasket Material | Soft Copper (Replaceable in some models) |
| Military Standard | MIL-C-7992 / MS20813 |
| Application | Landing Gear Struts, Accumulators, Air Bottles |
| Operating Temp | -65°F to +250°F |
Aircraft Compatibility Section
The MS20813-1 is a "fleet-wide" part used on virtually every pressurized airframe in the AirParts Aero inventory:
General Aviation: Standard cap for the nose and main gear struts of Cessna, Piper, and Beechcraft aircraft.
Corporate Jets: Utilized on the hydraulic accumulators and emergency gear-blowdown bottles of Learjet, Gulfstream, and Citation aircraft.
Commercial Transports: Used for nitrogen charging on the landing gear of Boeing and Airbus fleets.
Military Aircraft: Standard hardware for high-pressure pneumatic systems on various tactical and transport airframes.
Technical Applications & Safety
In a high-pressure system, the internal valve core (the "Schrader" valve) is the primary seal. However, these cores can develop slow leaks due to extreme temperature cycles or contamination. The MS20813-1 acts as the final barrier. Without this cap, a slow leak in a nose gear strut could lead to a "flat" strut during taxi, potentially causing prop-strike or structural damage.
Safety Warning: Never substitute an MS20813-1 with a standard plastic or light-duty metal tire cap. Standard caps are not designed to hold pressure and will pop off or leak at the 500+ PSI levels common in aircraft struts. Always ensure the internal gasket is present before installation.
Technical Engineering Insight
The engineering value of the MS20813-1 lies in its Gasket Compression and Thread Engagement. High-pressure gases (especially Nitrogen) are prone to "micro-leaks" through the threads of the valve stem.2 The MS20813-1 solves this through its internal copper gasket. When the cap is tightened to the specified torque (usually finger-tight plus a 1/8 to 1/4 turn), the gasket is crushed against the top rim of the valve stem, creating a metal-to-metal seal that is significantly more reliable than a standard rubber seal over long periods.
Furthermore, the cadmium plating on the MS20813-1 acts as a sacrificial lubricant. In the harsh environment of a landing gear well—exposed to de-icing fluids, brake heat, and salt—this plating prevents the cap from "cold-welding" to the valve stem. Choosing the genuine MS-standard cap through AirParts Aero ensures that your maintenance team can always remove the cap for servicing without damaging the expensive charging valve assembly.
Installation & Maintenance Addendum
Gasket Check: Before every installation, look inside the cap to ensure the copper gasket is seated and hasn't fallen out.
Cleanliness: Ensure the threads on the valve stem are clean. Dirt on the threads can prevent the cap from seating fully against the gasket.
Torque: Do not over-torque with pliers. Over-tightening can deform the valve stem or "mushroom" the copper gasket, making the cap difficult to remove later.