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AN840-4D – Aerospace Hose Fitting Adapter
Available now on the Airparts.aero Marketplace.
Overview
The AN840-4D is a military-standard, straight adapter fitting, a fundamental component for creating versatile connections in aerospace and high-performance fluid systems. This adapter is specifically engineered to transition from a male AN 37-degree flared tube fitting to a male NPT (National Pipe Taper) thread. Machined from high-strength aluminum alloy, the AN840-4D is designed to connect a flexible hose assembly (which typically terminates in a female AN swivel nut) to a rigid component with a female NPT port, such as a fuel pump, filter housing, or hydraulic valve.
Key Features
- Dual-Interface Design: Provides a highly reliable transition between two distinct sealing methods: the AN 37-degree flare for hoses and tubing, and the NPT taper for threaded ports on rigid components.
- AN 37-Degree Flare Seal: The flared end is machined with a precision 37° seating surface. This is the universal standard for aerospace fluid fittings, creating a highly reliable metal-to-metal seal that is resistant to vibration and can be reconnected multiple times.
- Tapered Pipe Thread Seal: The NPT threaded end is tapered, creating a seal through the principle of thread deformation when tightened into a female port. This interference fit provides a strong mechanical joint and a fluid-tight seal when used with a proper thread sealant.
- Lightweight Aluminum Alloy: The "D" suffix specifies that the fitting is machined from a solid piece of high-strength 2024-T351 aluminum alloy. This material provides an excellent combination of strength and light weight for most general aviation fluid applications.
- AN Standard Certified: As a certified Air Force/Navy (AN) standard part, it guarantees perfect interchangeability with any other standard AN/MS fitting of the same size, ensuring dimensional accuracy, thread quality, and material integrity.
Technical Specifications
- Standard: AN840 (Adapter, Straight, Pipe To Tube)
- Full Part Number: AN840-4D
- Component Type: Male AN Flare to Male NPT Pipe Thread Adapter
- Material (D): 2024-T351 Aluminum Alloy
- Finish: Blue Anodized
- Flared Tube End:
- Tube Size (Dash Size): -4 (for 1/4 inch O.D. tubing systems)
- Flare Angle: 37°
- Thread Specification: 7/16-20 UNF
- Pipe Thread End:
- Nominal Size: 1/4 inch
- Thread Standard: NPT (National Pipe Taper)
Aircraft Compatibility
The AN840-4D is a universal fluid fitting used on nearly every type of certified and experimental aircraft ever built. Its use is determined by the plumbing requirements of a specific system.
- Common System Applications:
- Fuel Systems: Connecting flexible fuel lines to rigid components like fuel pumps, selectors, and carburetors/fuel servos.
- Hydraulic Systems: Routing hydraulic lines from rigid plumbing to flexible hoses connected to brake calipers and actuators.
- Oil Systems: Used for connecting flexible oil cooler lines and lines for pressure/temperature sensors.
- Pneumatic & Instrument Lines: Connecting vacuum system lines and instrument pressure lines to pumps and gauges.
Applications
The AN840-4D is the industry-standard solution for adapting a hose to a rigid component:
- Component Connection: Allows a standard female AN hose end to be securely connected to a female NPT port on a system component.
- Fluid System Plumbing: A fundamental building block for creating reliable, serviceable, and leak-proof fluid systems.
Why Choose AN840-4D?
In aircraft fluid systems, where leaks can lead to catastrophic failures such as engine stoppage or loss of hydraulic control, the integrity of every fitting is paramount. The AN840-4D is manufactured and certified to a military standard, guaranteeing that it has the correct material strength, precise flare angle, and thread tolerances to create a dependable, high-pressure seal. Using non-certified, commercial-grade fittings (like automotive 37° or plumbing 45° flare fittings) can introduce parts with incorrect flare angles or poor thread quality, leading to leaks and potential system failure. For any airworthy installation, trust only certified AN/MS hardware.
Order Your AN840-4D Fittings Now
Ensure the integrity of your aircraft's critical fluid systems with certified AN-standard hardware. Visit Airparts.aero to order your AN840-4D adapter fittings today.
The AN840-4D fitting masterfully combines two different sealing technologies. The AN flare end utilizes a mechanical seal. The 37-degree cone on the fitting provides a surface against which a flared tube or a female hose fitting is clamped by a B-nut, creating a metal-to-metal seal that is highly resistant to vibration and can be reconnected multiple times. The NPT (National Pipe Taper) end, conversely, creates a seal through a principle called "thread interference." Both the male threads on the fitting and the female threads in the port are tapered. As the fitting is tightened, the threads are wedged together, creating a seal through the deformation of the threads themselves. This is why a thread sealant (like Titeshal or Loctite 567) is required on NPT threads to fill any microscopic voids and ensure a perfect, leak-proof seal.
Note: Installation must be performed by a certified A&P mechanic. The flared tube end should never have sealant applied. The NPT end requires an appropriate, fuel/oil-resistant thread sealant. Support the fitting with a wrench during tightening to prevent stress on the component it is threaded into. Do not overtighten NPT fittings, as this can crack the port casting.
