Understanding AN Fittings and Common Contaminants

AN (Army-Navy) fittings are a standardized type of flare fitting used extensively in aviation, motorsports, and high-performance automotive systems. They are prized for their reliability, vibration resistance, and ability to handle high pressures. Typically made from aluminum, steel, or brass, AN fittings can become fouled over time with oil, fuel residue, road grime, oxidation, and even hard mineral deposits from coolant or water systems. The choice of cleaning method depends heavily on both the material of the fitting and the nature of the contamination. Aluminum fittings, for example, require careful handling to avoid caustic damage, while steel fittings may need rust removal.

Common contaminants include:

  • Hydrocarbon residues – motor oil, transmission fluid, fuel varnish, and brake fluid.
  • Oxidation and corrosion – white powdery deposits on aluminum (aluminum oxide) or red rust on steel.
  • Dirt and road grime – abrasive particles that can embed in soft aluminum.
  • Mineral scale – hard, chalky deposits from coolant or hard water rinse.
  • Old thread sealant or tape – leftover PTFE or anaerobic compounds.

Identifying the contaminant is the first step to selecting an effective cleaner, as using the wrong chemical can worsen corrosion or damage the fitting’s sealing surface.

Choosing the Right Cleaning Solution

There is no one-size-fits-all cleaner for AN fittings. Below are the most effective categories, each with specific strengths and cautions.

Degreasing Solvents

For removing oil, grease, and fuel residues, a high-quality solvent is your first line of defense. Common choices include:

  • Brake cleaner – fast-evaporating and very effective at cutting petroleum-based grime. Non-chlorinated versions are safer for aluminum and the environment.
  • Isopropyl alcohol (90% or higher) – gentle on metals yet effective at dissolving light oils and residues. It is safe for all AN fitting materials and leaves no residue.
  • Mineral spirits or kerosene – good for heavy grease but require thorough rinsing and proper ventilation.

Caution: Never use chlorinated solvents on aluminum fittings as they can cause pitting and stress corrosion cracking.

Acidic Solutions for Rust and Scale

Mild acids can safely dissolve rust and mineral deposits without damaging base metal if used correctly.

  • White vinegar (acetic acid) mixed 1:1 with water is an excellent household option for light rust and calcium deposits. Soak for 15–30 minutes, then scrub.
  • Citric acid solution (powdered citric acid dissolved in warm water) is less aggressive than vinegar and works well on aluminum without discoloring it.
  • Commercial rust removers such as Evapo-Rust are safe for steel and some aluminum alloys but always test an inconspicuous area first.

Important: Acidic solutions must be neutralized with a baking soda-water rinse afterward to prevent ongoing corrosion.

Alkaline Cleaners for Aluminum Safety

Aluminum is sensitive to strong acids and strong alkalis. However, mild alkaline cleaners (pH 9–11) formulated for aluminum are effective against oxidation and organic films without etching the surface. Products like Simple Green Aircraft & Precision Cleaner or Mean Green are widely trusted. Always follow dilution instructions and avoid soaking longer than necessary.

Ultrasonic Cleaning Fluids

When using an ultrasonic cleaner (discussed below), the choice of cleaning solution is critical. Dedicated ultrasonic cleaning fluids for metals (e.g., Kester 5760 or L&R #111) provide the right balance of cavitation efficiency and metal safety. Alternatively, a dilute ammonia-free glass cleaner or a 5% Simple Green solution works for many applications.

Step-by-Step Cleaning Guide

Safety First

Before handling any chemical, wear nitrile gloves, safety glasses, and work in a well-ventilated area. If you have anodized AN fittings, be aware that strong cleaners can strip the anodizing, so test a hidden area first.

Disassembly

If the fittings are still installed, remove them from hoses or components carefully. Use two wrenches to avoid twisting the hose. Wrap soft-jawed pliers or use aluminum-friendly tools to prevent marring the hex surfaces. Inspect threads and sealing cones for damage during disassembly.

Pre‑Cleaning

Rinse the fittings with warm water or blast them with a parts washer to remove loose dirt and grit. This step prevents abrasives from being ground into the metal during scrubbing.

Application and Soaking

Apply your chosen cleaning solution. For parts with stubborn deposits, submerge them in a plastic or stainless steel container with the solution. Soaking times vary:

  • Light oil/grease: 5–10 minutes in solvent.
  • Rust or scale: 15–30 minutes in vinegar or citric acid, checking progress every 5 minutes.
  • Heavy carbon/varnish: up to 1 hour in a degreaser, possibly with ultrasonic assist.

For tight internal passages, use a small brush or a syringe to force solution into the fitting’s bore.

Scrubbing

After soaking, scrub all surfaces with a soft-bristle brush (nylon or brass) – avoid steel wire brushes on aluminum. Pay special attention to the sealing cone and threads. A dental pick or small brass wire brush can help remove caked-on debris inside the flare seat.

Rinsing and Drying

Thoroughly rinse the fittings with clean water – deionized or distilled water is ideal to avoid spotting. If you used an acid or alkali, follow with a baking soda neutralizer (1 tablespoon per quart of water) and then a final water rinse. Blow dry with compressed air, or place in a low-temperature oven (150°F) for 10–15 minutes. Never air-dry without blowing out internal cavities – water trapped in a fitting can cause internal rust.

Final Inspection

Inspect each fitting under good light. Look for:

  • Cracks or stress marks on the hex or tube nut.
  • Pitting or deep corrosion on sealing surfaces.
  • Stripped or damaged threads.
  • Any leftover anodizing flaking.

If you find severe damage, replace the fitting – cleaning cannot restore structural integrity.

Ultrasonic Cleaning – The Professional Approach

An ultrasonic cleaner is the most thorough method for restoring AN fittings, especially when multiple fittings are involved. The microscopic cavitation bubbles generated by high-frequency sound waves penetrate every crevice, including the 37‑degree sealing cone and internal bore. This method is ideal for removing baked-on varnish, thread locker residue, and fine scale that brushing cannot reach.

For best results:

  • Use a basket to keep fittings separated – avoid contact that can dampen cavitation.
  • Heat the cleaning solution to 120–140°F (most units have built-in heating).
  • Run a 10–15 minute cycle, then rinse and inspect.
  • For heavily soiled fittings, repeat with fresh solution.

Always use a cleaning solution compatible with your fittings’ material. Ultrasonic cleaners can quickly damage metals if aggressive chemistry is used. Never use flammable solvents in an ultrasonic bath – the ultrasonic action can vaporize the solvent and create a fire hazard.

For more on ultrasonic cleaning best practices, the CTG Clean blog offers detailed guidance.

Maintenance After Cleaning

Restoring AN fittings is only half the battle – keeping them clean extends their life and prevents future failures.

  • Apply a corrosion inhibitor: After drying, spray or wipe a light coating of Boeshield T‑9 or WD‑40 Specialist Corrosion Inhibitor onto the fitting. Wipe off excess; you only need a thin film.
  • Lubricate threads: For reassembly, apply a small amount of anti-seize compound (copper-based for steel fittings, nickel-based for stainless) or a lightweight oil. Never use grease that can contaminate the fluid system.
  • Store properly: Place clean fittings in sealed plastic bags or a parts cabinet with desiccant packs. Avoid storing them in damp garages or basements.
  • Reinspect periodically: If used in a race vehicle or aviation application, inspect fittings every season or annually – look for any sign of corrosion, thread galling, or sealing surface wear.

For maintenance of assembled systems, consider using a line of specialized AN fitting maintenance products from AN Plumbing.

When to Replace vs. Clean AN Fittings

Not every dirty fitting can or should be restored. Replace a fitting if:

  • There are visible cracks or deformation.
  • Threads are cross-threaded, stretched, or missing more than two consecutive threads.
  • The sealing cone is deeply scratched, gouged, or has a raised burr.
  • The fitting shows signs of intergranular corrosion (a white powdery dust that cannot be scrubbed away).
  • Anodizing is completely worn and the bare aluminum is heavily pitted.

Fittings that pass inspection can almost always be cleaned and reused. In fact, many vintage aircraft and hot-rod builders routinely restore 40‑year‑old AN fittings to like-new condition using the methods above. A helpful guide on AN fitting restoration can be found at AvWeb’s article on AN fitting restoration.

By selecting the right cleaning solution and following a disciplined process, you can bring old or dirty AN fittings back to full service life – saving money, reducing waste, and maintaining the high standards your system requires. Whether you are a professional mechanic or a weekend project builder, investing a little time in proper cleaning pays off in reliability and safety.