Aerospace Fluid Fittings: AN Flared, Flareless and Port Connections
Aerospace fluid fittings join rigid tube to tube, hose and ports. The two main systems are 37 degree flared fittings, the AN818 nut and AN819 sleeve family bought under SAE AS4841, and flareless fittings, the former MS21921 nut and MS21922 sleeve now under SAE AS21921, AS21922 and the AS18280 procurement specification.
Updated
Fitting types and where they are used
Pick the fitting system to match the tube and the mating hardware already on the drawing. Flared and flareless joints seal in different ways, and the parts of one system do not belong in the other.
| System | How it seals | Where it is used |
|---|---|---|
| 37 degree flared (AN and AS) | Tube end flared to 37 degrees, clamped metal to metal between the fitting cone and a sleeve by the nut | Hydraulic, fuel, oil and pneumatic lines on aircraft, launch vehicles, GSE and test stands |
| Flareless (MS, now AS) | A sleeve is preset onto unflared tube and bites or seats as the nut is tightened | Higher pressure lines and thin wall or hard tube that does not flare well |
| Straight thread port (AS5202) | Fitting threads into a port and seals with a boss packing O-ring | Manifolds, valve bodies, pumps and actuators |
| Welded and brazed joints | Permanent joint, no separable seal | Lines where leak paths must be minimized; covered in our tube and fluid systems guide |
Fitting shapes within each system include unions, elbows, tees, crosses, reducers, bulkhead fittings, plugs and caps, and tube to boss adapters. A 37 degree AN fitting and a 45 degree SAE flare fitting are not interchangeable because the flare angles differ.
The specifications that govern fluid fittings
Most MS fitting sheets have been superseded by SAE AS sheets with the same base number, so an MS21921 nut is now bought to AS21921. The table shows the status we could verify on SAE and EverySpec copies of DLA records.
| Document | What it covers | Status as verified |
|---|---|---|
| SAE AS4841 | Fittings, 37 degree flared, fluid connection: requirements for 37 degree flared tube or machined cone fittings | Revision D, April 2021 |
| SAE AS4395 | Fitting end, flared tube connection, design standard | Revision C, June 2025; superseded MS33656 |
| AN818 / AN819 | 37 degree flare coupling nut and sleeve | Still bought by AN number; one nut and one sleeve per flared tube end |
| SAE AS18280 | Fittings, flareless tube, fluid connection: requirements for flareless fittings, nuts and sleeves | Revision H, May 2021 |
| MS21921 | Nut, sleeve coupling, flareless | Superseded by SAE AS21921 (revision D, November 2025) |
| MS21922 | Sleeve, coupling, flareless | Superseded by SAE AS21922 |
| MS21913, MS21914, MS21915, MS21916 | Flareless plug, pressure seal cap, straight tube to boss adapter, reducer | Each superseded by the SAE AS sheet with the same number |
| SAE AS5202 | Port or fitting end, internal straight thread, design standard | Replaced MS33649 |
Flare geometry, port design and the AN versus JIC substitution question are already covered on our fluid systems guide, so this page focuses on the parts themselves and how to buy them.
Have a part like this?Send it in under a minute. A person replies within 1 business day.How the part number is built: AN818-4D
AN fitting part numbers carry the fitting type, the tube size and a material code. Aircraft Spruce's AN818 table lays it out clearly, and it is a good model for reading the family.
- AN818 is the 37 degree flare coupling nut.
- -4 is the tube outside diameter in sixteenths of an inch, so 1/4 in tube, which the table pairs with a 7/16-20 thread. The table runs from -2 (1/8 in tube, 5/16-24 thread) to -16 (1 in tube, 1-5/16-12 thread).
- D is the material code for aluminum alloy. The same table lists the nut with no letter as steel, and lists a J suffix for its stainless steel version.
So AN818-4D is an aluminum nut for 1/4 in tube, and AN818-4 is the steel nut of the same size. Do not assume a letter means the same material in every standard; read the standard sheet for the part you are buying, and require the cert to name the alloy. The matching sleeve is AN819 in the same dash size, and the flared tube end is formed to AS4395 geometry.
Flareless fittings: MS21921 and MS21922 parts
A flareless joint uses a nut and a sleeve on unflared tube. The sleeve is preset onto the tube with a presetting tool before final assembly, and the preset is what makes the joint repeatable. Flareless parts formerly bought as MS21921 nuts and MS21922 sleeves are now covered by SAE AS21921 and AS21922, under the AS18280 procurement specification.
- Preset matters. Ask how the source presets sleeves and how it inspects the preset, since an underset or overset sleeve may seal at assembly and leak at proof or in vibration.
- Do not mix manufacturers' sleeves and nuts on critical lines without approval. Parts made to the same AS sheet are interchangeable by design, but many programs qualify a specific combination.
- Bulkhead and cluster fittings. Many flareless bulkhead, tee, elbow and cluster fittings were also MS sheets and now carry AS numbers; check each line on the drawing.
Ports and seals
Where a fitting threads into a component, the port design decides the seal. AS5202 straight thread ports seal with a boss packing O-ring, not a cone, and the fitting end must be made to suit the port. Ports machined to the wrong profile will not seal, so machined manifolds need port inspection, not just thread gauging.
Boss packings such as AS28778 (formerly MS28778) parts are covered on our gaskets and packings page, and O-ring compounds on the O-rings page. Order the packing with the fitting so the dash sizes match.
Materials, finishes and the trade-offs
Fitting material follows the tube and the system: pressure, temperature, fluid, weight, and what the fitting touches. The usual choices are aluminum alloy, carbon or alloy steel, corrosion resistant steel and titanium.
| Material | Strengths | Watch for |
|---|---|---|
| Aluminum alloy | Light weight, matches aluminum tube | Lower strength and temperature limit; galling and thread damage if overtorqued; galvanic attack when coupled to stainless or steel in a wet environment |
| Steel | Strength and low cost | Needs a corrosion protective finish; finish choice affects torque and galvanic behavior |
| Corrosion resistant steel | Corrosion resistance, higher temperature, common on GSE and test stand lines | Weight; galling on threads without the right lubricant; specialty metals rules on defense contracts |
| Titanium | High strength to weight | Cost and availability; galling; compatibility limits in some fluids, including oxygen service, which needs engineering review |
Two practical rules. Match the fitting material to the tube unless the drawing deliberately mixes them, because the nut and sleeve see the same corrosion environment as the tube. And treat finish as part of the part number: a different plating or anodize changes friction, torque and galvanic behavior. The aluminum alloys, stainless steels and titanium alloys pages go deeper on the materials.
Color coding
Anodized aluminum AN fittings are often blue, and some buyers read color as a material code. We could not find a current fitting standard that defines color as the way to identify material, so do not accept or reject fittings by color. Read the part number suffix, check the cert, and use an alloy check at receiving when the source is unfamiliar.
Cleanliness for oxygen service
Fittings for oxygen, propellant and pressurant systems must be cleaned to a stated level and stay clean until assembly. Machining oils, thread lubricants and particles trapped in threads and sleeve bores are the usual contaminants.
- Put the cleaning specification and level on the PO, not "clean for oxygen".
- Specify any thread lubricant by name, and only one approved for oxygen service.
- Require individual bagging with the cleanliness level and date on the label, and caps or plugs on open ports.
- Expect fittings to be recleaned if a bag is opened before use.
Our precision oxygen cleaning service handles cleaning and certification, and the cleaning reference explains the levels.
Ordering checklist for fluid fittings
- Full part number with dash size and material code, for example AN818-4D, and the standard and procurement specification revision you accept, such as AS4841 or AS18280
- Current SAE successor for any MS number on the drawing, confirmed with your engineering authority
- Manufacturer and lot or heat traceability, with material certs for the bar or forging
- Finish and finish specification
- DFARS specialty metals clause flowed down for steel, stainless and titanium parts where the contract carries it; see our specialty metals page
- Matching nuts, sleeves and boss packings called out as separate lines
- Cleaning specification, level and packaging for oxygen or propellant service
- Packaging that protects threads, flare faces and sleeve edges
- Certificate of conformance to the PO and standard; see our C of C guide
Counterfeit and quality risks, and how to receive fittings
The common problems are commercial JIC parts sold as AN or AS parts, wrong alloys, parts with remarked numbers, and surplus fittings with no traceability. Because fittings look alike across systems, the part number and cert carry most of the proof.
- Buy from the manufacturer or authorized and franchised distributors, and ask for chain of custody otherwise.
- Check marking against the standard, thread size and class with gauges, and flare cone or sleeve seats for nicks and tool marks.
- Verify alloy by a portable analyzer or conductivity check when the source is not the manufacturer.
- Reject parts with damaged sealing surfaces even if dimensions pass; a scratched cone leaks.
- Quarantine mixed or unlabeled lots.
See our counterfeit parts avoidance process and the receiving inspection guide.
How we source fluid fittings
We do not stock fittings or publish prices. We source them for your order from the manufacturer or through authorized and franchised distributors, flow down the standard, material, finish, specialty metals and cleaning requirements, and check the paperwork before shipment.
Fittings rarely ship alone, so we can put them on one purchase order with the tube assemblies, hose, seals and machined manifolds they connect, kit them by line or by panel, and have the kit cleaned and bagged together. Non standard fittings can be machined to your drawing by qualified shops. For obsolete MS or vendor fittings, we search under the controls on our obsolete parts page. The rest of the family is on the seals and fluid parts page, including hose assemblies.
Request fluid fittings
Send the part numbers or drawings, quantities, materials, finish and any oxygen cleaning requirement through the fluid systems quote form. Listing the procurement specification and revision up front avoids substitutes. A person replies within one business day, and rush requests are prioritized. Use our confidentiality process for proprietary drawings.
Send the job. A person replies within 1 business day.
Part, quantity and date are enough to start. Drawings and the mutual NDA follow by reply, and controlled programs never send technical data through this site.
We have it.
Reference . A person replies within 1 business day. Watch for our email from [email protected].
Questions
What replaced MS21921 and MS21922 flareless fittings?
EverySpec copies of the DLA records show MS21921 (nut, sleeve coupling, flareless) superseded by SAE AS21921 and MS21922 (sleeve, coupling, flareless) superseded by SAE AS21922. SAE lists AS21921 revision D from November 2025. Flareless fittings, nuts and sleeves are procured under SAE AS18280, revision H from 2021. Programs often still write the MS number on the drawing, so confirm the accepted successor before ordering.
What does the D mean in AN818-4D?
In the AN818 table published by Aircraft Spruce, D is the material code for aluminum alloy, and a nut with no letter is steel. The -4 is the tube size in sixteenths of an inch, so 1/4 inch tube with a 7/16-20 thread. Material letters are not universal across every standard, so read the sheet for the part you are buying and make the cert name the alloy.
Can I use JIC fittings in place of AN fittings?
Not where the drawing calls AN or AS parts. Both use a 37 degree flare, but AN and AS fittings are made to aerospace standards with tighter thread and quality requirements and are bought under procurement specifications such as AS4841. JIC industrial fittings are not built to those requirements. Substituting them on flight or pressure hardware needs engineering approval, and on many programs it is simply not allowed.
Are 37 degree and 45 degree flare fittings interchangeable?
No. AN and AS flared fittings use a 37 degree flare, while SAE automotive and refrigeration flare fittings use 45 degrees. The cone angles do not match, so the joint will not seat or seal correctly. Keep tooling separate as well, because a 45 degree flaring tool produces the wrong tube end for an AN fitting.
Can fitting color tell me what material it is?
Do not rely on it. Aluminum AN fittings are often anodized blue, but we could not find a current fitting standard that makes color the material identifier, and finishes vary by manufacturer and specification. Identify material by the part number suffix, the certificate of conformance and material certs, and use an alloy check at receiving when the source is not the manufacturer or its authorized distributor.
Do fittings for oxygen systems need special cleaning?
Yes. Oxygen system fittings must be cleaned to a specified level with documented verification, then bagged and capped until assembly. Thread lubricants must be approved for oxygen service. Put the cleaning specification and level on the purchase order rather than a general note, and expect fittings to be recleaned if a bag is opened early. Our precision oxygen cleaning page explains the process and paperwork.
Related
Sources
- SAE AS4841: Fittings, 37 Degree Flared, Fluid Connection
- SAE AS4395: Fitting End, Flared Tube Connection, Design Standard
- SAE AS18280: Fittings, Flareless Tube, Fluid Connection
- SAE AS21921: Nut, Sleeve Coupling, Flareless
- EverySpec: MS21910 to MS21921 listing (MS21921 superseded by SAE AS21921)
- EverySpec: MS21922 to MS21939 listing (MS21922 superseded by SAE AS21922)
- Aircraft Spruce: AN818 coupling nut size and material table
- SAE AS5202: Port or Fitting End, Internal Straight Thread, Design Standard
- Wikipedia: AN thread (37 versus 45 degree flares, dash sizes)
- SAE AS28778A: Packing, Preformed, Straight Thread Tube Fitting Boss
Ready to
source it?
A person replies within 1 business day. NDA first.
Request a quote