Aerospace Gaskets and Packings: Port Seals, Crush Gaskets and Static Seals

Updated October 12, 2026

Gasket and packing types and where they are used

Choose the seal type from the joint, not the other way round. A straight thread port, a pipe flange and an avionics cover each need a different seal, and the drawing or the fitting standard usually decides it.

Plain O-rings in glands, including AS568 sizing and compound choice, are on our O-rings page. This page covers the rest of the static seal family.

Seals for flared and flareless fittings

A 37 degree flared or flareless tube joint seals metal to metal and needs no gasket at the tube end. The seal question comes where the fitting screws into a port: straight thread ports to AS5202 seal with a boss packing, an O-ring sized for the port and compressed by the fitting's jam nut or shoulder.

The most common boss packing is the nitrile part formerly called MS28778. SAE AS28778 replaced MS28778, kept the MS28778 part numbers, and added visual quality, lot, packaging and labeling requirements for O-rings molded from AMS-P-5510 nitrile rubber. AMS-P-5510 is titled for Type I hydraulic service from -65 to 160°F. The current AS28778 revision we found is C, dated November 2021.

Higher temperature or different fluid service uses other boss packing standards: AS83461/2, which replaced AMS-P-83461/2 in 2009 for M83461 nitrile boss packings, and AS3208 for fluorocarbon boss parts, which the MIL-R-83248 cancellation notice names for straight thread and boss gasket parts. Boss backup rings exist too, such as MS28773 PTFE retainers.

Decoding a real part number: MS28778-4

• MS28778 identifies the boss packing family, now procured under SAE AS28778 with the part number retained.
• -4 is the tube size in sixteenths of an inch. Boeing Distribution lists MS28778-4 for a 1/4 in tube OD and states the family supersedes AN6290 gaskets.
• Textron Aviation's parts listing gives MS28778-4 an inside diameter of 0.351 in, the same nominal ID as AS568-904, which Freudenberg lists at 0.351 by 0.072 in.

That dimensional match explains why boss packings are often cross referenced to AS568 900 series sizes. The compound is not the same, though: an AS568-904 ring could be fluorosilicone or anything else. Order by the part standard. Port and fitting end design standards are covered on our fluid fittings page.

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The specifications that govern gaskets and packings

The table lists the documents we could verify, with status from SAE, EverySpec copies of DLA records, ASME or manufacturer technical pages.

Crush gaskets and bonded seals

Crush gaskets and bonded seals both seal a flat face under a fastener or fitting head, but they work differently and are not interchangeable.

Crush gaskets are soft metal rings, usually copper or aluminum, that flow into the surface finish as they are torqued. MS35769 copper gaskets are the common military part; distributor listings show a 1/8 in ring width and 3/32 in thickness across many dash numbers, varying in inside diameter. Treat them as single use. A crushed gasket has already work hardened and will not reseal reliably. Confirm the temper if the drawing requires annealed material, since listings we read do not state it.

Bonded seals, often called Dowty washers, are a metal washer with an elastomer ring molded to its inside edge. Parker's NAS1523 Stat-O-Seal uses a 4130 steel retainer to AMS 6350, heat treated to Rockwell C26 to C33, and works by controlled confinement: the rubber is compressed only down to the retainer thickness, so no groove is needed in the mating flange. Parker notes the fastener head diameter must exceed the seal's maximum E diameter for the principle to hold.

Because the bonded seal has an elastomer, it carries a compound choice and a cure date just like an O-ring. Galvanic contact matters for both: a steel retainer or copper gasket against aluminum in a wet environment needs a finish or isolation that the drawing specifies.

Flat, molded, metal and spiral wound gaskets

Flat and molded elastomer gaskets are bought to a part drawing plus a material specification, so the drawing has to carry everything: material and revision, hardness, thickness tolerance, flash limits and cure date marking. Fluorosilicone gaskets, for example, are often bought to AMS-R-25988, whose scope covers molded parts, sheet, strip and extrusions for oil and fuel resistant service.

Metal and spiral wound gaskets show up mostly on ground support equipment, test stands and facility piping, where flanges follow ASME B16.5 or B16.47. ASME B16.20 sets materials, dimensions, tolerances and markings for ring joint, spiral wound and grooved metal gaskets for those flanges. On a spiral wound gasket, specify the winding metal, the filler and the inner and outer ring materials, because a stainless winding with a carbon steel outer ring is a different part from an all stainless one.

For oxygen piping, gasket fillers and any lubricant must be compatible with oxygen at the system pressure, and the gasket must be cleaned with the line. Our precision oxygen cleaning service page covers how cleaned parts are bagged and certified. Ground side builds are covered on our ground support equipment page.

PTFE and spring energized seals

When an elastomer cannot take the temperature, the fluid or the pressure, the usual step is a PTFE based seal: a plain PTFE backup ring next to an O-ring, or a spring energized seal with a PTFE or polymer jacket held against the gland by a metal spring.

Spring energized seals keep sealing force at cryogenic temperatures, where PTFE contracts and an elastomer would go glassy. Published ratings vary by profile and jacket: Marco Rubber lists standard spring energized seals from -280 to 500°F, and James Walker cites cryogenic duty to -260°C for one of its families. Those are manufacturer figures for specific products. For your joint, get the rating for the exact part number and confirm it was tested in your medium.

Most spring energized seals are bought to a manufacturer part number or a customer drawing rather than an SAE part standard. That makes the PO more important: name the manufacturer and part number, the jacket material and filler, the spring alloy and the cleanliness level if the seal goes into oxygen or propellant service. For backup rings, use the standards on our O-rings page.

Conductive EMI gaskets

EMI gaskets seal an enclosure joint against moisture and pressure while giving a low resistance electrical path across the flange. MIL-DTL-83528 is the military specification for conductive elastomeric shielding gaskets, currently at revision G from January 2017, with slash sheets for shapes such as the O-ring sheet MIL-DTL-83528/2.

The base elastomer is silicone or fluorosilicone, filled with silver or silver plated particles, and the lettered material types set hardness, shielding effectiveness and temperature range. A supplier stating that a material "meets" MIL-DTL-83528 is not the same as being on QPL-83528; if your program requires qualified product, check the QPL. Mating flange finish matters as much as the gasket: a chromate conversion coated aluminum flange behaves differently from an anodized one. Our surface finishing reference covers those finishes, and connector and harness parts are covered under electrical parts.

Cure dating and storage for elastomer gaskets

Any gasket or packing with an elastomer element needs a cure date and controlled storage, the same as an O-ring. SAE AS5316 covers recording, packaging and storage of elastomer seals and seal assemblies with an elastomer element before hardware assembly, which includes bonded seals and boss packings. Shelf life by compound and the storage conditions are summarized on our O-rings page; we do not repeat them here.

Two gasket specific points. Molded gaskets cut from sheet should trace to the sheet's batch and cure date, not to the date they were cut. And kitted seal sets for a rebuild should keep each part in its own labeled bag inside the kit, so the oldest cure date in the kit is visible.

Ordering checklist for gaskets and packings

• Full part number and the standard and revision you accept, for example MS28778-4 procured to AS28778, or the drawing number and revision for a custom gasket
• Material specification and compound, with QPL or QPD source where one exists, such as QPL-83528 for EMI gaskets
• Manufacturer, lot or batch, and cure date by quarter and year for any elastomer
• Certificate of conformance, plus material certs for metal gaskets and bonded seal retainers
• DFARS specialty metals clause flowed down where the contract carries it and the part contains a covered metal, such as a steel retainer; see our specialty metals page
• Finish on metal retainers or gaskets, and any galvanic isolation requirement
• Packaging per AS5316 for elastomer parts, individual bags for cleaned parts, and the cleanliness level on the label
• Single use or reuse instruction for crush gaskets on the installation drawing

Counterfeit and quality risks, and how to receive them

Gaskets and packings are cheap, which is exactly why off spec substitutes appear: a commercial boss O-ring in place of an AS28778 part, an aluminum crush washer in place of copper, a bonded seal with a commercial retainer.

• Buy from the qualified manufacturer or its authorized distributor, and ask for chain of custody when you cannot.
• Match the bag label to the cert: part number, lot, compound and cure date.
• Check a sample's dimensions and, for elastomers, hardness. For metal gaskets, check material by a quick conductivity or alloy check when the source is unfamiliar.
• On EMI gaskets, a volume resistivity check on a sample catches unfilled or underfilled material.
• Reject loose, unlabeled or mixed lot parts for flight and pressure hardware.

Our counterfeit parts avoidance process and receiving inspection guide go further.

How we source gaskets and packings

We do not hold stock or publish prices. We source gaskets and packings for your order through the manufacturer or authorized and franchised distributors, flow down the standard, revision, cure date and packaging requirements, and check the paperwork before shipment.

Where it helps, we put the seals on one purchase order with the parts they seal, such as a machined manifold, a tube assembly or a GSE panel, or build them into kits by assembly. Custom molded and EMI gaskets can be made to your drawing by qualified molders. For discontinued legacy gaskets, we search under the controls on our obsolete parts page. Browse the rest of the family on the seals and fluid parts page, including hose assemblies.

Request gaskets and packings

Send the part numbers or drawings, quantities, cure date limit and any cleaning or QPL requirement through the fluid systems quote form. If the seals go with machined or tube hardware, include that too, so it can all be quoted on one PO. A person replies within one business day, and rush requests are prioritized. Use our confidentiality process for proprietary drawings.

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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.

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Questions

Related

• Aerospace O-Rings: Sizes, Compound Specifications and Cure Dating
• Aerospace Fluid Fittings: AN Flared, Flareless and Port Connections
• Aerospace Hose Assemblies: Hose Families, Specifications, Testing and Age Control
• Aerospace Tube Assemblies, Fluid Lines and Pressure System Hardware

Sources

• SAE AS28778A: Packing, Preformed, Straight Thread Tube Fitting Boss (revision history)
• SAE AMS-P-5510: Boss packing, Type I hydraulic, -65 to 160 F
• SAE AMS-P-83461/2A record (replaced by AS83461/2)
• EverySpec: MIL-R-83248/1A Notice 5 (cancellation, AS3208 and AS3209)
• Boeing Distribution: MS28778-4 O-ring listing
• Textron Aviation parts: MS28778-4
• Freudenberg: O-ring -904, 8.915 x 1.829 mm
• Boeing Distribution: MS35769-11 copper crush gasket
• Aircraft Spruce: NAS1523 Stat-O-Seal
• GlobalSpec: ASME B16.20 Metallic Gaskets for Pipe Flanges
• EverySpec: MIL-DTL-83528G
• SSP: MIL-DTL-83528 and QPL 83528
• Marco Rubber: Spring energized seals
• James Walker: Seals in PTFE and engineered thermoplastics
• SAE AS5316: Storage of Elastomer Seals and Seal Assemblies
• SAE AMS-R-25988: Fluorosilicone rubber