Aerospace O-Rings: Sizes, Compound Specifications and Cure Dating

Updated October 12, 2026

O-ring types and where they are used

Aerospace O-rings are sorted first by compound, because the compound decides which fluids and temperatures the seal survives. Size comes second and is almost always an AS568 dash number.

O-rings used on oxygen, propellant and pressurant systems sit in the same families but carry extra material qualification, covered in the compound section below. For the tube, fittings and test side of those systems, see our tube and fluid systems guide.

How AS568 dash numbers work

AS568 is the SAE size standard. It sets inside diameters, cross sections, tolerances and dash number codes for O-rings and for straight thread tube fitting boss gaskets, and says those dimensions suit any elastomer where tooling exists. The current issue is AS568F, revised in 2020 and reaffirmed in January 2026.

The first digit of a three digit dash number tells you the cross section. Global O-Ring and Seal's AS568 chart lists the series as follows:

The remaining digits step the inside diameter up within the series. An AS568 number alone is only a size. It says nothing about compound, hardness or quality controls, which is why aerospace drawings call a part standard instead.

Decoding a real part number: AS3209-012

• AS3209 is the SAE packing standard for O-rings molded from AMS7276 fluorocarbon rubber. The cancellation notice for MIL-R-83248/1A points future buys to it.
• -012 is the size. Boeing Distribution's listing for AS3209-012 states that it corresponds to AS568-012, with a 0.364 in inside diameter and a 0.070 in cross section, 75 ±5 Shore A.

So one number carries the size, the compound and the controls of the part standard. Not every series maps its dash numbers this neatly, so when you cross a legacy M or MS number to a new standard, match the inside diameter and cross section from the standard sheet rather than copying digits.

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The specifications that govern aerospace O-rings

Many O-ring specifications on older drawings are cancelled or inactive for new design, and the replacement is usually an SAE document that kept the old part numbers. The table shows the status we could verify on SAE, EverySpec copies of DLA notices, or the issuing body.

The practical rule: if a drawing calls a cancelled MIL or MS number, the parts are usually still bought by the same part number under the SAE successor. Confirm which document and revision your program accepts before the PO goes out, because a supplier can legitimately certify to either.

Choosing the compound: fluid, temperature and oxygen service

Pick the compound from the fluid first, then check the temperature range and the compression set the joint can tolerate. Swapping families to save time is the most common way an O-ring fails in service.

• Petroleum hydraulic systems. Nitrile to AMS-P-83461 is the default. Its scope covers MIL-H-5606 and MIL-H-83282 type fluids from -65 to 275°F.
• Fuel and hot oil. Fluorocarbon to AMS7276 handles higher temperatures, but read the revision. The G revision said the material was not recommended in high temperature stabilized (HTS) engine oils, and the H revision kept a warning about HTS oils, naming MIL-PRF-23699 Class HTS among them. Check each application.
• Phosphate ester hydraulics. EPDM is the usual answer, and it must be kept away from petroleum oils and greases. Parker states its NAS1613 EPDM compound is not compatible with mineral oil products. A single wrong lubricant at assembly can ruin a phosphate ester seal.
• Cold fuel and oil service. Fluorosilicone to AMS-R-25988 is the usual choice where nitrile is too stiff at low temperature and fuel resistance is still needed.

Oxygen and LOX service

No elastomer is oxygen compatible by family name alone. Compatibility comes from test data for the specific compound, at your pressure and in your configuration. NASA-STD-6001, currently Revision B with Change 3, sets the materials testing NASA uses for flight hardware and ground support equipment, including mechanical impact tests in liquid and gaseous oxygen. ASTM G86 covers mechanical impact ignition in LOX and GOX from ambient to 10,000 psig, and ASTM itself notes the method ranks materials and may not directly predict behavior in the end use configuration.

For oxygen systems, name the qualified compound on the drawing (by manufacturer compound number if your program approved one), require cleaning to the stated level and keep hydrocarbon lubricants out. Our precision and oxygen cleaning reference covers cleanliness levels, and the engineering plastics page covers PTFE and PCTFE seats used next to O-rings in oxygen valves.

Backup rings

Backup rings are PTFE retainers placed beside an O-ring to stop it extruding into the clearance gap at high pressure. Specify them to the same gland standard as the O-ring, because ring dimensions are tied to the groove.

Two buying notes. First, a scarf cut or spiral ring installs without stretching but can be put in backwards or on the wrong side of the O-ring; drawings should show which side faces pressure. Second, solid rings need the gland designed for them. Do not substitute one style for the other without engineering approval.

Cure date, shelf life and storage

Every elastomer seal should arrive with a cure date, and that date plus the storage history decides whether it can still be installed. SAE AS5316 is the current document for recording, packaging and storing elastomer seals before they go into hardware. It succeeded ARP5316, which was withdrawn, and its packaging requirement exists to keep positive identity from manufacture to installation.

Seal manufacturers commonly treat the last day of the cure quarter as the time of manufacture, so a part cured in the first quarter starts its clock on March 31. Global O-Ring and Seal publishes these AS5316 shelf life recommendations:

The same summary gives storage conditions of 59 to 100°F, relative humidity below 75 percent unless parts are in sealed moisture proof bags, no direct sunlight or UV rich light, and no ozone sources such as mercury vapor lamps or high voltage equipment. "Unlimited" assumes those conditions. Your customer's specification overrides the recommended practice, so put any shelf life limit your program uses on the PO.

Ordering checklist for O-rings

Put the part standard number and the revision on the PO, not a generic description. A line that says "O-ring, Viton, 2-012" invites a commercial part with no traceability.

• Full part number, for example AS3209-012 or M83461/1-xxx, and the revision of the part and material standard you accept
• QPL or QPD source where the specification has one, and the qualified manufacturer's name on the cert
• Manufacturer, compound number and batch or lot, with cure date by quarter and year on the bag label
• Certificate of conformance to the part standard, and the manufacturer's material test report when your quality plan requires it
• Packaging per AS5316: sealed, labeled bags, one lot per bag, no loose parts or rubber bands
• Maximum age at receipt if your program limits it, such as no more than a set fraction of shelf life used
• Oxygen or cleanliness requirements, including the cleaning specification and level, if the parts go into an oxygen or propellant system
• Backup rings called out separately by their own standard

Our certificate of conformance guide shows what a usable cert contains, and the material certs and traceability page covers lot traceability.

Counterfeit and quality risks, and how to receive O-rings

The risk with O-rings is less outright counterfeits and more substitution: a commercial compound in the right size and color, sold with a generic cert, or old stock relabeled with a new date. Visual inspection cannot tell two black elastomers apart.

• Buy from the qualified manufacturer or its authorized distributor. Ask for the chain of custody back to the molder when the source is anyone else.
• Check the bag, not just the cert. The label should carry the part number, compound, lot and cure date, and match the cert exactly.
• Check age at receipt. Compare the cure date to the shelf life your program uses before accepting.
• Measure a sample. Inside diameter and cross section on an optical comparator or with a cone and pi tape catch wrong sizes quickly.
• Hardness check. A Shore A check on a sample is a cheap screen for a wrong compound family.
• Quarantine anything unlabeled. Loose O-rings with no lot identity should not go into flight or pressure hardware.

For broader controls on suspect parts, see our counterfeit parts avoidance process and the receiving inspection guide.

How we source O-rings

We do not stock O-rings or sell from a catalog. We source them for your order through the qualified manufacturer or authorized and franchised distributors, with the paperwork your program requires.

• We flow your part standard, revision, QPL, cure date and packaging requirements to the source and check the cert and bag labels before shipment.
• We can put O-rings and backup rings on the same purchase order as the machined manifold, valve body or tube assembly they seal, or pack them into kits by assembly.
• For legacy part numbers, we work out the current SAE successor with you, and for discontinued compounds we search under the controls on our obsolete parts page.

We do not promise that every compound or size is available, and we will tell you if a qualified source cannot be found. Related parts are on the seals and fluid parts page, including hose assemblies and fluid fittings.

Request O-rings with the right paperwork

Use the fluid systems quote form and list the part standard, dash numbers, quantities, cure date limit and any oxygen cleaning requirement. Sending those up front lets us go straight to qualified sources instead of trading questions. A person replies within one business day, and rush requests are prioritized. If the request ties to export controlled hardware, read our controlled programs process first.

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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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Related

• Aerospace Gaskets and Packings: Port Seals, Crush Gaskets and Static Seals
• Aerospace Hose Assemblies: Hose Families, Specifications, Testing and Age Control
• Aerospace Fluid Fittings: AN Flared, Flareless and Port Connections
• Aerospace Tube Assemblies, Fluid Lines and Pressure System Hardware

Sources

• SAE AS568F: Aerospace Size Standard for O-Rings
• Global O-Ring and Seal: AS568 size chart
• SAE AMS7276J: Fluorocarbon (FKM) rubber for seals
• SAE AS83248/1A: O-Ring Molded from AMS7276 Material
• EverySpec: MIL-R-83248/1A Notice 5 (cancellation)
• SAE AMS-P-83461: Petroleum Hydraulic Fluid Resistant Packing, 275 F
• EverySpec: SAE AS83461/1 adoption notice
• EverySpec: MIL-P-25732C Notice 2
• SAE AMS-P-25732A (cancelled August 2003)
• EverySpec: MS28775E Notice 2 (superseded by SAE AS28775)
• SAE AMS-R-25988: Fluorosilicone rubber
• SAE AS5316: Storage of Elastomer Seals
• Global O-Ring and Seal: O-ring shelf life and AS5316
• Parker: Ethylene Propylene 80 Durometer O-ring EM163-80 (NAS1613)
• EverySpec: MS28774M backup retainer
• EverySpec: MS28782E Notice 1
• CDI Products: Aerospace backup ring standards chart
• ASTM G86: Ignition sensitivity to mechanical impact in oxygen
• NASA-STD-6001B with Change 2
• Boeing Distribution: AS3209-012 O-ring listing