Buying aircraft O-rings and seals: part numbers, compounds, cure dates and what to put on the RFQ

The short answer

• Order the full part number from the IPC or CMM, prefix and dash number included. The prefix carries the compound and the fluid; the dash number carries the size.
• An AS568 dash number is a size, not a part. MS28775-012, MS29513-012 and M83248/1-012 are the same nominal size in three different rubbers.
• Hydraulic fluid families do not mix. The FAA's airframe handbook says seals for petroleum-based and phosphate ester-based fluids are not usable with or tolerant of each other's fluid.
• Ask for the cure date and the certificate on every seal line. In the supplier quotes we received, most seal lines stated neither.

Packing, gasket, O-ring: the words on the part number

The FAA's Aviation Maintenance Technician Handbook – Airframe (FAA-H-8083-31B) uses the words this way: hydraulic seals on a sliding or moving assembly "are normally called packings", seals between non-moving fittings and bosses "are normally called gaskets", and "most packings and gaskets used in aircraft are manufactured in the form of O-rings." That is why the title on an MS or NAS O-ring sheet usually reads "packing, preformed" rather than "O-ring".

For a buyer, the practical split is between two kinds of ring:

• Gland O-rings, sized by the AS568 dash numbers (-001 to -4xx), used in grooves in pistons, rods, covers and fittings.
• Boss gaskets for straight-thread tube fittings, sized by tube diameter. SAE AS568 lists them in a separate 900 series, where the dash number "designates the tube size in 16th's of an inch". MS28778, MS29512, NAS1612 and AS3208 are boss-gasket families.

Back-up rings, which stop an O-ring extruding under pressure, "are not color coded or otherwise marked and must be identified from package labels", the FAA handbook notes.

Why "same size" is not interchangeable

SAE AS568, the aerospace size standard for O-rings, is explicit about what it covers. Its scope specifies "the inside diameters, cross-sections, tolerances, and size identification codes (dash numbers)", and says the dimensions "are suitable for any elastomeric material". Each dash number "should be appended to an appropriate Drawing or Standard number". In other words, AS568 tells you the ring's dimensions. The drawing or standard number in front of the dash, such as MS28775, is what tells you what the ring is made of.

The compound matters because each elastomer resists some fluids and is attacked by others. Parker's O-Ring Handbook, for example, lists ethylene propylene (EPR, EPDM) as "particularly useful when sealing phosphate-ester hydraulic fluids" and, in the same entry, as not compatible with "mineral oil products (oils, greases and fuels)". Nitrile (NBR) is the reverse: resistant to petroleum and mineral oils and to many fuels.

The FAA handbook draws the conclusion for aircraft hydraulics:

• "Seals for systems that use MIL-H-5606 are made of neoprene or Buna-N. Seals for phosphate ester-based fluids are made from butyl rubber or ethylene-propylene elastomers."
• Petroleum-based and phosphate ester-based fluids "will not mix; neither are the seals for any one fluid usable with or tolerant of any of the other fluids."
• On brakes: contaminated brake or hydraulic systems "must have all of the fluid evacuated and all seals replaced before the aircraft is released for flight."

So a nitrile MS28775-012 and an ethylene propylene ring of the same -012 size are not alternates, however well one fits the groove. MIL-PRF-5606 (the current designation of the red petroleum-based fluid) and phosphate ester fluids such as Skydrol sit in different systems, and a ring meant for one will fail in the other. Fuel and engine oil have their own compounds again.

Color marks do not settle it either. The FAA handbook describes the stripe and dot codes some makers apply (blue for MIL-H-5606 compatibility, a green stripe for phosphate ester, red for hydrocarbon fuel) and then warns that color coding "is not a reliable or complete means of identification", does not identify size, and "provides no means to establish the age of the O-ring or its temperature limitations." Its advice: "the basic part number on the sealed envelope provides the most reliable compound identification."

The common families at a glance

The table below summarizes what the governing documents and manufacturer references say about the families that show up most often on airline and MRO seal lists. It is a map for reading part numbers, not a substitution chart: the part to buy is the one your IPC, CMM or engineering data calls out.

Many documents behind these rings have been cancelled or handed to SAE while the MS part number stays in use, so a certificate citing an AMS or AS document for an MS part is normal if it is the document that governs the part ordered. A replacement listed in one catalog is that manufacturer's statement for its own aircraft; whether you may use it is decided by your approved data (see alternate and interchangeable part numbers).

Kits, singles and pack quantities

Seals reach a buyer in three forms, and each is quoted differently.

• Singles by standard part number (MS28775-214, NAS1611-012). Small rings are often sold by the bag or with a minimum quantity, so a line for four rings can come back as a pack of more. Ask for the pack size and the minimum on the quote, and say whether a whole bag is acceptable; the manufacturer's original sealed package is also what carries the cure date and batch.
• Component manufacturers' own seal part numbers. A pump, actuator or valve CMM may call out a seal by the component maker's number rather than an MS or NAS number. Quote that number. If a standard-part equivalent is offered, it is an alternate and needs the same approval as any other.
• Seal kits under a single kit part number, typically sold for a component overhaul or a brake. A kit is convenient and keeps the right compounds together, but check what it contains against the CMM before relying on it, and ask for the cure date of the kit or its oldest item. For components with internal seals, see our guide to buying hydraulic pumps and valves.

The FAA handbook's advice on brake seals applies to all three: "Acquire seals by part number in a sealed package from a reputable supplier to avoid bogus seals and ensure the correct seals for the brake assembly in question."

NE or NS for a seal

Seals are almost always sold new, and the question is which kind of new. In our quote data, 64% of seal lines were offered NE and 28% NS. NE is new from the manufacturer or its distribution; NS, new surplus, is unused stock that has passed through other owners, as our guide to condition codes explains.

For most parts the difference is mainly about paperwork. For a seal it is also about age. Elastomers age on the shelf, and neither code says when the ring was made. An NS ring from a sealed, labeled manufacturer's package with a recent cure date can be a perfectly good buy; an NS ring from an unmarked bag has no cure date anyone can prove and no compound identification the FAA handbook would accept. The condition code narrows the question; the cure date and the package answer it.

The cure date: the field quotes leave out

Elastomer seals are dated by the quarter in which the rubber was cured, written like 2Q24. The FAA handbook tells technicians to check that new brake seals "have not exceeded their shelf life, which is typically three years from the cure date"; the limit that applies to you comes from your quality manual, the CMM or your customer's contract. How the quarter is counted, which standards apply (MIL-STD-1523A is cancelled; SAE AS5316 covers seal storage) and how to set a minimum remaining shelf life are in our guide to aircraft parts shelf life.

The date is usually printed on the package and simply not typed into the quote. But it means two seal quotes are rarely comparable as they arrive: the cheaper one may be the older one. Ask for the cure date before you compare prices.

What to put on a seal RFQ

A seal line gets a usable answer when it leaves nothing to guess. An example:

And a checklist before you send it:

1. Part number exactly as in the IPC or CMM, prefix, slash and dash number included. Do not shorten MS28775-012 to "-012" or to an AS568 size.
2. A description that names the service (hydraulic, phosphate ester, fuel, boss), so a mismatch between number and description is caught by the seller before it is caught at your dock.
3. Condition: NE, NS or either.
4. Cure date and batch of the stock quoted, and a minimum remaining shelf life if your procedures set one.
5. The certificate you need: manufacturer's certificate of conformance, distributor CoC, 8130-3 or EASA Form 1.
6. Original sealed, labeled packaging, one batch per package.
7. Alternates: none, or the specific number you accept, line by line.
8. Pack size and minimum order per line, so a low unit price is not a large minimum in disguise.

Carry the same requirements onto the purchase order, and check them at receipt: part number on the envelope against the PO, batch and cure date against the certificate, package intact. Our guides to writing an aircraft parts RFQ and receiving inspection cover the rest of the line and the dock.

Sources

• FAA-H-8083-31B, Aviation Maintenance Technician Handbook – Airframe — FAA (chapter 12: seals, O-rings, O-ring color coding, seal materials, intermixing of fluids; chapter 13: brake seals).
• AS568, Aerospace Size Standard for O-Rings — SAE International (current revision F; scope and 900-series wording quoted from revision B, 2001).
• AS28775A, Packing, Preformed – MS28775 O-Ring — SAE International, reaffirmed February 2025.
• MS28775E, Packing, Preformed, Hydraulic, +275 °F — EverySpec (cancelled, superseded by SAE AS28775).
• MIL-P-25732C, Packing, Preformed, Petroleum Hydraulic Fluid Resistant, 275 °F and Notice 2 (7 August 2003) — EverySpec.
• MIL-P-5315B, Packing, Preformed, Hydrocarbon Fuel Resistant — EverySpec (sizes of MS29512 and MS29513; superseded by SAE AMS-P-5315).
• MS29513C, Packing, Preformed, Hydrocarbon Fuel Resistant, O-Ring — EverySpec (superseded by SAE AS29513).
• MIL-R-83248/1A Notice 5, cancellation (24 July 2008) — EverySpec.
• NAS1613, Seal Element, Packing, Preformed, Ethylene Propylene Rubber — NTIS record (abstract).
• Common Military Specifications — Apple Rubber (material, series and dash-number system per specification).
• A seal distributor's page on military-specification O-rings (NAS1611, NAS1612 material and range); not named.
• Parker O-Ring Handbook, ORD 5700 — Parker Hannifin, 2007 (sections 2.2 and 2.8; specifications section).
• NAS1593-015, O-ring — Textron Aviation parts catalog (replacement M83248/1-015).