What a dash number like -214 actually specifies, what it deliberately leaves out, and how one small rubber ring ends up with a dash number, a mil-spec part number, and a National Stock Number all at once.
AS568 is an SAE standard that does one narrow thing extraordinarily well: it defines the standard inch-series o-ring sizes. Before it, every manufacturer sized seals their own way; after it, a "-214" means the same inside diameter and cross-section to every seal house, distributor, and design engineer in the U.S. industrial and aerospace world. The standard assigns each size a dash number — written -006, -112, -214 and so on — and each dash number fixes exactly two dimensions: the ring's inside diameter and its cross-section (the thickness of the cord), with tolerances.
That is the whole scope. The dash number is a size, full stop — a point the standard's users forget at their peril, because everything else that matters about a seal (material, hardness, temperature and fluid compatibility) lives in other documents. That split between size and material is the key to reading every o-ring callout you will ever meet, and it is the same design-vs-item split that runs through the military part-numbering world generally — see our pillar guide to MS, NAS & AN part numbers for the broader pattern.
Dash numbers are not arbitrary — they are grouped into series by cross-section. The first digit of the dash number tells you roughly how fat the cord is; within a series, ascending numbers step through increasing inside diameters. Conceptually:
-0xx series: the thin end. Most of the series carries a nominal 1/16 in. (0.070 in.) cross-section, with the very smallest dash numbers thinner still. Example: -010 is a 0.239 in. ID ring with a 0.070 in. cross-section — instrument-scale sealing.
-1xx series: nominal 3/32 in. (0.103 in.) cross-section. Example: -112 is 0.487 in. ID × 0.103 in.
-2xx series: nominal 1/8 in. (0.139 in.) cross-section — the workhorse series for fittings, cylinders, and general hydraulics. Example: -214 is 0.984 in. ID × 0.139 in.
-3xx series: nominal 3/16 in. (0.210 in.) cross-section, for larger glands.
-4xx series: nominal 1/4 in. (0.275 in.) cross-section, the heaviest standard cord.
There is also a -9xx group of special sizes, best known for the boss gaskets used on straight-thread tube fittings, whose dimensions follow the fitting sizes rather than the regular ID progression. Two practical consequences of the series structure: first, a dash number with a different first digit is never a substitute — a -112 and a -212 share nothing but digits; second, when a print gives only ID and cross-section, the series logic usually lets you name the dash number without a chart, and a chart confirms it.
Order "a -214" and you have specified a shape, not a seal. The compound does the sealing, and the compound callout lives outside AS568 entirely:
Industrial drawings typically pair the dash size with a material family and a Shore A hardness — "NBR 70" (nitrile, the general-purpose petroleum-service default), "FKM 75" (fluorocarbon, for higher temperatures and aggressive fluids), EPDM for phosphate-ester and water service, silicone for temperature extremes at low mechanical duty. Same -214 size, radically different seals: put an EPDM ring in petroleum hydraulic fluid and it will swell to failure regardless of how perfect the size match was.
Mil-spec o-ring part numbers bundle a governing material specification with the AS568 size. MIL-PRF-25732 covers nitrile rings for aircraft hydraulic service, ordered under part numbers whose dash matches the AS568 size; fluorocarbon equivalents ride on the M83248-series numbers the same way. So MS28775-214 and M83248/1-214 are the same physical size — the -214 — in two different qualified materials. When a maintenance manual calls one of these numbers, the material decision has already been made for you; substitute across specs only with engineering authority.
The buying rule that falls out of all this: a complete o-ring order is always size plus material spec. A dash number alone is an incomplete question, which is also why one dash size fans out into many part numbers — and, as we will see next, many stock numbers.
In the federal supply system, o-rings live in Federal Supply Class 5331 — one of the larger classes in our catalog, with roughly 600 stocked stock numbers. The cataloging logic follows directly from the size-plus-material rule: each combination of size and governing specification that the supply system needs becomes its own item of supply with its own NSN. A -214 in MIL-PRF-25732 nitrile and a -214 in fluorocarbon are different NSNs, because for supply purposes they are different items — even though both trace to the same AS568 row. (What those 13 digits encode is covered in What Is a National Stock Number?.)
The practical effect for a buyer holding a maintenance manual: the NSN in the parts table has already resolved size and material, which makes it the safest single identifier to order against. The cataloging record links it back to the mil-spec part numbers, so translation runs both directions — our part number lookup handles part-number-to-NSN, and our guide on how to look up an NSN covers the free public routes for everything else.
Adjacent to the plain o-ring is the packing with retainer — an elastomer sealing element bonded into a metal retainer ring, making a one-piece sealing washer that goes under a bolt head or fitting where there is no machined o-ring groove. The common standard is NAS1523, and our NAS1523 page lists 18 stocked variants across its size and configuration codes. These catalog under FSC 5330, packing and gasket materials, rather than 5331 — a distinction worth knowing when a class-by-class browse comes up empty: if your "o-ring" has metal in it, look one class over.
Because one seal carries up to three identities, you can buy it three ways, and all three land on the same shelf:
By NSN — the fully-resolved identifier. Browse FSC 5331 in our catalog; every page shows the item name, the reference part numbers, and the published unit price. By mil-spec part number — enter the MS or M-number in the part number lookup and land on the NSN and price. By dash number plus material — when you hold a commercial drawing rather than a cataloged number, send us the dash size and the compound callout and we will quote against stock: get a price on it below, and paste the whole seal kit list if you are working a rebuild.
Same disclosure we make everywhere: our stock seals and hardware are resale items, country of origin Türkiye (Turkey), sold at published prices with source documentation on every order and no government contract required — the full argument for that model is in buying NSN parts without a government contract. And when your sealing problem is not the ring but the groove — a gland to cut, a boss to machine, a manifold to make — that is our U.S.-produced custom CNC machining line, built through an ISO 9001:2015 certified production network.
A size designation from SAE's AS568 standard, which defines the standard inch o-ring sizes used across U.S. industry and aerospace. The dash number selects a specific inside diameter and cross-section from the standard's tables — for example, -214 is a 0.984 in. ID ring with a 0.139 in. cross-section. The dash number specifies size only, never material or hardness.
No. Material, durometer, and performance requirements live in a separate specification — a compound callout such as nitrile (NBR) 70A, fluorocarbon (FKM) 75A, or a military specification like MIL-PRF-25732. Part numbering systems such as MS28775 or M83248/1 combine a governing material spec with the AS568 dash size, which is why the same -214 size exists in many different part numbers.
Each cataloged combination of size and governing specification becomes its own item of supply in Federal Supply Class 5331. One dash size can therefore carry several NSNs — one per material spec — and the cataloging record links each NSN back to its reference part numbers.
A packing with retainer bonds an elastomer sealing element into a metal retainer ring, making a one-piece sealing washer that seals under a bolt head or fitting without an o-ring groove. NAS1523 is the common standard — we stock 18 variants — and these items catalog under FSC 5330 rather than FSC 5331.
Yes — a dash number plus a material spec fully defines the part. Find our stock by NSN in the FSC 5331 catalog, by mil-spec number through the part number lookup, or send the dash number and material callout on an RFQ. Stock is country of origin Türkiye (Turkey) with published pricing.
The military-standard families decoded.
The 13-digit anatomy, decoded with a worked example.
Free methods that actually work.
Stocked o-ring NSNs with published pricing.
Packings with retainer, gasket materials.
MS, NAS, AN, and OEM cross-reference to NSN.
