Size & Standards

AS568 Dash Numbers Decoded: How the -0XX to -9XX Series Work

2026-10-266 min read

If you have ever ordered seals from a maintenance catalog, you have seen labels like -214, -111 or -326. These AS568 dash numbers are not arbitrary part codes — they encode the ring's cross-section and inside diameter in a compact convention dating back decades. Once you can decode them, you can read any O-ring drawing, compare brands, and select a dash size directly from your groove dimensions.

Anatomy of a Dash Number

A dash number has two parts: the hundreds digit(s) identify the series — meaning the fixed cross-section — while the last two digits identify the inside diameter within that series. Example: in -214, the leading ‘2’ places the ring in the 0.139 in (3.53 mm) cross-section family, and ‘14’ identifies the 14th ID step in that family.

The Series Map

Series	CS (in)	CS (mm)	Role
-001 to -003	0.070	1.78	Miniature seals for instruments and tiny fittings
-1XX	0.103	2.62	Small hydraulic and pneumatic sections
-2XX	0.139	3.53	The workhorse family — most standard designs
-3XX	0.210	5.33	Medium cylinders and process equipment
-4XX	0.275	6.99	Heavy industrial, large grooves
-5XX to -8XX	0.103 – 0.275	2.62 – 6.99	Extended and face-seal ID ranges
-9XX	0.139	3.53	Very large diameter rings

Some Familiar Examples

Dash no.	Nominal ID (in)	CS (in)	Commonly found in
-006	0.114	0.070	Small fittings, instrumentation
-010	0.239	0.070	Compact connectors and valves
-111	0.487	0.103	Small pneumatic/hydraulic gear
-214	0.984	0.139	One of the most widely used general-purpose sizes

Within a series, ID steps are intentionally dense — typically a few hundredths of an inch apart at the small end and wider at the large end — so the catalog can cover thousands of hardware sizes with a manageable number of molds.

What the Dash Number Does NOT Specify

Material. A -214 can be NBR, FKM, EPDM, silicone or FFKM. The compound is always a separate line item — this is the #1 cause of wrong orders.
Hardness. -214 70A and -214 90A are different products for different pressures.
Quality class or tolerance. Revision letters (AS568A/B/C) updated the document over time; tighter-tolerance and aerospace versions exist under additional specification classes and should be called out explicitly.
Color, approvals or packaging. FDA compliance or color coding never follows from the dash number.

A complete order line therefore always combines the dash size with compound and hardness, e.g. ‘AS568 -214, FKM 75 Shore A’. See our custom ordering guide for the full specification checklist.

How to Select a Dash Size From Your Groove

From the groove geometry, identify the required cross-section family (which of the five CS values the gland was designed around).
Determine the required ID from the seal type: for piston seals the ID is chosen for roughly 1–5% stretch over the gland; for rod and face seals the OD/ID is chosen for small interference. Full formulas appear in our ID stretch and OD interference article.
Open that series in the AS568 table and pick the nearest dash ID — exact matches are rare; the bands are designed for nearest-neighbor selection.
Recompute effective squeeze and gland fill with the chosen dash, per our compression ratio guide, and run the tolerance stack.

Read the hundreds digit for the cord, the trailing digits for the circle — then add material and hardness yourself. That is the entire AS568 language.

Need help finding the right dash? Send us the groove or hardware dimensions and we will return the nearest AS568 option plus the equivalent metric/JIS alternatives — or start from the O-ring size chart yourself. All standard dash sizes are available in our O-ring range.

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