O-Ring Gland Design: Squeeze and Groove Sizing with AS568

An O-ring does not seal because it fits the groove — it seals because it does not. A properly designed static seal is deliberately squeezed: the groove is cut smaller than the ring’s cross-section so the elastomer is compressed and pushed against every sealing surface. Too little squeeze and it leaks; too much and it takes a permanent set, extrudes, or tears. Getting that compression into the right window — typically 15% to 30% for a static seal — is the entire game, and it starts with the AS568 dash number.

The AS568 Standard: Where Every O-Ring Groove Begins

AS568 is the SAE standard that defines the inch-series O-ring sizes almost every North American design uses. Each ring is identified by a dash number (for example -214) that fixes two dimensions: the inside diameter (ID) and the cross-section diameter (CS, often called the “W” dimension). The cross-section is the number that matters for sealing, and AS568 organizes rings into families by CS:

Dash series	Nominal CS (W)	Typical use
-0xx (004–050)	0.070″	Small bores, fittings
-1xx (102–178)	0.103″	General purpose
-2xx (201–284)	0.139″	Most common, medium bores
-3xx (309–395)	0.210″	Large bores, hydraulics
-4xx (425–475)	0.275″	Very large diameters

Pick the CS first based on your available radial space, then choose the dash number whose ID matches your bore or rod. The groove is designed around the CS — not the other way around.

Squeeze: The Number That Makes or Breaks the Seal

Squeeze (also called compression) is the percentage the cross-section is flattened when the ring is installed:

Design targets depend on the application:

• Static seals: 15–30% squeeze. More squeeze means a more robust seal, but also higher compression set over time.
• Dynamic seals (reciprocating): 8–16% squeeze. Too much squeeze here means friction, heat, and rapid wear.
• Rotary seals: lower still — excess squeeze generates heat that destroys the elastomer.

There is a lower bound you cannot violate: below roughly 0.007″ of absolute squeeze, surface finish and micro-irregularities start to leak regardless of the percentage. Small cross-sections need a higher squeeze percentage just to clear that absolute floor.

Worked Example: Designing a Groove for a -214 O-Ring

Take a common -214 ring: ID = 0.984″, CS = 0.139″ (nominal). We want a static face seal at about 22% squeeze.

Parameter	Calculation	Result
Groove depth (for 22% squeeze)	0.139 × (1 − 0.22)	0.108″
Groove width (for ~85% fill)	1.3 × CS (rule of thumb)	0.180″
Gland fill check	ring volume / groove volume	~75–85% (OK)

Two constraints govern that groove width. It must be wide enough that the ring, when squeezed, has room to bulge sideways — and it must never let gland fill exceed 100%. Elastomers are nearly incompressible; they change shape, not volume. If the groove is too narrow, thermal expansion and swell have nowhere to go, and the ring can force the gland apart or extrude past the gap. Aim for gland fill in the 60–85% range to leave headroom for thermal growth and fluid swell.

Don’t Forget Extrusion Gap and Surface Finish

Even a perfectly squeezed ring fails if the diametral clearance gap is too large for the system pressure — the elastomer nibbles into the gap and shreds. Higher pressures and softer durometers demand tighter gaps, and above roughly 1,500 psi a back-up ring is standard practice. Surface finish matters too: static seals want 16–32 µin Ra on the sealing faces, dynamic seals 8–16 µin on the moving surface. A groove designed to three decimals will still weep if it is machined rough.

Match the Material to the Fluid and Temperature

Geometry seals the joint, but only if the elastomer survives the environment. The dash number tells you the size, not the compound — the same -214 groove might take Buna-N (NBR) for general petroleum oil and air, Viton (FKM) for high temperature and aggressive fluids, EPDM for brake fluid and hot water, or silicone for extreme cold. Each compound has a service temperature band and a chemical-compatibility list, and stepping outside either one causes hardening, swelling, or rapid compression set no groove geometry can rescue. Two practical reminders: elastomers swell in compatible fluids, so leave gland-fill headroom for it; and every compound has a maximum working temperature above which it takes permanent set and stops springing back. Confirm both the temperature range and the fluid compatibility before you commit the durometer — a 70-durometer NBR ring in a 400°F line will fail no matter how perfect the squeeze.

Identify First, Then Design

Half of real-world seal work starts with an unmarked ring pulled from a failed assembly. If you can measure ID and CS with a caliper or an optical comparator, our O-ring size identifier maps those dimensions back to the AS568 dash number so you order the right replacement instead of guessing. From there, the O-ring groove calculator takes your dash number and application type and returns groove depth, width, squeeze percentage, and gland fill — with the extrusion-gap and fill checks built in so you cannot accidentally over-fill the gland.