O-Ring Groove Dimensions (AS568)
AS568 standard O-ring sizes with recommended groove width, depth, and radii for static face, static radial, dynamic piston, and dynamic rod seals. Includes metric BS / JIS equivalents and squeeze-percentage guidelines.
Reference
AS568 is the US aerospace / industrial standard for O-ring dimensions. It defines cross-section diameters in 5 sizes: 0.070", 0.103", 0.139", 0.210", and 0.275". Each size has dash-number ranges (e.g. -2xx are all 0.139" cross-section). Groove design targets a specific squeeze ratio — static face / radial uses 20-30% squeeze, dynamic reciprocating uses 8-20%, rotary uses 0-8%. This chart gives the groove width and depth for each scenario plus the recommended hardware dimensions.
AS568 standard cross-section sizes
| Dash range | CS nominal | CS tolerance | Used for |
|---|---|---|---|
| −001 through −050 | 0.070" (1.78 mm) | ±0.003" | Small bores, precision instruments |
| −102 through −178 | 0.103" (2.62 mm) | ±0.003" | General purpose up to ~1.5" bore |
| −201 through −284 | 0.139" (3.53 mm) | ±0.004" | General purpose, 1.5 - 10" bore |
| −309 through −395 | 0.210" (5.33 mm) | ±0.005" | Larger bores, hydraulic cylinders |
| −425 through −475 | 0.275" (6.99 mm) | ±0.006" | Very large bores, heavy hydraulics |
Recommended groove dimensions by cross-section
| CS | Static face groove W | Static face depth | Dynamic piston groove W | Dynamic piston depth | Dynamic rod groove W | Dynamic rod depth |
|---|---|---|---|---|---|---|
| 0.070" | 0.094-0.104" | 0.050-0.057" | 0.093-0.098" | 0.055-0.057" | 0.094-0.099" | 0.055-0.057" |
| 0.103" | 0.138-0.146" | 0.074-0.081" | 0.140-0.144" | 0.083-0.085" | 0.141-0.144" | 0.083-0.085" |
| 0.139" | 0.188-0.198" | 0.101-0.108" | 0.187-0.190" | 0.113-0.115" | 0.188-0.190" | 0.113-0.115" |
| 0.210" | 0.281-0.290" | 0.152-0.163" | 0.281-0.284" | 0.170-0.173" | 0.281-0.284" | 0.170-0.173" |
| 0.275" | 0.375-0.387" | 0.201-0.213" | 0.375-0.379" | 0.224-0.228" | 0.375-0.379" | 0.224-0.228" |
Squeeze percentage guidelines
- Static face seal (gland against flat)
- 20-30% squeeze. Groove depth = CS × 0.70-0.80.
- Static radial (inside a bore or on OD)
- 15-25% squeeze. Groove depth = CS × 0.75-0.85.
- Dynamic reciprocating (piston / rod)
- 8-16% squeeze. Too much squeeze → seal rolls and wears.
- Rotary (shaft seal)
- 0-8% squeeze. Too much heat from friction destroys nitrile; consider X-ring or rotary seal instead.
- Vacuum
- 20-30% squeeze + low-outgassing compound (FFKM or VMQ).
Groove stretch & compression rules
- ID stretch
- Max 5% for long life; 2% is ideal. > 5% stretch accelerates aging.
- OD compression
- Max 3% compression (free vs installed OD).
- Groove fill
- 60-85% of groove volume. Too much fill → extrusion; too little → loss of sealing force.
- Back-up rings
- Add for pressures > 1500 psi on dynamic seals or > 3000 psi static.
- Surface finish groove
- 32 Ra μin max static; 16 Ra μin for dynamic seals. Lay perpendicular to seal motion.
Common AS568 sizes (most-stocked static)
| Dash # | ID (in) | OD (in) | CS (in) |
|---|---|---|---|
| -004 | 0.070 | 0.210 | 0.070 |
| -008 | 0.171 | 0.311 | 0.070 |
| -014 | 0.489 | 0.629 | 0.070 |
| -018 | 0.739 | 0.879 | 0.070 |
| -022 | 1.114 | 1.254 | 0.070 |
| -112 | 0.487 | 0.693 | 0.103 |
| -115 | 0.612 | 0.818 | 0.103 |
| -118 | 0.862 | 1.068 | 0.103 |
| -122 | 1.114 | 1.320 | 0.103 |
| -210 | 0.734 | 1.012 | 0.139 |
| -214 | 0.984 | 1.262 | 0.139 |
| -218 | 1.234 | 1.512 | 0.139 |
| -222 | 1.484 | 1.762 | 0.139 |
| -226 | 2.239 | 2.517 | 0.139 |
| -230 | 2.614 | 2.892 | 0.139 |
| -232 | 2.864 | 3.142 | 0.139 |
| -236 | 3.239 | 3.517 | 0.139 |
| -240 | 3.734 | 4.012 | 0.139 |
| -244 | 4.234 | 4.512 | 0.139 |
| -326 | 2.000 | 2.420 | 0.210 |
| -328 | 2.125 | 2.545 | 0.210 |
| -334 | 2.750 | 3.170 | 0.210 |
| -338 | 3.125 | 3.545 | 0.210 |
| -342 | 3.625 | 4.045 | 0.210 |
| -346 | 4.125 | 4.545 | 0.210 |
Metric / JIS equivalents
- ISO 3601 / BS1806
- European standard. Sizes partially overlap AS568 but use metric IDs.
- JIS B 2401 P-series
- Japanese reciprocating / static seal (e.g. P-10 = 9.8 × 2.4 mm).
- JIS B 2401 S-series
- Static seal only, thinner CS (e.g. S-10 = 9.5 × 1.5 mm).
- JIS B 2401 G-series
- Stationary fixing (large ID, 3.1 mm CS).
- JIS B 2401 V-series
- Vacuum flange (e.g. V-75 = 74.5 × 5.7 mm).
Compound selection quick guide
- Nitrile (NBR, "Buna-N")
- −40 to +250°F. Oil, grease, hydraulic fluid, water. Cheapest general-purpose. Poor in ozone / sun.
- Viton (FKM)
- −20 to +400°F. Fuels, oils, most solvents. Not for hot water / steam / ketones / amines.
- EPDM
- −60 to +300°F. Hot water, steam, brake fluid, mild acids/bases. Fails immediately in petroleum oils.
- Silicone (VMQ)
- −100 to +450°F. Food-grade, inert. Poor abrasion resistance; not for dynamic seals.
- Kalrez / FFKM
- −20 to +600°F. Nearly universal chemical resistance. $$$$ — use only when nothing else works.
- Polyurethane (AU / EU)
- −40 to +200°F. Excellent abrasion resistance for dynamic seals. Hydrolysis in hot water.
Notes
- Dimensions from SAE AS568 and Parker Handbook ORD 5712. Always cross-check against the seal manufacturer's gland-design spec for your specific application (dynamic rate, temperature, media, pressure).
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