O-Ring Cross-Sections and Groove Depths
AS568 and metric O-ring cross-sections with static and dynamic groove depth ranges.
An O-ring is a rubber ring that seals by being squeezed in a groove. How much it is squeezed, called the squeeze or compression, decides whether it seals: too little leaks, too much wears and sets. The usual guidance is 15 to 30 percent for a static seal and 10 to 20 percent for a moving (dynamic) seal.
The O-ring is chosen by its cross-section and inside diameter. AS568 dash numbers fall in five series by cross-section (0xx to 4xx); metric sizes use cross-sections such as 1.5, 2, 2.5, 3 and 4 mm. This table gives each cross-section and the groove depth that produces the usual squeeze.
The table
| O-ring series or size | Cross-section (in) | Cross-section (mm) | Static groove depth, 15 to 30 percent squeeze: smallest (mm) | Static groove depth: largest (mm) | Dynamic groove depth, 10 to 20 percent squeeze: smallest (mm) | Dynamic groove depth: largest (mm) |
|---|---|---|---|---|---|---|
| AS568 0xx (-001 to -050) | 0.07 | 1.78 | 1.24 | 1.51 | 1.42 | 1.6 |
| AS568 1xx (-102 to -178) | 0.103 | 2.62 | 1.83 | 2.22 | 2.09 | 2.35 |
| AS568 2xx (-201 to -284) | 0.139 | 3.53 | 2.47 | 3 | 2.82 | 3.18 |
| AS568 3xx (-309 to -395) | 0.21 | 5.33 | 3.73 | 4.53 | 4.27 | 4.8 |
| AS568 4xx (-425 to -475) | 0.275 | 6.99 | 4.89 | 5.94 | 5.59 | 6.29 |
| Metric 1 mm | 0.039 | 1 | 0.7 | 0.85 | 0.8 | 0.9 |
| Metric 1.5 mm | 0.059 | 1.5 | 1.05 | 1.28 | 1.2 | 1.35 |
| Metric 1.78 mm | 0.07 | 1.78 | 1.25 | 1.51 | 1.42 | 1.6 |
| Metric 2 mm | 0.079 | 2 | 1.4 | 1.7 | 1.6 | 1.8 |
| Metric 2.4 mm | 0.094 | 2.4 | 1.68 | 2.04 | 1.92 | 2.16 |
| Metric 2.5 mm | 0.098 | 2.5 | 1.75 | 2.13 | 2 | 2.25 |
| Metric 2.62 mm | 0.103 | 2.62 | 1.83 | 2.23 | 2.1 | 2.36 |
| Metric 3 mm | 0.118 | 3 | 2.1 | 2.55 | 2.4 | 2.7 |
| Metric 3.53 mm | 0.139 | 3.53 | 2.47 | 3 | 2.82 | 3.18 |
| Metric 4 mm | 0.157 | 4 | 2.8 | 3.4 | 3.2 | 3.6 |
| Metric 5 mm | 0.197 | 5 | 3.5 | 4.25 | 4 | 4.5 |
| Metric 5.33 mm | 0.21 | 5.33 | 3.73 | 4.53 | 4.26 | 4.8 |
| Metric 6 mm | 0.236 | 6 | 4.2 | 5.1 | 4.8 | 5.4 |
| Metric 6.99 mm | 0.275 | 6.99 | 4.89 | 5.94 | 5.59 | 6.29 |
| Metric 8 mm | 0.315 | 8 | 5.6 | 6.8 | 6.4 | 7.2 |
Source and notes: AS568 standard cross-sections (the dash number sets the cross-section and the inside diameter; 1.78, 2.62, 3.53, 5.33 and 6.99 mm are the metric equivalents of the 0xx to 4xx series) and the other common metric sizes. Groove depths are computed from the usual squeeze guidance (static 15 to 30 percent, dynamic 10 to 20 percent of the cross-section); the groove width, the gland fill and the fluid, temperature and pressure change the final design, so use the seal maker's gland tables.
How to use this table
- Find the cross-section of the O-ring you have or want.
- Read the groove depth range for a static or dynamic seal.
- Check the groove width and the stretch with the maker’s gland tables: depth is only one part of the design.
Worked example: a 3.53 mm (AS568 2xx series) O-ring in a static groove
- The cross-section is 3.53 mm (0.139 inch). The static groove depth range for 15 to 30 percent squeeze is 2.47 to 3.00 mm.
- A groove depth of 2.8 mm gives a squeeze of (3.53 − 2.8) ÷ 3.53 = 21 percent, inside the static range.
- For a moving seal the same O-ring would need a groove 2.82 to 3.18 mm deep.
Notes and cautions
- The groove depth in a face seal is measured from the sealing surface; in a radial (piston or rod) seal the depth is the radial gap that the ring is squeezed into, which includes the clearance.
- High pressure needs a back-up ring to stop the rubber being extruded into the clearance gap.
- The material (nitrile, FKM, silicone, EPDM) must suit the fluid and temperature: see the seals guide.
Related tools and guides
- Seals, O-Rings and Gaskets (guide)
- Metric Hex Bolt, Nut and Washer Dimensions
- Inch Fraction to Decimal and Millimeter Chart
- All reference tables
FAQ
What does the dash number mean?
In AS568, the number after the dash (-210, for example) identifies the cross-section and the inside diameter: -210 is a 2xx series ring (3.53 mm cross-section) with a 1.25 inch inside diameter.
How much should an O-ring be squeezed?
About 15 to 30 percent for a static seal and 10 to 20 percent for a dynamic seal. Below that it leaks; above it wears and takes a permanent set.
Sources
- SAE AS568 (aerospace size standard for O-rings); ISO 3601-1 (O-ring inside diameters and cross-sections); the squeeze guidance is from the seal makers’ design guides (Parker O-Ring Handbook, Trelleborg).
- AS568 standard cross-sections (the dash number sets the cross-section and the inside diameter; 1.78, 2.62, 3.53, 5.33 and 6.99 mm are the metric equivalents of the 0xx to 4xx series) and the other common metric sizes. Groove depths are computed from the usual squeeze guidance (static 15 to 30 percent, dynamic 10 to 20 percent of the cross-section); the groove width, the gland fill and the fluid, temperature and pressure change the final design, so use the seal maker's gland tables.