What a dash number identifies
AS568 organizes inch-family O-ring dimensions under dash numbers such as -014 or -224. The designation identifies nominal inside diameter and cross-section. It does not specify elastomer family, compound, hardness, colour, cure system or regulatory approval; those must be stated separately.
The familiar cross-section families are approximately 0.070, 0.103, 0.139, 0.210 and 0.275 inches, commonly shown as 1.78, 2.62, 3.53, 5.33 and 6.99 mm. The metric values are conversions of an inch-based system, not a metric replacement standard.
Read the table in the right order
- Locate the exact dash number, including leading zeros.
- Read nominal inside diameter and cross-section in the source units.
- Read the applicable tolerances rather than treating the nominal value as exact.
- Calculate nominal outside diameter as
ID + 2 × CS. - Specify material, hardness and compound requirements separately.
A nearby dash number can share a cross-section while having a different inside diameter. Never infer the dimensions from the first digit alone.
Why conversion and rounding matter
Converting a dimension to millimetres does not change the tolerance basis. Repeatedly rounding inch values can create a false exact match with an ISO or JIS size. Preserve sufficient precision in calculations, and show the original standard designation on drawings and purchase orders.
Size selection is not groove design
A standard O-ring still needs a gland designed for the actual seal type. Radial piston, radial rod, reciprocating and face seals use different squeeze, stretch, fill and surface requirements. Choose the hardware model first, then evaluate the standard size across its tolerance limits.
Use the searchable AS568 size chart to identify candidates. Then open the relevant calculator and compare the standard row separately from any supplier-specific part number.
Confirm before release
This site provides a preliminary engineering subset, not a substitute for the controlled standard. Before releasing a drawing or purchase specification, verify the current AS568 edition and the supplier’s compound-specific dimensional data. If a vendor part uses an AS568 dash number, keep both the standard designation and the vendor part number in the bill of materials.