The main difference
AS568 is an inch-origin dash-number system. ISO 3601-1 includes metric size series commonly designated by inside diameter × cross-section. Both describe O-ring dimensions, but their preferred sizes, tolerances and associated quality requirements come from different standard systems.
| Family | Common cross-sections |
|---|---|
| AS568 inch family | 1.78, 2.62, 3.53, 5.33, 6.99 mm (converted) |
| ISO metric family | 1.80, 2.65, 3.55, 5.30, 7.00 mm |
The values look close by design. “Close” does not mean automatically interchangeable.
Why a near match can still change the result
A difference of a few hundredths of a millimetre changes squeeze and O-ring volume. Its effect may be small at nominal conditions but larger when ring tolerance and groove tolerance combine at the same extreme. Inside-diameter differences also change stretch and installed cross-section.
For an interchange review, compare at least:
- nominal ID and cross-section;
- positive and negative tolerances;
- resulting outside diameter;
- minimum and maximum squeeze;
- maximum gland fill;
- stretch or compression of the installed circumference;
- compound, hardness and quality grade.
Which standard should lead?
Use the standard required by the customer, industry, legacy drawing or equipment family. North American aerospace and industrial drawings frequently use AS568; internationally standardized metric hardware frequently uses ISO 3601. Availability in the target supply chain also matters, but should not silently override the drawing requirement.
A safe substitution workflow
Start with the controlled original designation. Enter the original and proposed dimensions as separate candidates in the same seal calculator. Compare the complete tolerance results and confirm the groove, not just the ring. Then document the substitution, compound equivalence, inspection method and purchasing designation.
The O-ring size standards hub provides separate AS568 and ISO 3601 tables. Keeping standard rows and supplier rows separate makes it clear whether a match is dimensional, commercial or both.
Avoid hybrid designations
Do not describe an ISO size as “equivalent to AS568” without the exact dimensional comparison. Do not round 2.62 mm to 2.65 mm and reuse the original groove calculation. A controlled cross-reference should state both designations and the engineering basis for interchange.