Two standards, two logics
AS568 (SAE/ANSI) sizes O-rings in inches and groups them into five cross-section series identified by a three-digit dash number. ISO 3601 sizes in millimetres, uses a different tolerance and cross-section progression, and is the default across Europe, Asia and most non-US OEM drawings.
The mistake we see most often
Procurement teams frequently substitute the 'nearest' AS568 dash number for an ISO 3601 callout, or vice versa, assuming a millimetre-to-inch rounding is close enough. In most cases the cross-section mismatch is small enough to fit the groove but large enough to under- or over-compress the seal — leading to slow leaks or early extrusion that's hard to diagnose.
What to check before you substitute
Confirm actual ID and CS in the same unit system, not nominal fraction labels — AS568's '1/16 series' is actually 1.78mm, not 1.5875mm. Recalculate groove fit and squeeze percentage for the substituted size rather than assuming it. Flag the substitution on the drawing and BOM so future maintenance doesn't reorder the wrong standard.
FITCO tools to standard on both
We manufacture to both AS568 and ISO 3601 tolerance classes from the same compound library, so customers working from either standard, anywhere in the world, get a verified match rather than a rounded approximation.