An o-ring that is nearly the right size is not the right size. Because different regions and industries standardised separately, several sizing systems are in circulation and they do not map cleanly onto each other.
AS568 — the American standard
The most widely known, published by SAE. Uses dash numbers — a 214, a 325, a 010. Grouped in series by cross-section, so all 200-series rings share one cross-section and vary in diameter.
Dominant in North American equipment, and the sizing used for ORFS hydraulic fittings.
SAE 900 series — the boss seal exception
Straight-thread O-Ring Boss fittings use their own numbering, the 900 series. A dash 6 ORB ring is a #906, and it does not correspond to a 006 in the standard AS568 range. This catches people constantly.
See our ORB guide.
Metric
Specified directly as inside diameter × cross-section in millimetres — a 140 × 4 mm ring is exactly that. No lookup table required, which is arguably the most sensible system of the lot.
ISO 3601
The international standard, metric-based, widely used in European and global equipment.
JIS B2401 — Japanese
Used across Japanese equipment. Series designations such as P, G and S denote application type rather than simply size, so a JIS number carries information an AS568 dash number does not.
BS 4518 — British
The British metric standard, found on older UK-built equipment.
SMS 1588 — Swedish
Common in Scandinavian equipment and some hydraulic applications.
Why they will not substitute
The differences are often small — a fraction of a millimetre in cross-section — and that is exactly the problem. A ring that is slightly undersized in cross-section will seal on the bench and fail under pressure. One slightly oversized will be over-compressed, take a set early, or be pinched on assembly.
Which do you have?
Work from the equipment. Japanese machine, look at JIS. European hydraulics, ISO 3601 or metric. North American, AS568. When the equipment tells you nothing, the safest route is to order by equipment model rather than by size.
We stock all six ranges in Buna-N 70A and 90A, FKM, EPDM and silicone.
Beaver Island Parts Company is not affiliated with, endorsed by, or sponsored by any equipment manufacturer named on this page. All brand names, model numbers and part numbers are the property of their respective owners and are used solely to identify the equipment our replacement parts fit.