Size & Standards

AS568 vs Metric (ISO 3601) vs JIS B 2401: Which O-Ring Standard Do You Need?

2026-11-026 min read

Maintenance departments inherit machines from three engineering traditions — American-inch, European-metric and Japanese-metric — and each came with its own O-ring standard. The good news: the standards share five cross-section families. The trap: shared cross-section does not mean interchangeable size. This article explains how to choose and when conversion is safe.

Origin and Design Logic

Feature	AS568	ISO 3601-1 metric	JIS B 2401
Origin	USA	International / Europe	Japan
Units	Inch	Millimetres	Millimetres
Designation	Dash number (-214)	ID × CS (20 × 3.55)	Series + number (P20, G25...)
Series logic	Cross-section families (-0XX…-9XX)	Cross-section steps in mm	P (motion), G (fixed), V (vacuum) duties
Typical equipment	US-origin machinery, global hydraulics	European and modern international machines	Japanese vehicles and industrial equipment

The Five Shared Cross-Sections

The standards were reconciled long ago at the cross-section level — metric and JIS sizes are essentially the same cords rounded to millimetres:

AS568 CS (in)	ISO/JIS CS (mm)	Difference	Practical effect
0.070	1.78 / 1.80	~0.02 mm	Effectively the same cord
0.103	2.62 / 2.65	~0.03 mm	Within normal tolerance
0.139	3.53 / 3.55	~0.02 mm	Grooves accept either cord
0.210	5.33 / 5.30	~0.03 mm	Grooves accept either cord
0.275	6.99 / 7.00	~0.01 mm	Nearly identical

The inside diameters, however, step through different increments. A ring whose CS matches perfectly may still be too loose or too tight in stretch/interference because its ID differs by a few tenths of a millimetre — enough to move squeeze outside the design window.

Three Rules for Choosing

Follow the machine's origin and documentation. US-built equipment and most global fluid-power components default to AS568; European machinery to ISO metric; Japanese machinery (especially automotive and machine tools) to JIS. The original spare-parts list names the standard.
Never mix standards within one groove without recalculating. Cross-check ID, stretch and squeeze against the actual charts — do not assume ‘metric equivalent’ means interchangeable.
For new designs, pick one standard and stay consistent. There is no universally ‘best’ standard; consistency across a machine simplifies spares, kits and documentation.

Conversion Pitfalls

Pitfall	Why it matters
Rounding inch sizes to the nearest millimetre	Compound rounding of ID and CS shifts stretch and squeeze in the same adverse direction in worst-case tolerances
Assuming identical tolerances	AS568, ISO 3601 and JIS define slightly different ID/CS tolerance bands; a precision replacement should use the native standard
Confusing JIS P and G series	P sizes are intended for motion grooves, G sizes for fixed grooves — they are not interchangeable selections
Ignoring the compound	All three standards specify geometry only; material and hardness remain separate choices

A Practical Cross-Reference Workflow

Measure the old ring or the groove (ID and CS) following the measuring procedure.
Identify the cord family and look up the nearest entry in all three charts.
Compare the candidate IDs: pick the candidate whose calculated stretch (piston: 0–5%) or interference (rod: 0–2%) lands in the window.
Confirm squeeze against the groove; only then place the order.

The cross-sections made peace decades ago; the inside diameters did not. A ring fits a groove on cord size but seals correctly only when the ID fit is also right.

We stock and cross-reference all three standards in the O-ring range — AS568, ISO metric and JIS B 2401, in NBR, FKM, EPDM and silicone. The full dash-number structure is explained in AS568 dash numbers decoded, and the overview of all three chart systems is in the O-ring size chart. Send us your measured sizes for a free cross-reference.

Back to All Posts