Metric vs Inch O-Rings: Can They Be Interchanged?

Introduction

Engineers often face the question: Can an AS568 (inch) O-ring be replaced with an ISO 3601 (metric) size? The short answer is sometimes — but only when the dimensional difference is smaller than the assembly tolerance. This article explains how to evaluate interchangeability safely, when substitution is acceptable, and when it risks seal failure.

The Standards at a Glance

• AS568: Inch-based dash numbers with fixed ID and CS dimensions (e.g., AS568-214 = 1.000″ ID × 0.139″ CS).
• ISO 3601: Metric sizes with ID and CS in millimeters (e.g., 25.00 × 3.53 mm).
• JIS B 2401: Japanese metric standard, dimensionally close to ISO 3601 but with slightly different size increments.

Conversion Basics

1 inch = 25.4 mm. To compare an AS568 size to a metric size:

• Multiply the AS568 ID by 25.4 to get the metric equivalent.
• Multiply the AS568 CS by 25.4 to get the metric equivalent.

For example, AS568-214 (1.000″ × 0.139″) converts to 25.40 mm ID × 3.53 mm CS.

When Are They Interchangeable?

Interchangeability depends on the groove tolerance and compression rate budget:

If the metric substitute differs by more than the allowable tolerance, the compression rate will fall outside the safe range.

Common Cross-References

Many popular AS568 sizes have exact or near-exact ISO 3601 equivalents because the ISO standard was intentionally aligned with common inch sizes.

When NOT to Interchange

1. Critical aerospace or medical assemblies: Tolerances are too tight; always use the specified standard.
2. Large cross-section differences: A 0.5 mm difference in CS changes compression rate significantly in a small groove.
3. Dynamic seals: Compression and groove-width budgets are tighter than in static flanges.
4. Gas seals: Gas molecules are small and can leak through minor imperfections that liquid seals would tolerate.
5. Vacuum applications: Outgassing and compression set requirements may differ between suppliers and standards.

How to Check Interchangeability

Follow this process before substituting a metric O-ring for an inch size (or vice versa):

Step 1: Convert Dimensions

Convert both sizes to the same unit (usually millimetres).

Step 2: Calculate the Difference

Find the difference in ID and CS between the two sizes.

Step 3: Check Groove Tolerance

Ensure the difference is smaller than the groove machining tolerance.

Step 4: Recalculate Compression Rate

Use the substitute size in your groove dimensions to verify compression remains within the safe range:

• Static seals: 15–25%
• Dynamic seals: 8–15%

Step 5: Verify Groove Width

Ensure the substitute O-ring fits the groove width with appropriate clearance:

• Static: groove width ≈ 1.1–1.2 × CS
• Dynamic: groove width ≈ 1.2–1.3 × CS

Example Calculation

Application: Hydraulic cylinder static port seal Groove: 25.50 mm bore, 3.20 mm deep, 4.00 mm wide Specified: AS568-214 (25.40 mm ID × 3.53 mm CS) Proposed substitute: ISO 3601 — 25.00 × 3.53 mm

• ID difference: 0.40 mm
• Groove tolerance: ±0.10 mm → too large
• Compression with substitute: (3.53 – 3.20) / 3.53 = 9.3% → below minimum for static seal

Conclusion: Do not substitute. Order the exact AS568-214 size.

Material and Hardness Compatibility

Even when dimensions match, the material and hardness must also be compatible:

• Do not substitute a 70 Shore A NBR for a 90 Shore A specification in a high-pressure application.
• FDA-grade requirements must be verified regardless of dimensional match.
• Low-temperature compounds may have different shrinkage characteristics that affect fit.

Best Practice

Always calculate the resulting compression rate with the substitute size before approving an interchange. If the compression falls outside 10–25% (static) or 8–15% (dynamic), order the correct size rather than substituting.

For critical applications, request a sample of the substitute size and perform a fit check in the actual groove before approving the change.

Conclusion

Metric and inch O-rings are often interchangeable in general industrial applications, but precision assemblies require exact-size conformance. The most popular AS568 sizes have direct ISO 3601 equivalents, making substitution straightforward for maintenance and repair. When in doubt, run the tolerance and compression calculations before making the switch.

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Frequently Asked Questions

Can I always replace an AS568 O-ring with an ISO 3601 size? No. While many sizes are dimensionally equivalent, you must verify the difference is within your groove tolerance and that the resulting compression rate is acceptable.

Why do some ISO sizes exactly match AS568 sizes? The ISO 3601 standard was intentionally harmonised with AS568 for common sizes to simplify global supply chains and cross-border maintenance.

What if I only have the groove dimensions and not the O-ring size? Calculate the required O-ring cross-section from the groove depth and target compression rate. Then select the standard CS closest to your calculation and verify the ID from the groove diameter.

Are JIS B 2401 and ISO 3601 interchangeable? Often yes, but JIS has some sizes that differ slightly from ISO. Always check the exact dimensions before substituting.

Does converting from inch to metric change the tolerance class? No. The tolerance class (e.g., ISO 3601 Class A vs Class B) is independent of the unit system. Specify the tolerance class separately.

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Need the exact size in any standard? We stock AS568, ISO 3601, and JIS B 2401 sizes. Request a quote or use our size converter.