Whether an AS568 (inch) O-ring can replace an ISO 3601 (metric) size arises constantly in global MRO procurement and equipment maintenance. The answer is context-dependent: many popular sizes are dimensionally equivalent or close enough for general industrial service, but others will cause compression-rate errors that produce leakage or premature failure.
Quick answer: If the ID difference between the AS568 and ISO 3601 candidates is ≤ 0.20 mm and the CS values are identical, substitution is usually acceptable for general industrial static seals. For dynamic, high-pressure gas, aerospace, or regulated medical device applications, always verify through the four-step compression calculation procedure below.
The Three Major O-Ring Standards
AS568 (SAE Aerospace Standard 568)
The United States O-ring sizing standard, maintained by SAE International. Dimensions are based on the inch system, with inside diameter (ID) and cross-section (CS) defined in inches to three decimal places.
- Dash number system:
- -001 to -004: CS 0.040" (1.02 mm)
- -006 to -050: CS 0.070" (1.78 mm)
- -102 to -178: CS 0.103" (2.62 mm)
- -210 to -284: CS 0.139" (3.53 mm)
- -309 to -395: CS 0.210" (5.33 mm)
- -425 to -475: CS 0.275" (6.99 mm)
- Tolerance: AS568 tolerance tables; tighter for smaller sizes; two tolerance classes (AS568 standard and AS568 Class 2 precision)
- Primary markets: United States, aerospace globally, hydraulic equipment with US-origin designs
ISO 3601 (International Standard)
The international metric O-ring standard published by ISO. Dimensions are in millimeters.
- Parts: ISO 3601-1 (dimensions and tolerances), ISO 3601-2 (housing dimensions and surface finish), ISO 3601-3 (quality acceptance criteria), ISO 3601-5 (material identification)
- Tolerance classes: Grade S (special — tighter, precision and dynamic service), Grade N (normal — general service), and Grade CS (close special) in some editions
- Primary markets: Europe, international OEM equipment, metric-system countries
JIS B 2401 (Japanese Industrial Standard)
The Japanese O-ring standard. Same concept as ISO 3601 but different size series and tolerance values.
- Size series: P-series (pistons and shafts), G-series (pipe fittings), V-series (vacuum flanges)
- Dimensions: Metric, but the size increment sequence differs from ISO 3601
- Primary markets: Japan, Korean and Taiwanese automotive supply chains, Asian industrial equipment
Why AS568 and ISO 3601 Are Often Interchangeable
Key fact: AS568 and ISO 3601 use near-equivalent cross-section values. The inch-based AS568 CS values convert to metric numbers that are close to, but not exactly the same as, the preferred ISO 3601 metric CS values. For most practical substitutions the difference is small enough to be ignored, but it must be checked:
| AS568 CS (mm) | AS568 Series | ISO 3601 Preferred CS (mm) | CS Difference | Notes |
|---|---|---|---|---|
| 1.02 | -001 to -004 | 1.00 / 1.20 | 0.02–0.18 mm | Miniature sizes; verify carefully |
| 1.78 | -006 to -050 | 1.80 | 0.02 mm | Essentially interchangeable |
| 2.62 | -102 to -178 | 2.65 | 0.03 mm | Check small IDs and dynamic service |
| 3.53 | -210 to -284 | 3.55 | 0.02 mm | Usually negligible |
| 5.33 | -309 to -395 | 5.30 | 0.03 mm | Verify on small IDs |
| 6.99 | -425 to -475 | 7.00 | 0.01 mm | Essentially interchangeable |
Because the CS values are close, the main question is whether the inside diameters are close enough for the target application.
AS568 to ISO 3601 Cross-Reference Table
| AS568 Dash | AS568 ID (mm) | AS568 CS (mm) | Nearest ISO 3601 ID (mm) | ISO CS (mm) | ID Difference | Interchangeable (general industrial)? |
|---|---|---|---|---|---|---|
| -006 | 2.90 | 1.78 | 2.80 | 1.80 | 0.10 mm | Yes |
| -008 | 4.47 | 1.78 | 4.50 | 1.80 | 0.03 mm | Yes (near-exact) |
| -010 | 6.07 | 1.78 | 6.00 | 1.80 | 0.07 mm | Yes |
| -012 | 9.25 | 1.78 | 9.25 | 1.80 | 0.00 mm | Yes (exact) |
| -020 | 21.95 | 1.78 | 22.40 | 1.80 | 0.45 mm | Verify calculation |
| -110 | 9.19 | 2.62 | 9.25 | 2.65 | 0.06 mm | Yes |
| -116 | 12.37 | 2.62 | 12.50 | 2.65 | 0.13 mm | Yes |
| -122 | 19.18 | 2.62 | 19.00 | 2.65 | 0.18 mm | Verify calculation |
| -210 | 12.37 | 3.53 | 12.50 | 3.55 | 0.13 mm | Yes |
| -214 | 24.99 | 3.53 | 25.00 | 3.55 | 0.01 mm | Yes (near-exact) |
| -218 | 29.84 | 3.53 | 30.00 | 3.55 | 0.16 mm | Yes |
| -221 | 34.59 | 3.53 | 34.50 | 3.55 | 0.09 mm | Yes |
| -228 | 44.12 | 3.53 | 44.50 | 3.55 | 0.38 mm | Verify calculation |
| -238 | 69.52 | 3.53 | 69.50 | 3.55 | 0.02 mm | Yes (near-exact) |
| -325 | 37.47 | 5.33 | 37.50 | 5.30 | 0.03 mm | Yes |
| -336 | 62.87 | 5.33 | 63.00 | 5.30 | 0.13 mm | Yes |
| -350 | 102.24 | 5.33 | 100.00 / 102.00 | 5.30 | 0.24–2.24 mm | No — verify exact ISO size |
| -425 | 113.67 | 6.99 | 112.00 / 115.00 | 7.00 | 1.33–1.67 mm | Verify calculation |
Note on large sizes: AS568 and ISO 3601 use different ID increment patterns above ~100 mm. A direct "nearest ISO" size can differ by several millimeters. At large IDs, always recalculate compression rate and fill rate rather than assuming interchangeability.
Interchangeability by Application Type
| Application | Max Acceptable ID Difference | Max Acceptable CS Difference | Notes |
|---|---|---|---|
| General industrial static seal | 0.30 mm | 0.10 mm | Most standard substitutions acceptable |
| Hydraulic cylinder (static port) | 0.20 mm | 0.05 mm | Verify compression calculation |
| Hydraulic cylinder (dynamic reciprocating) | 0.15 mm | 0.05 mm | Check both ID and CS carefully |
| High-pressure gas seal (static) | 0.08 mm | 0.03 mm | Gas exploits small gaps; tighter limits |
| Face seal (static, low pressure) | 0.50 mm | 0.10 mm | ID less critical in face seal orientation |
| Vacuum flange seal (KF, ISO-K, CF) | Do not substitute | Do not substitute | Use vacuum-hardware-specified sizes |
| Aerospace (AMS, MIL-spec) | No substitution without engineering review | No substitution | Certification is specification-specific |
| Medical device (FDA 510(k)) | Per drawing only | Per drawing only | Regulatory review required |
Four-Step Substitution Verification Procedure
Before substituting an AS568 O-ring for an ISO 3601 size (or vice versa), run this check:
Step 1: Compare cross-sections
Calculate the CS difference as a percentage of the original size's CS:
CS difference (%) = |CS_original − CS_substitute| / CS_original × 100If the difference is > 3%, the substitution will change compression rate significantly. For a 3.53 mm CS O-ring, 3% = 0.11 mm. A 3.55 mm ISO CS substituted for a 3.53 mm AS568 CS gives 0.6% difference — negligible.
Step 2: Compare inside diameters
Calculate the ID difference:
ID difference = |ID_original − ID_substitute|Compare against the application-specific limit in the table above.
Step 3: Recalculate compression rate
Use the substitute O-ring dimensions in the actual groove:
Squeeze (%) = (CS − groove depth) / CS × 100Verify the result remains within the acceptable range:
- Static seals: 15–22%
- Dynamic reciprocating: 10–15%
- Face seals: 20–30%
Step 4: Recalculate groove fill rate
Fill rate (%) = (π/4 × CS²) / (groove width × groove depth) × 100Ensure fill rate ≤ 85% at ambient, and ≤ 75% if significant thermal expansion or fluid swell is expected.
Worked example: Can AS568-214 replace ISO 3601 25.00×3.55 in a static hydraulic port?
- AS568-214: ID = 24.99 mm, CS = 3.53 mm
- ISO 3601: ID = 25.00 mm, CS = 3.55 mm
- Groove: depth = 2.90 mm, width = 4.25 mm
Step 1 — CS difference: |3.55 − 3.53| / 3.55 × 100 = 0.6% ✓ Step 2 — ID difference: |25.00 − 24.99| = 0.01 mm ✓ (well below 0.20 mm for static hydraulic) Step 3 — Squeeze: (3.53 − 2.90) / 3.53 × 100 = 17.8% ✓ (within 15–22% static range) Step 4 — Fill rate: (π/4 × 3.53²) / (4.25 × 2.90) × 100 = 79.5% ✓
Conclusion: AS568-214 and ISO 3601 25.00×3.55 are functionally interchangeable in this static hydraulic port.
Worked example: Can AS568-122 replace ISO 3601 19.00×2.65 in a dynamic rod seal?
- AS568-122: ID = 19.18 mm, CS = 2.62 mm
- ISO 3601: ID = 19.00 mm, CS = 2.65 mm
- Groove: depth = 2.25 mm, width = 3.40 mm
Step 1 — CS difference: |2.65 − 2.62| / 2.65 × 100 = 1.1% ✓ Step 2 — ID difference: |19.18 − 19.00| = 0.18 mm — borderline for dynamic (limit 0.15 mm) Step 3 — Squeeze: (2.62 − 2.25) / 2.62 × 100 = 14.1% ✓ (within 10–15% dynamic range) Step 4 — Fill rate: (π/4 × 2.62²) / (3.40 × 2.25) × 100 = 70.3% ✓
Conclusion: The ID difference of 0.18 mm exceeds the 0.15 mm recommended limit for dynamic reciprocating service. Substitution may cause slight stretch variation and is not recommended without testing. For static service, it would be acceptable.
Tolerance Stack-Up: Why Small Differences Matter
Even when nominal dimensions are close, tolerance stack-up can make a substitution marginal. Consider AS568-214 vs ISO 3601 25.00×3.55:
- AS568-214 ID tolerance: approximately ±0.21 mm
- ISO 3601 25.00×3.55 Grade N ID tolerance: approximately ±0.22 mm
- Worst-case combined ID offset: up to ±0.43 mm from nominal match
In practice, this means two production lots that both meet their respective standards could differ by nearly 0.5 mm in ID. For general industrial static seals this is still acceptable; for high-pressure gas or precision dynamic seals it is not.
JIS B 2401 Conversion Notes
JIS B 2401 P-series sizes are metric but use a different size progression than ISO 3601. Common P-series cross-sections are 1.0, 1.5, 1.9, 2.4, 3.1, 3.5, 5.7, and 8.4 mm.
| JIS P-Series CS (mm) | Nearest AS568 CS (mm) | Nearest ISO CS (mm) | Interchangeability |
|---|---|---|---|
| 1.0 | 1.02 (-001 to -004) | 1.00 | Verify carefully |
| 1.5 | 1.78 (-006 to -050) | 1.80 | Generally interchangeable |
| 1.9 | 1.78 (-006 to -050) | 1.80 | 0.12 mm CS difference — verify |
| 2.4 | 2.62 (-102 to -178) | 2.65 | Generally interchangeable |
| 3.1 | 3.53 (-210 to -284) | 3.55 | Generally interchangeable |
| 3.5 | 3.53 (-210 to -284) | 3.55 | Essentially interchangeable |
| 5.7 | 5.33 (-309 to -395) | 5.30 | 0.37 mm CS difference — verify |
| 8.4 | 6.99 (-425 to -475) | 7.00 | Not equivalent; different CS |
JIS G-series (pipe fittings) and V-series (vacuum flanges) should not be substituted with AS568 or ISO 3601 sizes without engineering review — these series have application-specific dimensions.
When You Should Never Substitute
There are situations where metric and inch O-rings must not be interchanged, regardless of dimensional similarity:
- Aerospace flight hardware: AS568 parts carry material and batch certifications (AMS-R-83485, etc.) that ISO 3601 parts do not automatically satisfy.
- Medical devices: FDA 510(k) clearances are drawing-specific; material and dimensions must match the approved design.
- Vacuum systems: KF, ISO-K, and CF flanges use size-specific O-rings designed for precise groove fill and permeation limits.
- High-pressure gas: Small dimensional differences create leak paths that liquid seals tolerate but gases do not.
- Reciprocating dynamic seals: ID and CS differences alter stretch, compression, and friction heat generation.
Metric-to-Inch Conversion Formulas
For quick reference:
mm = inch × 25.4
inch = mm / 25.4Common cross-section conversions:
| Inch CS | mm CS |
|---|---|
| 0.040" | 1.02 mm |
| 0.070" | 1.78 mm |
| 0.103" | 2.62 mm |
| 0.139" | 3.53 mm |
| 0.210" | 5.33 mm |
| 0.275" | 6.99 mm |
FAQ
Q1: Can I use an AS568 O-ring in a metric groove designed for ISO 3601?
Often yes, if the AS568 size is dimensionally close to the ISO 3601 size and you verify compression rate and fill rate in the actual groove. The most common interchangeable sizes are the medium cross-sections (1.78 mm ≈ 1.80 mm, 2.62 mm ≈ 2.65 mm, 3.53 mm ≈ 3.55 mm). Always run the four-step verification for dynamic, high-pressure, or critical static applications.
Q2: What is the most common mistake when substituting metric for inch O-rings?
Assuming that "close enough" ID and CS are always acceptable without recalculating compression. A 0.5 mm ID difference may be irrelevant in a large face seal but catastrophic in a high-pressure gas seal. The application determines whether a substitution is safe, not the dimensions alone.
Q3: Are AS568 and ISO 3601 tolerance classes the same?
No. AS568 uses standard and Class 2 (precision) tolerances. ISO 3601 uses Grade N (normal), Grade S (special), and Grade CS (close special) tolerances. A precision AS568 Class 2 O-ring is roughly comparable to ISO 3601 Grade S, but the numerical tolerance bands are not identical. For critical applications, compare the actual tolerance tables rather than assuming equivalence.
Q4: How do I convert an AS568 dash number to metric dimensions?
Multiply the inch dimensions by 25.4. For example, AS568-214 has ID 0.984" and CS 0.139":
- ID = 0.984 × 25.4 = 24.99 mm
- CS = 0.139 × 25.4 = 3.53 mm
Most suppliers publish conversion tables; always use the supplier's table for the specific dash number rather than relying on rounded approximations.
Q5: Can I use JIS B 2401 O-rings in place of ISO 3601?
Only if the JIS size has the same ID and CS as the ISO size. JIS P-series is metric but uses a different size progression and slightly different cross-sections (e.g., 1.9 mm vs 1.8 mm). JIS G-series and V-series are application-specific and should not be substituted without verification.
Q6: Why do some AS568 and ISO 3601 sizes have exactly the same dimensions?
Historical convergence. Many ISO 3601 sizes were developed to align with common AS568 sizes so that global equipment could use interchangeable seals. Sizes like AS568-214 (24.99×3.53 mm) and ISO 3601 25.00×3.55 mm are intentionally matched. However, the standards do not cover identical size ranges, so not all sizes have a direct counterpart.
Q7: What tools can I use to verify a substitution quickly?
- Compare ID and CS differences against the application table above.
- Use our O-ring compression calculator to check squeeze and fill rate.
- Refer to our AS568 size chart, ISO 3601 size chart, or general metric O-ring size chart for exact dimensions.
- For critical applications, request sample seals and measure them before approving the substitution.
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Need help finding a metric equivalent for an AS568 dash number? Contact our engineering team with the original size, groove dimensions, and application — we confirm interchangeability, recommend the correct ISO 3601 or JIS B 2401 equivalent, and supply NBR, FKM, EPDM, HNBR, PTFE, and FFKM O-rings in both inch and metric sizes.