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 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 -049 (CS 1.78 mm), -100 to -149 (CS 2.62 mm), -200 to -249 (CS 3.53 mm), -300 to -349 (CS 5.33 mm), -400 to -449 (CS 6.99 mm), -450 to -475 (CS 8.74 mm), plus larger sizes to -932
- 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: Class A (tighter — precision and dynamic service), Class N (normal — general service)
- 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 share the same six standard cross-section values. ISO 3601 was designed to be compatible with common AS568 cross-sections. These CS values are identical in both standards:
| CS (mm) | CS (inches) | AS568 Series | ISO 3601 ID Range |
|---|---|---|---|
| 1.78 | 0.070" | -001 to -049 | 1.00 mm to ~30 mm ID |
| 2.62 | 0.103" | -100 to -149 | 2.90 mm to ~45 mm ID |
| 3.53 | 0.139" | -200 to -249 | 6.00 mm to ~100 mm ID |
| 5.33 | 0.210" | -300 to -349 | 22.40 mm to ~200 mm ID |
| 6.99 | 0.275" | -400 to -449 | 82.14 mm to ~400 mm ID |
| 8.74 | 0.344" | -450 to -475 | 101.32 mm to ~400 mm ID |
Because CS values are shared, the only remaining 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) | ID Difference | Interchangeable (general industrial)? |
|---|---|---|---|---|---|
| -008 | 3.68 | 1.78 | 3.55 | 0.13 mm | Yes |
| -012 | 6.07 | 1.78 | 6.00 | 0.07 mm | Yes |
| -020 | 10.77 | 1.78 | 10.78 | 0.01 mm | Yes (near-exact) |
| -108 | 3.68 | 2.62 | 3.55 | 0.13 mm | Yes |
| -110 | 9.19 | 2.62 | 9.25 | 0.06 mm | Yes |
| -116 | 12.37 | 2.62 | 12.37 | 0.00 mm | Yes (exact) |
| -122 | 19.18 | 2.62 | 18.72 | 0.46 mm | Verify calculation |
| -210 | 12.37 | 3.53 | 12.37 | 0.00 mm | Yes (exact) |
| -214 | 25.40 | 3.53 | 25.00 | 0.40 mm | Verify — borderline |
| -218 | 29.84 | 3.53 | 30.00 | 0.16 mm | Yes |
| -221 | 34.59 | 3.53 | 34.65 | 0.06 mm | Yes |
| -228 | 44.12 | 3.53 | 44.12 | 0.00 mm | Yes (exact) |
| -238 | 69.52 | 3.53 | 69.50 | 0.02 mm | Yes (near-exact) |
| -314 | 31.12 | 5.33 | 31.50 | 0.38 mm | Verify calculation |
| -325 | 37.47 | 5.33 | 37.50 | 0.03 mm | Yes |
| -336 | 62.87 | 5.33 | 63.00 | 0.13 mm | Yes |
| -350 | 102.24 | 5.33 | 101.20 | 1.04 mm | No — ID difference too large |
| -425 | 204.95 | 6.99 | 206.00 | 1.05 mm | No — verify first |
Note on -214: The 0.40 mm ID difference may or may not be acceptable depending on groove design and application type. Apply the substitution verification procedure below.
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 |
Substitution Verification Procedure
Use this five-step process before approving any O-ring substitution across standards.
Step 1: Record Both Dimensions in Millimeters
Convert all dimensions to millimeters:
- AS568-214: ID = 1.000" × 25.4 = 25.40 mm, CS = 0.139" × 25.4 = 3.53 mm
- ISO substitute candidate: ID = 25.00 mm, CS = 3.53 mm
Step 2: Calculate the Dimensional Difference
- ID difference: 25.40 − 25.00 = 0.40 mm
- CS difference: 3.53 − 3.53 = 0.00 mm (identical)
Step 3: Compare to Groove Tolerance
The groove ID machining tolerance determines how much O-ring ID variation is acceptable. If the groove ID is 25.50 mm ±0.10 mm:
- The groove accommodates O-rings from ID 25.40 to 25.60 mm with correct compression
- The substitute at 25.00 mm ID is 0.40 mm smaller than nominal — outside the ±0.10 mm tolerance
- This substitution requires compression calculation before approving
Step 4: Calculate Compression Rate With the Substitute
Compression (%) = (CS − Groove Depth) / CS × 100If groove depth = 3.05 mm (designed for ~13.6% compression with CS = 3.53 mm):
- Original AS568-214 compression: (3.53 − 3.05) / 3.53 = 13.6% — within 12–15% dynamic target
- ISO substitute (same CS = 3.53 mm): compression unchanged = 13.6%
In this example the CS is identical, so compression rate is unaffected. However, the 0.40 mm smaller ID means the O-ring ID is smaller than nominal — it will sit in the groove with more circumferential stretch in a piston/rod groove. Calculate the stretch:
Stretch (%) = (Groove ID − O-ring ID) / O-ring ID × 100- AS568-214 in a 25.5 mm groove ID: (25.5 − 25.4) / 25.4 × 100 = 0.4% stretch — negligible
- ISO 25.0 mm in the same groove: (25.5 − 25.0) / 25.0 × 100 = 2.0% stretch — acceptable for static; verify for dynamic
Step 5: Identify Seal Orientation
Whether ID matters depends on how the seal is loaded:
- Radial seal (piston or rod): O-ring ID must match the groove ID closely — the ID difference directly affects the circumferential stretch
- Face seal (axial pressure): O-ring ID determines only whether the ring fits in the groove — larger ID differences are acceptable because the primary sealing load is axial, not radial
- Tube fitting or boss seal: ID determines fit onto the thread boss — specify the ISO size matching the thread standard
Effect of ID Difference on Compression Rate
This table shows how a fixed ID difference translates into compression change for radial seals, where the O-ring ID affects the diametral contact geometry:
| ID Difference | ID Smaller than Nominal (over-stretched) | ID Larger than Nominal (under-stretched) | Effect on Static Seal | Effect on Dynamic Seal |
|---|---|---|---|---|
| 0.05–0.10 mm | Slight over-stretch | Slight looseness in groove | Negligible | Negligible |
| 0.15–0.20 mm | Measurable over-stretch | O-ring slightly loose | Acceptable for general service | Verify; acceptable for most |
| 0.25–0.40 mm | Noticeable over-stretch; compression slightly reduced | O-ring may not seat fully | Verify compression calculation | Not recommended without calculation |
| > 0.40 mm | Significant over-stretch; risk of cross-section reduction | Possible gap at groove side wall | Calculate compression carefully | Not recommended |
| > 0.80 mm | Excessive stretch; cross-section reduces (Poisson effect) | O-ring may migrate in groove | Not acceptable | Not acceptable |
Poisson effect on CS under stretch: When an O-ring is stretched circumferentially by more than ~5%, the cross-section diameter decreases by roughly half the stretch percentage due to material incompressibility. A 6% circumferential stretch reduces effective CS by approximately 3%. For a 3.53 mm CS ring, this yields an effective CS of approximately 3.42 mm — potentially reducing compression by 0.5–1.0 percentage points.
JIS B 2401 Interchangeability
JIS B 2401 uses a different size progression. JIS P-series (piston/shaft) and G-series (pipe) sizes do not always correspond to AS568 or ISO 3601.
| Property | ISO 3601 | JIS B 2401 | Practical Effect |
|---|---|---|---|
| CS values | Same as AS568 (1.78, 2.62, 3.53, 5.33, 6.99, 8.74 mm) | Similar; some JIS-specific CS values | Most JIS P-series CS match ISO |
| ID increment sequence | ISO tables | Different increment sequence | Not all JIS IDs exist in ISO |
| Tolerance class | Class A and Class N | JIS Grade 1, Grade 2 | Different values at same CS |
| Groove (gland) dimensions | ISO 3601-2 | JIS-specific tables | Do not assume groove interchangeability |
For JIS-origin equipment (Japanese-designed machinery, automotive systems), use JIS sizes as specified. JIS P-7 (7 mm ID, 1.9 mm CS) is a common size with no direct AS568 or ISO 3601 equivalent — specifying the nearest ISO size would require a 1.9 mm CS ring that does not exist in the six standard CS values.
Tolerance Class Comparison
| Dimension | AS568 | AS568 Class 2 | ISO 3601 Class A | ISO 3601 Class N | JIS Grade 1 |
|---|---|---|---|---|---|
| CS tolerance (CS 1.78 mm) | ±0.08 mm | ±0.05 mm | ±0.05 mm | ±0.09 mm | ±0.07 mm |
| CS tolerance (CS 2.62 mm) | ±0.10 mm | ±0.06 mm | ±0.06 mm | ±0.10 mm | ±0.09 mm |
| CS tolerance (CS 3.53 mm) | ±0.10 mm | ±0.07 mm | ±0.06 mm | ±0.10 mm | ±0.09 mm |
| CS tolerance (CS 5.33 mm) | ±0.13 mm | ±0.08 mm | ±0.08 mm | ±0.13 mm | ±0.11 mm |
| ID tolerance (ID = 25 mm) | ±0.20 mm | ±0.14 mm | ±0.14 mm | ±0.26 mm | ±0.22 mm |
| ID tolerance (ID = 50 mm) | ±0.25 mm | ±0.18 mm | ±0.18 mm | ±0.35 mm | ±0.26 mm |
| ID tolerance (ID = 100 mm) | ±0.35 mm | ±0.25 mm | ±0.25 mm | ±0.50 mm | ±0.36 mm |
ISO 3601 Class A is tighter than or equal to AS568 across most dimensions. An ISO 3601 Class A O-ring in the equivalent size will meet or exceed the AS568 dimensional requirement. ISO 3601 Class N has similar or slightly looser tolerances than AS568 for most sizes.
For precision hydraulic cylinders, aerospace, and medical device applications, specify tolerance class explicitly (ISO 3601-1 Class A) when ordering — do not rely on supplier default.
When Substitution Is Never Acceptable
Regardless of dimensional similarity, do not substitute across standards without formal engineering approval in these cases:
- Aerospace regulated assemblies (AMS, MIL-spec, FAA-approved maintenance): Material certification (AMS-R-83485 for FKM, for example), batch traceability, and dimensional conformance to the specified standard are all required. Dimensional equivalence from a different standard does not satisfy certification requirements.
- Medical device manufacturing (FDA 510(k), EU MDR): Material and dimensional specifications are fixed as part of the device design history file. Changing from one standard to another is a design change requiring design verification testing and regulatory review.
- High-pressure gas systems (> 100 bar): Gas leakage is far more sensitive to small dimensional variations than liquid leakage. A 0.2 mm ID difference that is inconsequential in a static oil seal may produce detectable gas leakage. Require a compression rate calculation and pressure leak test validation.
- High-cycle dynamic seals (> 100,000 cycles/year): Compression rate accuracy is critical — incorrect squeeze increases friction, heat generation, and wear rate. Small ID or CS differences that fall within "general industrial" tolerance may still shorten service life in high-cycle applications.
- Vacuum flange seals (KF/ISO-K, CF/ConFlat): These flanges use specific O-ring sizes defined by vacuum hardware standards (ISO 2861), not by AS568 or ISO 3601. Do not substitute standard O-rings for vacuum flange sizes.
Material and Hardness Considerations
Dimensional compatibility is necessary but not sufficient for a valid substitution. Also verify:
- Material: Same elastomer family (NBR, FKM, EPDM, etc.) from both specifications
- Hardness (Shore A): Same durometer within ±5 Shore A — a 70 Shore A AS568 seal should be replaced with a 70 Shore A ISO 3601 seal unless the application specifies different hardness
- Certification: If the original seal carries FDA, AMS, USP, or NACE certification, the substitute must carry the same certification — a dimensionally identical O-ring without the required certification is not an acceptable substitute
- Temperature grade: A standard NBR should not be substituted with high-ACN NBR without verifying TR10 cold temperature performance
How to Find the Right O-Ring From Groove Dimensions Only
When the original part number is unknown, calculate from groove dimensions:
Required CS from groove depth:
CS = Groove Depth / (1 − Target Compression / 100)- Static seal target compression: 15–25%
- Dynamic seal target compression: 12–15%
Example: Groove depth = 3.05 mm, static seal target 18% compression: → CS = 3.05 / 0.82 = 3.72 mm → nearest standard CS is 3.53 mm (next smaller standard value) → Verify resulting compression: (3.53 − 3.05) / 3.53 = 13.6% — within range for static use
Required ID for radial piston seal:
O-ring ID ≈ Groove bore diameter − (1–3% × groove bore diameter) for slight pre-stretchExample: Groove bore ID = 25.0 mm → target O-ring ID = 25.0 × 0.98 = 24.5 mm → nearest standard size is 25.00 mm (0.0% stretch) or search for a 24.5 mm ISO size
FAQ
Q1: Can I always replace an AS568 O-ring with an ISO 3601 size?
No — always check the dimensional difference against your groove tolerance and verify the resulting compression rate. Many popular sizes are equivalent or close enough for general industrial use, but others differ by more than the allowable tolerance. Follow the five-step verification procedure before approving any substitution.
Q2: Why do some ISO 3601 sizes exactly match AS568 sizes?
ISO 3601 was developed with intentional alignment to common AS568 sizes to facilitate global interoperability in industrial applications. The six standard cross-sections are identical in both standards, and many inside diameters are equivalent or within 0.1 mm. The ISO size series also includes additional smaller metric sizes not in AS568 (especially below 10 mm ID where AS568 has fewer sizes).
Q3: What if I only have the groove dimensions and need to find the right O-ring?
Calculate required CS from groove depth and target compression (CS = Groove Depth / (1 − compression%)). Then find required ID from groove bore diameter, accounting for 1–3% pre-stretch for radial seals. Match calculated values to the nearest standard size in AS568 or ISO 3601 tables and verify that the resulting compression falls within the 12–25% acceptable range.
Q4: Are JIS B 2401 and ISO 3601 interchangeable?
For common P-series JIS sizes, many ISO 3601 sizes are dimensionally close — but not identical in every case. Tolerance classes differ between the two standards, and JIS G-series (pipe) sizes follow a different sequence from ISO sizes. Always compare actual nominal dimensions and verify tolerance compliance before substituting JIS for ISO or vice versa in precision applications.
Q5: Does the conversion between inch and metric change the tolerance class?
No. Tolerance class (ISO 3601 Class A vs Class N, or AS568 vs AS568 Class 2) is a specification choice independent of the unit system. When ordering a metric equivalent of an AS568 O-ring, specify the ISO 3601 class explicitly — Class A for precision and dynamic applications, Class N for general industrial static service.
Q6: How do I identify whether an O-ring on existing equipment is inch or metric?
Measure the uncompressed O-ring cross-section diameter (CS) with digital calipers. If CS is very close to 1.78, 2.62, 3.53, 5.33, 6.99, or 8.74 mm, it is likely a standard AS568/ISO 3601 cross-section (both standards share these values). Measure the inside diameter and look up both AS568 and ISO 3601 tables — the equipment's country of origin usually indicates which standard was used. If the CS is approximately 1.9, 2.4, or 3.1 mm, it may be a JIS B 2401 P-series size.
Q7: What happens if I use an O-ring that is too large (ID too big)?
An O-ring with ID larger than the groove bore diameter will sit loosely in the groove without circumferential tension. In a piston groove, this means the O-ring may roll or spiral during assembly, and may not seat correctly. It will also have lower compression against the bore wall due to the lack of pre-stretch. For face seals, a slightly oversized ID is less critical — but if the ID is too large, the O-ring may buckle or fold. Maximum acceptable ID over-size for a piston seal: approximately 0.5% of groove diameter; for face seals, approximately 5%.
Q8: Is it safe to use a metric O-ring purchased from an overseas supplier in a US-designed hydraulic system?
Yes, if the dimensions have been verified against the procedure above, the material meets the original specification (compound, hardness, certification if applicable), and the tolerance class is equal to or tighter than the original. A metric ISO 3601 Class A O-ring in the dimensionally equivalent size is technically interchangeable with an AS568 O-ring for general industrial hydraulic service. For aerospace, medical, or safety-critical applications, the original standard must be maintained regardless of dimensional equivalence.
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Need to match a metric or inch O-ring to your groove dimensions? Request a quote with your measured CS, ID, groove bore, and groove depth — we will cross-reference AS568, ISO 3601, and JIS B 2401 and confirm the correct size and material. MOQ from 1 piece; stocked in AS568 and common ISO metric sizes across all major elastomer families.