AS568 vs ISO 3601 O-Ring Sizes: Engineering Guide to Standard Selection & Interchange
Aug 11, 2026
BLUF: Two Standards, One Critical Distinction
AS568 and ISO 3601 are the two dominant O-ring dimensional standards governing global seal procurement. ISO 3601 Class A is dimensionally identical to AS568B-the same inside diameters, same cross-sections, same dash numbers-42-. ISO 3601 Class B provides a separate metric framework with different cross-section series and tolerance calculations-42. The practical consequence: an AS568 -214 O-ring and an ISO 3601 Class A -214 O-ring are the same physical part. An AS568 -214 and an ISO 3601 Class B 37.69 × 3.53 mm are not.
This distinction determines whether your seal fits the gland, compresses correctly, and holds pressure through thermal cycling.
Core Definitions: AS568 and ISO 3601
AS568 is the Aerospace Size Standard published by SAE International, first issued in 1971 and most recently updated as AS568D in 2014-17-1. It defines O-ring dimensions in inch-based fractional measurements and assigns each size a unique dash number (e.g., -006, -112, -214, -325). The dash number encodes both cross-section series and inside diameter-1.
ISO 3601 is the international standard for fluid power system O-rings, structured in five parts-42:
| Part | Title | Function |
|---|---|---|
| ISO 3601-1 | Inside diameters, cross-sections, tolerances, designation codes | Defines O-ring dimensions |
| ISO 3601-2 | Housing dimensions for general applications | Specifies gland geometry- |
| ISO 3601-3 | Quality acceptance criteria | Defines surface imperfection grades (N, S, CS)- |
| ISO 3601-4 | Retaining ring specifications | Back-up ring standards |
| ISO 3601-5 | Material specifications | Elastomer selection for industrial applications |
The 2008 revision of ISO 3601-1 introduced the Class A / Class B distinction to harmonize the American and metric systems-42.
Dimensional Comparison: Cross-Section Series
AS568 Cross-Section Series
AS568 organizes O-rings into five primary series based on cross-sectional diameter-17-1:
| Series | Dash Range | CS (inch) | CS (mm) | Tolerance (inch) |
|---|---|---|---|---|
| 000 | -001 to -099 | 0.070 | 1.78 | ±0.003 |
| 100 | -100 to -199 | 0.103 | 2.62 | ±0.003 |
| 200 | -200 to -399 | 0.139 | 3.53 | ±0.004 |
| 300 | -300 to -399 | 0.210 | 5.33 | ±0.005 |
| 400 | -400 to -499 | 0.275 | 6.99 | ±0.006 |
The 900 series covers boss gasket seals with variable cross-sections-17.
ISO 3601 Class A vs Class B
Class A is identical to AS568B-42-. The ISO 3601-1 size code for Class A O-rings is the same as the AS568 dash number--. Dimensional tolerances for Class A match AS568 specifications and are tighter than Class B-27.
Class B uses metric measurements and covers cross-sections from 0.8 mm to 8.4 mm-42. It includes both metric and inch sizes that fit into metric grooves, with tolerances calculated using a different methodology-42.
Key Dimensional Translation
| AS568 Dash | CS (inch) | CS (mm) | ID Range (inch) |
|---|---|---|---|
| -006 | 0.070 | 1.78 | 0.114 |
| -112 | 0.103 | 2.62 | 0.487 |
| -214 | 0.139 | 3.53 | 0.984 |
| -325 | 0.210 | 5.33 | 1.475 |
| -437 | 0.275 | 6.99 | 3.734 |
Conversion formula: ID (mm) = ID (inch) × 25.4; CS (mm) = CS (inch) × 25.4-2.
Quality Grades and Tolerances
ISO 3601-3 Quality Acceptance Criteria
ISO 3601-3 defines three grades for surface imperfections-flash, offset, parting-line projection, flow marks, and non-fills-27:
| Grade | Application | Description |
|---|---|---|
| Grade N | General-purpose | Standard acceptance for most industrial O-rings- |
| Grade S | Optimum performance | Tighter requirements for industrial and aerospace applications- |
| Grade CS | Critical service | Special requirements for aerospace and critical oxygen service |
Global O-Ring and Seal, for example, defines all standard O-rings to ISO 3601-3 Grade N, with stricter grades available upon request-27.
Dimensional Tolerances
AS568 dimensional tolerances are in accordance with ISO 3601-1 Class A-17-. For the 1/16 inch (1.78 mm) series, the cross-section tolerance is ±0.08 mm-. Tolerances widen as cross-section increases-the 0.275 inch (6.99 mm) series carries ±0.006 inch (±0.15 mm)-17.
Gland Design: The Critical Link
The O-ring standard you choose must align with your gland (groove) dimensions. This is where interchangeability failures occur.
Gland Design Standards
| Standard | Gland Reference |
|---|---|
| AS568 | Parker O-Ring Handbook ORD 5700 tables-33 |
| ISO 3601 Class A | ISO 3601-2 housing dimensions- |
| ISO 3601 Class B | ISO 3601-2 with preferred metric-dimensioned hardware |
Gland Design Calculation Example
For an AS568 -214 O-ring (CS = 0.139 inch / 3.53 mm)-:
| Parameter | Value |
|---|---|
| Gland depth | 0.111 – 0.113 inch |
| Squeeze | 0.022 – 0.032 inch (approx. 15–25% compression) |
| Gland width | 0.189 inch |
| Free volume | 6–15% of gland volume |
A common error is ordering by nominal bore alone without confirming cross-section-. For an AS568 -214 gland, using an ISO 3601 Class B metric O-ring with a 3.55 mm cross-section-only 0.02 mm larger-changes compression by approximately 0.6%, potentially causing extrusion or installation damage-.
Interchangeability: What Works, What Doesn't
Direct Interchange (Safe)
AS568 ↔ ISO 3601 Class A: Fully interchangeable-42. The ISO 3601-1 size code matches the AS568 dash number-. Example: AS568 -214 = ISO 3601 Class A -214.
No Direct Interchange (Risk)
AS568 ↔ ISO 3601 Class B: Not directly interchangeable-1. The cross-section series differ. A 3.53 mm AS568 cross-section does not equal a 3.55 mm ISO Class B cross-section, despite the numerical proximity.
Cross-Reference Tools
International cross-reference tables map AS568 to ISO 3601, BS 1806, DIN 3771, JIS B 2401, and other standards-41. These tables do not account for tolerances or materials-use them as screening tools, not final approval documents-41.
Selection Decision Matrix
| Your Situation | Recommended Standard | Rationale |
|---|---|---|
| North American equipment, AS568 drawings | AS568 (or ISO 3601 Class A) | Direct fit, documented gland dimensions |
| European/Asian equipment, metric drawings | ISO 3601 Class B | Metric hardware, ISO-specified glands |
| Global procurement, mixed installed base | ISO 3601 Class A | Serves as AS568 equivalent with ISO documentation |
| New design, no legacy constraints | Either-but document the choice | Gland design must match the selected standard |
Procurement Specifications: What to Include in Your RFQ
When sourcing O-rings, provide:
Standard and designation: "AS568 -214" or "ISO 3601 Class A -214" or "ISO 3601 Class B 37.69 × 3.53 mm"
Material: Elastomer type, hardness (Shore A), and any compliance requirements (FDA, NSF, WRAS, etc.)
Quality grade: ISO 3601-3 Grade N, S, or CS-
Quantity: Annual volume and order frequency
Gland dimensions: If available, for verification
Xiamen Yuefeng Seal Co., Ltd. supplies custom O-rings in AS568 and ISO 3601 standards, with NBR, FKM, EPDM, Silicone, and FFKM compounds. Standard sizes such as AS568 (Parker sizes) and metric sizes including BS1516 are available, with inner diameters from 0.5 mm to 150 mm and custom tooling available without mold fees for many common sizes-.
FAQ
Q: What is the minimum order quantity (MOQ) for AS568 and ISO 3601 O-rings?
A: MOQ depends on size and material. Standard AS568 sizes in NBR 70 Shore A typically have no MOQ for stock sizes. Custom metric sizes or specialty compounds (FKM, FFKM) generally require 100–500 pieces per size. Request a quote with your specific dimensions for exact MOQ.
Q: What are the tooling costs for non-standard O-ring sizes?
A:For cross-sections matching standard series (1.78, 2.62, 3.53, 5.33, 6.99 mm), tooling costs are typically waived for production quantities. Non-standard cross-sections or diameters outside standard ranges require custom molds; tooling costs range from $200–$800 depending on complexity, amortized over the order quantity.
Q: What is the lead time for AS568 and ISO 3601 O-ring orders?
A: Stock AS568 sizes in NBR and FKM ship within 3–5 business days. Custom metric sizes or non-standard compounds require 10–15 business days for production. FFKM and specialty materials may extend to 20–25 business days. Air freight options are available for expedited requirements.
