GBT, ISO, or AS568: Which O-Ring Standard Should You Use? Wuxi ChuncoTech Rubber Helps You Choose

In our previous article, "Why Do Rubber Diaphragms Fail from Pressure Reversal? How Wuxi ChuncoTech Rubber Provides Bidirectional Sealing Solutions," we explored how pressure direction affects diaphragm design. Today, we tackle a fundamental question in O-ring selection that causes endless confusion in procurement, engineering, and maintenance: What is the difference between GB/T, ISO, and AS568 O-ring standards? If you have ever received a rejection because you specified "standard O-ring" without a dash number, or ordered AS568-214 only to find your local supplier offered a "similar metric size," you have asked yourself: Can I substitute a metric O-ring for an imperial dash number? Which standard applies to my equipment? How do I write a specification that suppliers actually understand?

The short answer is: You must simultaneously check the dimensional standard, gland standard, material standard, quality standard, and application conditions.Domestic hydraulic and pneumatic equipment in China typically follows the GB/T 3452 series. International projects typically follow ISO 3601 series. North American imported equipment, aerospace components, and SAE threaded fittings follow SAE AS568 dash numbers.

I. What Are the Three Major O-Ring Standards?

GB/T 3452 (Chinese National Standard) is the O-ring standard system for Chinese hydraulic and pneumatic applications. It consists of multiple parts: dimensions, gland design, quality inspection, backup rings, and material specifications. GB/T 3452.1-2005 modifies ISO 3601-1:2002. GB/T 3452.3 for groove design is currently under revision. For domestic equipment, this is your primary reference.

ISO 3601 (International Standard) is the O-ring system for international fluid power systems. ISO 3601-1:2012 is the current version (confirmed valid as of 2022 review). It includes dimension codes, tolerance classes A and B, and size identification. Export projects and international OEMs typically specify ISO rather than GB/T.

SAE AS568 (American Standard) is the SAE inch-system O-ring dimension standard. AS568-xxx dash numbers are three-digit codes that identify specific inner diameters and cross-sections in inches. AS568F (reaffirmed January 29, 2026) is the current version. North American equipment, aerospace, automotive, and SAE straight thread tube fitting boss gaskets use AS568 dash numbers.

The most important rule: Never substitute a "close metric size" for an AS568 dash number without engineering approval. A 2.62mm cross-section is not the same as a 2.5mm cross-section. A 3.53mm cross-section is not the same as 3.55mm. The gland is designed for the inch dimension.

II. GB/T 3452 Series: How to Use It Correctly

GB/T 3452 is not a single dimension table. It is a family of standards. In practice, you should reference the appropriate part:

GB/T 3452.1-2005 covers dimension series and tolerances. Select O-ring dimensions: inner diameter, cross-section, tolerance, and size identification code. This standard modifies ISO 3601-1:2002.

GB/T 3452.2-2007 covers visual quality inspection. Defect acceptance criteria for cracks, flash, voids, and surface imperfections. This standard is equivalent to ISO 3601-3:2005.

GB/T 3452.3-2005 covers groove dimensions for hydraulic and pneumatic O-rings. Use this when designing or checking gland dimensions. The standard is currently marked as "under revision."

GB/T 3452.4-2020 covers anti-extrusion rings (backup rings). Specify this for high pressure, large extrusion gaps, or reciprocating motion. This standard modifies ISO 3601-4:2008.

GB/T 3452.5-2022 covers elastomeric material specifications for industrial O-rings. This standard modifies ISO 3601-5:2015.

For domestic hydraulic and pneumatic equipment, design by GB/T 3452.1 for dimensions, GB/T 3452.3 for grooves, GB/T 3452.2 for quality, and GB/T 3452.5 or your company material specification for the rubber compound.

III. SAE AS568 Dash Numbers: How to Read Them

An AS568 dash number is a three-digit size identifier. For example, AS568-214 or simply -214. This is NOT a millimeter dimension and NOT the inner diameter value.

The AS568 dash series are organized by cross-section:

Zero / 0xx series has cross-section approximately 0.070 inches (1.78 mm). Used for small sizes and general small cross-section applications.

1xx series has cross-section approximately 0.103 inches (2.62 mm). Common for small to medium sizes.

2xx series has cross-section approximately 0.139 inches (3.53 mm). Very common for hydraulic systems, pumps, valves, and industrial equipment.

3xx series has cross-section approximately 0.210 inches (5.33 mm). Used for larger cross-section requirements.

4xx series has cross-section approximately 0.275 inches (6.99 mm). For large cross-section applications.

9xx series has varying cross-sections approximately 1.42-3.00 mm. Primarily used for tube fitting boss gaskets. Do not substitute with ordinary O-rings.

The SAE AS568F standard specifies inner diameter, cross-section, dimensional tolerances, and dash number identification codes for O-rings used in sealing applications and straight thread tube fitting boss gaskets.

IV. Which Standard Should You Choose for Your Application?

For imported equipment repair — First check the OEM manual, bill of materials, exploded parts diagram, or old part packaging. North American equipment, aerospace parts, and SAE port seals commonly use AS568 dash numbers. European or international OEMs more commonly use ISO 3601, DIN ISO, or customer proprietary standards.

Critical warning: Old parts can have dimensional drift due to compression set, swelling, or wear. When repairing, measure the old seal AND check the gland dimensions. Do not simply replace with the "closest metric size." A 2.62mm cross-section AS568 seal is NOT the same as a 2.5mm GB/T seal. Gland depths differ. Installation may damage the seal or cause immediate leakage.

For export products — If the customer drawing specifies AS568, quote and supply AS568 dash numbers. If the customer drawing specifies ISO 3601, quote the ISO dimension code, tolerance class, quality grade, and material specification.

ISO 3601-2 (2025 edition now published) includes groove dimensions for Class A O-rings (general industrial applications), Class B O-rings (mounting dimensions on specific metric hardware), and hydraulic/pneumatic applications with or without backup rings.

For domestic Chinese hydraulic and pneumatic equipment — For new designs, use GB/T 3452.1 for dimensions, GB/T 3452.3 for groove verification, GB/T 3452.2 for quality acceptance, and GB/T 3452.5 or your company material specification for the compound.

V. How to Write a Proper O-Ring Specification

A complete O-ring specification requires more than just "O-ring" and a size. Suppliers need standard, dimensions, material, hardness, color, quality grade, quantity, and any special requirements.

GB/T specification example : O-ring, GB/T 3452.1-2005, ID 25.0 × CS 3.55 mm, NBR 70 Shore A, black, quality per GB/T 3452.2, quantity 5,000 pieces.

ISO specification example : O-ring, ISO 3601-1, dimension code and tolerance class per drawing, ID × CS as shown on drawing, FKM 75 Shore A, quality grade per ISO 3601-3, quantity 2,000 pieces.

AS568 specification example : O-ring, AS568-214, FKM 75 Shore A, color as specified, quality grade per ISO 3601-3 or customer drawing, quantity 1,000 pieces.

Imported equipment repair inquiry : Provide equipment brand/model, old part photo, suspected dash number, measured ID and CS (millimeters), fluid type, temperature, pressure, installation position, and annual quantity. Only then can a supplier determine whether you need GB/T, ISO, AS568, or a customer proprietary part.

VI. Common Mistakes and How to Avoid Them

Mistake #1: Assuming AS568 and metric O-rings are interchangeable — A 2.62mm CS AS568 is NOT the same as 2.5mm. A 3.53mm CS is NOT the same as 3.55mm. Gland dimensions are designed for the specific cross-section. Using a metric O-ring in an AS568 gland will result in too much compression (too large CS) or too little compression (too small CS). Either way, the seal fails.

Mistake #2: Writing only "GB/T O-ring" with no part number — GB/T 3452 has multiple parts. Without specifying the part number and dimensions, the supplier cannot know which standard applies.

Mistake #3: Ignoring quality and material standards — The same AS568-214 can be made from NBR, FKM, EPDM, silicone, HNBR, or FFKM. Material determines chemical compatibility, temperature range, and service life. Always specify material and hardness.

Mistake #4: Using the old seal as the only reference — Old seals have taken compression set. A flattened, swollen, or shrunken seal does not represent the original dimensions. Always verify with gland measurements.

Mistake #5: Mixing ISO and GB/T versions — GB/T 3452.1-2005 modified ISO 3601-1:2002. The current ISO version is ISO 3601-1:2012. For export projects, specify the exact ISO version or follow the customer's requirement.

VII. Wuxi ChuncoTech Rubber: Your Standard-Compliant O-Ring Supplier

At Wuxi ChuncoTech Rubber , we manufacture O-rings to all three major standards:

• GB/T 3452 series for domestic Chinese equipment

• ISO 3601 series for international and export applications

• SAE AS568 dash numbers for North American and imported equipment

We also provide:

• Dimensional verification — we measure to ensure your AS568 seal matches the dash number tolerance, not just "looks close"

• Gland design review — we can check whether your groove is designed for metric or inch cross-sections

• Material traceability — full documentation of compound, hardness, and curing parameters

• Quality inspection — visual inspection per GB/T 3452.2 or ISO 3601-3

Do not guess. Do not substitute. Specify correctly.

VIII. Summary: Know Your Standard Before You Order

O-ring standards exist because the right seal depends on precise dimensions, material properties, and quality control. GB/T, ISO, and AS568 are not interchangeable without engineering review.

GB/T 3452 is for domestic Chinese hydraulic and pneumatic equipment. Use the full series: dimensions (part 1), quality (part 2), groove (part 3), backup rings (part 4), and material (part 5).

ISO 3601 is for international projects and export equipment. Specify the version (current is 3601-1:2012), dimension code, tolerance class, and quality grade.

SAE AS568 is for North American equipment, aerospace, and SAE ports. Use the three-digit dash number. Never substitute a "close metric size."

For procurement , write a complete specification: standard, part number or dimensions, material, hardness, color, quality grade, and quantity.

For repair , provide equipment brand, old part photo, measured dimensions, and application conditions.

For expert guidance and standard-compliant O-rings, contact Wuxi ChuncoTech Rubber today.

Take action now:

1. Visit our website https://www.chuncotechrubber.com/ to download our "O-Ring Standard Cross-Reference Guide."

2. Contact our engineering team — provide your standard requirement, dimensions, application, and quantity. We will provide free O-ring recommendations within 48 hours.

3. Request free samples or dimensional verification for your current O-ring needs.

Wuxi ChuncoTech Rubber — Sealing by standard. Quality by design.