AS568 Standard Complete Guide: O-Ring Sizes, Tolerances, and Cross-Reference Tables for Engineers

The AS568 standard is the most widely referenced O-ring sizing system in North America and is used globally in aerospace, automotive, hydraulics, and industrial equipment. Originally developed by the Society of Automotive Engineers (SAE) and now maintained under SAE AMS-R-83485 and SAE AMS 3216/3217, the AS568 system defines 400+ dash sizes across five cross-sectional series. This guide provides the technical details engineers need to specify, procure, and inspect AS568 O-rings correctly.

History and Scope of AS568

The AS568 standard was first published in the 1950s to eliminate the proliferation of non-interchangeable O-ring sizes in military and aerospace equipment. The standard defines:

• Inside diameters (ID) and cross-sectional diameters (CS) for each dash number
• Dimensional tolerances for both ID and CS
• Five cross-sectional series based on CS
• Material and hardness callouts (typically referenced alongside the size)

While AS568 is a U.S. customary (inch-based) standard, it is cross-referenced to the metric ISO 3601-1 standard, allowing engineers to specify either system depending on their supply chain and customer requirements.

The Five Cross-Sectional Series

AS568 organizes O-rings into five series based on cross-sectional diameter. Each series is suited to different pressure, gland size, and application constraints.

Series 400 (Aerospace/Miniature)

• Cross-section: 0.070 ± 0.003 inch (1.78 mm)
• Dash numbers: −001 to −004 (non-standard IDs), −005 to −050
• Applications: Aerospace fittings, fuel systems, small-bore hydraulics, instrumentation
• Notes: The 400 series uses the same CS as the 200 series but includes smaller IDs not covered by the 200 series numbering convention.

Series 200 (Industrial Standard)

• Cross-section: 0.103 ± 0.003 inch (2.62 mm)
• Dash numbers: −102 to −178
• Applications: General industrial hydraulics, pneumatics, automotive, plumbing
• Notes: The most commonly used series in industrial equipment. Sizes −110 to −178 cover the majority of standard hydraulic cylinder and valve applications.

Series 300 (Heavy-Duty Industrial)

• Cross-section: 0.139 ± 0.004 inch (3.53 mm)
• Dash numbers: −210 to −395
• Applications: Heavy machinery, large hydraulic cylinders, process equipment
• Notes: The 300 series is the second most common series after the 200 series. It offers a good balance of seal contact area and gland space requirements.

Series 400 (Large Cross-Section)

• Cross-section: 0.210 ± 0.005 inch (5.33 mm)
• Dash numbers: −425 to −475
• Applications: Large-diameter flanges, pipelines, heavy construction equipment
• Notes: Used where larger seal volume is needed for pressure resistance and conformability on rough or large-diameter surfaces.

Series 500 (Extra Large Cross-Section)

• Cross-section: 0.275 ± 0.006 inch (6.99 mm)
• Dash numbers: −901 to −932
• Applications: Very large flanges, shipbuilding, off-road equipment, wind turbines
• Notes: The largest standard cross-section. Often used in split-gland or large-bore static sealing applications.

AS568 Size Structure and Nomenclature

A complete AS568 O-ring designation includes the prefix "AS568" followed by the dash number and the material specification. Example:

AS568-210 NBR 70 Shore A

This indicates:

• Standard: AS568
• Dash number: −210 (ID = 0.734 inch, CS = 0.139 inch)
• Material: Nitrile Butadiene Rubber
• Hardness: 70 Shore A

Common AS568 Sizes and Dimensions

Series 400 / 200 Cross-Section (CS = 0.070 inch / 1.78 mm)

Note: Dash −006 through −050 use 0.070 inch CS. Dash −102 upward use 0.103 inch CS in the 200 series.

Series 200 Cross-Section (CS = 0.103 inch / 2.62 mm)

Series 300 Cross-Section (CS = 0.139 inch / 3.53 mm)

Series 400 Cross-Section (CS = 0.210 inch / 5.33 mm)

Series 500 Cross-Section (CS = 0.275 inch / 6.99 mm)

Dimensional Tolerances

AS568 specifies tolerances for both inside diameter and cross-sectional diameter. Tolerances vary with size because larger O-rings are more difficult to mold with the same absolute precision as small ones.

Inside Diameter Tolerance Example (Selected Sizes)

Cross-Section Tolerance

Tighter tolerances are available for precision aerospace and semiconductor applications but must be specified as custom requirements. Standard commercial-grade AS568 O-rings meet the tolerances above.

Relationship to ISO 3601-1

ISO 3601-1 is the international metric standard for O-rings. While AS568 and ISO 3601-1 are not identical, there is substantial overlap. Many global manufacturers produce O-rings that conform to both standards simultaneously.

Key Differences

1. Primary units: AS568 uses inches; ISO 3601-1 uses millimeters.
2. Size range: ISO 3601-1 includes metric sizes not found in AS568, particularly for European machinery.
3. Tolerance expression: ISO 3601-1 grades tolerance classes A, B, and C, where Class A is the tightest.
4. Dash numbering: ISO uses a different coding system (e.g., 18.77 × 1.78 mm) rather than dash numbers.

AS568 to ISO 3601-1 Cross-Reference (Common Sizes)

For most engineering purposes, the dimensional differences between AS568 and the nearest ISO size are negligible (typically <0.5%). However, in critical aerospace or high-precision hydraulic systems, always specify the exact standard and verify supplier conformance.

Gland Design Reference for AS568 Sizes

The AS568 standard is frequently paired with SAE ARP 1231 and SAE ARP 5681 for gland design. These references provide groove dimensions for static and dynamic applications. General guidance:

Piston (Outer) Gland Dimensions

Rod (Inner) Gland Dimensions

Rod gland widths are typically 0.005–0.010 inch wider than piston glands to reduce friction and accommodate thermal expansion in dynamic service.

Material and Hardness Specification

AS568 defines sizes and tolerances but does not specify material. The material callout is added by the design engineer based on the application. Common material-hardness combinations include:

• NBR 70 Shore A: General-purpose hydraulic and pneumatic seals
• NBR 90 Shore A: High-pressure and abrasive environments
• FKM 75 Shore A: High-temperature and fuel-resistant seals
• FKM 90 Shore A: High-temperature, high-pressure static seals
• EPDM 70 Shore A: Water, steam, and brake fluid applications
• Silicone 70 Shore A: Low-temperature and food-grade applications

For aerospace applications, material specifications such as SAE AMS-R-83485 (FKM) or SAE AMS 3216 (NBR) are called out alongside the AS568 dash number.

Inspection and Quality Verification

Dimensional Inspection

• Measure ID using a tapered cone gauge or optical comparator for large sizes
• Measure CS using a dial thickness gauge at three points around the circumference
• Verify conformance to AS568 tolerance tables

Visual Inspection

• No flash exceeding 0.005 inch (0.13 mm)
• No porosity, blisters, or foreign inclusions
• No surface cracks or tears

Batch Testing

For critical applications, request batch certification including:

• Hardness (ASTM D2240)
• Tensile strength and elongation (ASTM D412)
• Compression set (ASTM D395)
• Fluid swell (ASTM D471, if required)

At O-Ring Supply Co., all AS568 O-rings are inspected to dimensional and visual standards with batch traceability available upon request. Custom sizes outside the AS568 tables are manufactured to customer drawings with no mold fees and MOQ of one piece.

FAQ

Q1: What does the dash number mean in AS568?

The dash number is a size identifier, not a dimension. For example, AS568-210 has an inside diameter of 0.734 inch and a cross-section of 0.139 inch. The dash number allows engineers to specify an exact size without quoting full dimensions.

Q2: Can I use an AS568 O-ring in a metric gland designed for ISO 3601-1?

Usually yes, for common sizes. AS568 and ISO 3601-1 share many near-identical dimensions. However, always verify the gland fill ratio and squeeze percentage using the actual measured dimensions rather than assuming interchangeability.

Q3: What is the largest AS568 size available?

The largest standard AS568 dash number is −932 in the 500 series, with a nominal ID of 25.975 inches (659.77 mm) and a cross-section of 0.275 inch (6.99 mm). Larger sizes are available as custom-molded or spliced/vulcanized rings.

Q4: Are AS568 tolerances tight enough for aerospace use?

Standard AS568 tolerances are acceptable for most aerospace applications, but some critical systems require Class A or tighter custom tolerances. Aerospace specifications such as SAE AMS-R-83485 may impose additional inspection and material requirements beyond the basic dimensional standard.

Q5: How do I specify a custom O-ring size that does not match an AS568 dash number?

Specify the inside diameter and cross-sectional diameter in inches or millimeters, along with the material, hardness, and tolerance requirements. Example: "O-ring, ID = 45.00 ± 0.25 mm, CS = 4.00 ± 0.10 mm, FKM 75 Shore A, black." Custom molds can be produced without tooling fees at O-Ring Supply Co. with 7–15 day lead time.