MRO Magazine

Global O-Ring Standard Revised

MRO Magazine   

ISO3601, the standard covering O-rings, the most commonly used seal, has been revised to meet American (U. S.), European, as well as Asian needs. ISO 3601 consolidates the long-established AS 568B inc...

ISO3601, the standard covering O-rings, the most commonly used seal, has been revised to meet American (U. S.), European, as well as Asian needs. ISO 3601 consolidates the long-established AS 568B inch standard with the existing metric ISO 3601 and importantly, as it retains its original designation, means modification of documentation may not be required.

“Changes to the existing standard were needed to reflect what was happening in the marketplace,” says Bernd Murthum, O-ring product manager for Trelleborg Sealing Solutions, who is the convener on the ISO working group responsible for this latest revision of the ISO 3601 O-ring standard.

The American AS 568B standard dates back over 50 years. Originally, the high cost of tools to produce O-rings meant that inch O-rings were used in metric housings. Elastomer O-rings have a relatively large tolerance field between the minimum and maximum effective cross-section. At the lower limit there will be sufficient squeeze to fill the sealing groove and at the higher limit, it will not extrude beyond the sealing gap. This characteristic meant that AS 568B O-rings with widened tolerances could effectively fit into metric housings.

These wider tolerances were reflected in ISO 3601, initially drafted back in the early 1980s. The United States, which accounts for a third of the world’s market for O-rings, continued to use tolerances as specified by the AS standard, while the rest of the world worked to Europe’s widened tolerances.


“Discussions to harmonize the two standards have been ongoing for a number of years,” says Murthum.

The revised standard not only successfully embraces both the AS and ISO standard tolerances in a single standard but also extends the cross-sections covered. “ISO 3601 and AS 568B only describe five standard cross-sections. In reality in the marketplace, there are many more. Common practice was to use the principles of the ISO over a broader range of cross-sections. This is now recognized in the revised standard.”

Class A corresponds to the U. S. (American) standard AS 568B in its current format. Class B allows production of O-rings in technically acceptable and economical metric sizes and inch sizes, which can then fit into metric grooves. Also in Class B, there is now a table of tolerances covering cross-sections from 0.8 mm to 8.4 mm.

Until the harmonization between the AS and ISO standards, it was not possible to specify housing dimensions and retaining rings. Now two parts have been added to the standard that define these. Still under revision is a section on elastomer materials.

“One thing the working group ensured was that the ISO designation remained the same,” adds Murthum. “With such a long-established standard, this was important and means that documentation need not be changed, unless it includes the standard’s date.”

The five parts of the now valid O-ring standard ISO 3601 are as follows:

ISO 3601-1: 2008. Revised — inside diameters, cross-sections, tolerances and designation codes:

• Dimensions are expressed in both metric and inch measurements

• Class A is identical with AS568B

• Class B is similar to the tolerances of the previous ISO 3601-1: 2002 and includes an equation to calculate the tolerance for the O-ring inside diameter

• Range for O-ring cross-section diameter tolerances from 0.8 mm to 8.4 mm.

ISO 3601-2: 2008. New — housing dimensions for general applications:

• Covers technology for measuring surface roughness of O-ring housings

• Design recommendations for O-ring squeeze, elongation and compression, as well as maximum groove fill

• Equation for calculation of O-ring cross-section reduction caused by the elongation of the O-ring inner diameter.

ISO 3601-3: 2005. Partially revised — quality acceptance criteria for O-ring surfaces.

ISO 3601-4: 2008. New — retaining ring specifications: • Covers spiral Back-up Rings, solid type (cut/uncut), concave type (cut/uncut)

• Gives tolerances for outer and inner diameters depending on rod or piston applications

• Design recommendations but no size list for preferred housing dimensions.

ISO 3601-5: 2002. Currently in revision — suitability of elastomeric materials for industrial applications: • Will replace general information on material groups with detailed information of more practical use to design engineers

• Will comprise detailed material specifications for important material groups.

For more information, contact Trelleborg Sealing Solutions at

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