ISO 3601 O-Ring Standard: Sizes, Materials & Grades

O-rings are among the simplest and most widely used sealing components in hydraulic, pneumatic, automotive, aerospace, and industrial equipment. Despite their basic circular shape, O-rings must be manufactured and installed with carefully controlled dimensions. A small difference in inside diameter, cross-sectional diameter, groove depth, material hardness, or surface quality can significantly affect sealing performance.

When an O-ring is installed in a properly designed groove, it is compressed between two mating surfaces. This initial compression creates a sealing force that prevents liquid or gas from passing through the connection. As system pressure increases, the pressure pushes the elastomer toward the low-pressure side of the groove, strengthening the seal. However, excessive compression, insufficient groove space, incorrect clearance, or an incompatible elastomer can result in leakage or premature failure.

The ISO 3601 series provides internationally recognized requirements for selecting, designing, manufacturing, and inspecting O-rings used in fluid power systems. It addresses several important elements of an O-ring sealing system, including:

• Inside diameter and cross-sectional diameter
• Dimensional tolerances and size designation codes
• Housing and groove dimensions
• Surface quality and acceptance criteria
• Anti-extrusion or backup rings
• Elastomeric material specifications

Using ISO 3601 allows designers, manufacturers, and maintenance personnel to work with a consistent dimensional system. An O-ring ordered from one qualified supplier can therefore fit equipment designed by another manufacturer, provided that the size, material, tolerance class, and quality grade are specified correctly.

The standard is particularly important in hydraulic and pneumatic systems, where seals may be exposed to high pressure, repeated movement, wide temperature ranges, and different working fluids. ISO 3601 O-rings are used in cylinders, valves, pumps, manifolds, tube fittings, hydraulic ports, regulators, actuators, and many other fluid-handling components.

However, compliance with ISO 3601 dimensions alone does not guarantee successful sealing. The designer must also consider fluid compatibility, operating temperature, pressure, extrusion clearance, surface finish, compression set, installation method, and whether the application is static or dynamic.

This guide explains the structure of the ISO 3601 standard, its O-ring sizes and tolerances, groove-design requirements, quality grades, material specifications, and relationship with other commonly used O-ring standards.

1. What Is the ISO 3601 O-Ring Standard?

ISO 3601 is a series of international standards for O-rings used primarily in fluid power systems. It establishes a common framework for specifying O-ring dimensions, housing geometry, manufacturing quality, backup rings, and elastomeric materials.

The standard supports both general industrial and aerospace applications. It enables engineers to identify an O-ring using standardized dimensions and designation codes instead of relying only on a manufacturer’s proprietary part number.

An O-ring is defined mainly by two dimensions:

• Inside diameter (d1d_1): The diameter measured across the internal opening of the O-ring
• Cross-sectional diameter (d2d_2): The diameter or thickness of the circular elastomer section

The outside diameter can be estimated as:

OD=d1+2d2

For example, an O-ring with a 20 mm inside diameter and a 3 mm cross-sectional diameter has an approximate outside diameter of:

OD=20+(2×3)=26 mm

The inside diameter and cross-section must both match the intended groove. Selecting an O-ring based only on its outside diameter is unreliable because different combinations of d1 and d2 can produce similar outside dimensions.

Purpose of ISO 3601

The primary purpose of ISO 3601 is to promote dimensional consistency and interchangeability. It provides a standardized reference that can be used throughout the design, procurement, manufacturing, and inspection processes.

The series helps organizations:

• Select standardized O-ring sizes
• Define acceptable dimensional tolerances
• Design suitable grooves and housings
• Specify allowable surface imperfections
• Select standardized backup-ring configurations
• Identify appropriate elastomeric material specifications
• Communicate sealing requirements clearly to suppliers

This reduces the risk of installing an O-ring that appears to fit but does not provide the required squeeze, groove fill, or resistance to extrusion.

Applications of ISO 3601 O-Rings

ISO 3601 O-rings can be used in static and dynamic sealing arrangements. Common applications include:

• Hydraulic cylinder pistons and rods
• Pneumatic cylinders and actuators
• Pumps and motors
• Directional, pressure, and flow-control valves
• Hydraulic manifolds and ports
• Flanges and covers
• Tube and hose fittings
• Pressure regulators
• Mobile equipment
• Aerospace fluid systems
• Industrial processing equipment

In a static application, the mating parts do not move relative to the O-ring after assembly. Examples include flange seals, port seals, and manifold face seals. Static O-rings can often tolerate higher squeeze because they are not continuously exposed to friction.

In a dynamic application, one surface moves relative to the seal. Dynamic applications include reciprocating piston and rod seals and certain rotary arrangements. These applications require careful control of squeeze, lubrication, friction, surface finish, and clearance.

ISO 3601 does not mean that every listed O-ring is suitable for every fluid or operating condition. The dimensions may be standardized, but the elastomer compound must still be selected for the actual temperature, pressure, chemical exposure, and service environment.

2. ISO 3601 Standard Parts Explained

The ISO 3601 series consists of five complementary parts. Each part addresses a different aspect of O-ring standardization. Together, they provide a more complete specification than an O-ring size number alone.

ISO 3601-1: Inside Diameters, Cross-Sections, Tolerances and Designation Codes

ISO 3601-1 establishes O-ring inside diameters, cross-sectional diameters, dimensional tolerances, and designation codes. It covers O-rings intended for general industrial and aerospace fluid power applications.

This part is normally the starting point when identifying or ordering an ISO O-ring. It tells the user what dimensions correspond to a particular size code and how much dimensional variation is permitted.

The current published base edition is ISO 3601-1:2012, together with its technical corrigendum and 2019 amendment. The dimensions are applicable to different elastomeric materials when suitable manufacturing tooling is available. ISO 3601-1 catalogue

ISO 3601-2: Housing Dimensions for General Applications

ISO 3601-2 specifies the dimensions of the housing or groove in which the O-ring is installed. It covers both Class A O-rings for general industrial applications and Class B O-rings used with selected metric hardware, including fluid-power cylinder bores and piston rods.

The standard provides housing recommendations for general hydraulic and pneumatic applications, both with and without backup rings. The latest edition is ISO 3601-2:2025, which supersedes the withdrawn 2016 edition. ISO 3601-2:2025

This part addresses design factors such as:

• Groove diameter
• Groove width and depth
• Radial or axial compression
• Diametral clearance
• Lead-in chamfers
• Edge radii
• Surface-finish requirements
• Space for backup rings

The housing dimensions must be considered together with the tolerances of the metal components and the O-ring. Using the nominal O-ring dimensions without completing a tolerance analysis can result in too much or too little compression.

ISO 3601-3: Quality Acceptance Criteria

ISO 3601-3 establishes quality requirements for finished O-rings. It defines allowable limits for surface imperfections that can occur during molding, trimming, handling, or other manufacturing processes.

Typical imperfections include:

• Flash
• Parting-line projections
• Flow marks
• Non-fills
• Pits and voids
• Cuts and indentations
• Foreign-material inclusions
• Excessive surface roughness

An O-ring may have the correct dimensions but still be unsuitable if a cut, void, or molding defect crosses a critical sealing surface. ISO 3601-3 therefore helps manufacturers and purchasers apply consistent visual inspection and acceptance criteria.

The base publication is ISO 3601-3:2005, supplemented by Amendment 1:2018. ISO sealing-device catalogue

ISO 3601-4 specifies dimensions and tolerances for anti-extrusion rings, commonly called backup rings. These components support the O-ring and reduce the risk of the elastomer being forced into the clearance gap under pressure.

The standard covers five backup-ring types:

1. Spiral type
2. Angle-cut type
3. Solid type
4. Angle-cut concave type
5. Solid concave type

Backup rings are especially important when system pressure, temperature, or component clearance exceeds the practical extrusion resistance of the selected elastomer. ISO 3601-4

ISO 3601-5: Elastomeric Materials for Industrial Applications

ISO 3601-5 specifies selected standard elastomeric materials for O-rings used in general industrial applications. It provides a consistent basis for defining material properties rather than identifying an O-ring only by a generic polymer name.

For example, stating that an O-ring is “NBR” or “FKM” does not fully define its performance. Compounds within the same polymer family can have different hardness, temperature capability, compression set, chemical resistance, and mechanical properties.

ISO 3601-5:2015 helps standardize the material specification, but final selection must still be verified against the actual fluid, temperature range, pressure, and service conditions.

3. ISO 3601 O-Ring Sizes and Designation System

ISO 3601-1 standardizes the dimensions and designation codes used to identify O-rings. Each O-ring is primarily defined by its nominal inside diameter and cross-sectional diameter.

The two basic dimensions are:

• Inside diameter (d1d_1): the diameter of the opening inside the O-ring
• Cross-sectional diameter (d2d_2): the thickness of the circular elastomer section

The approximate outside diameter is calculated as:

d3=d1+2d

For example, an O-ring with an inside diameter of 25 mm and a cross-section of 3.55 mm has an approximate outside diameter of:

d3=25+(2×3.55)=32.10 mm

The outside diameter is useful for inspection, but O-rings should normally be specified using the inside diameter and cross-sectional diameter.

ISO 3601 size classes

ISO 3601-1 divides O-ring sizes into two main dimensional classes:

• Class A: O-rings based principally on the inch-size system used in general industrial and aerospace applications
• Class B: O-rings intended mainly for metric-dimensioned hardware

Class A O-rings generally have tighter inside-diameter tolerances than Class B O-rings. They may therefore be preferable when dimensional precision or compatibility with internationally standardized inch-based hardware is important.

Class B includes metric sizes selected for equipment such as hydraulic cylinder bores and piston rods. The class does not indicate the material, hardness, or surface-quality grade of the O-ring.

Size codes

ISO 3601 uses standardized size codes to simplify O-ring identification. The size code corresponds to a specific combination of nominal inside diameter and cross-sectional diameter.

A complete ordering description should not rely on the size code alone. It may also include:

• ISO 3601 reference
• Dimensional class
• Size code
• Elastomer type or compound
• Hardness
• Quality grade
• Additional application requirements

A typical specification may therefore identify an O-ring as an ISO 3601 size manufactured from 70 Shore A NBR with Grade N surface-quality requirements.

Nominal versus actual dimensions

The dimensions listed in a size chart are nominal values. The actual manufactured O-ring may be slightly larger or smaller within the tolerances permitted by ISO 3601-1.

For example, an O-ring with a nominal cross-section of 3.55 mm will not necessarily measure exactly 3.55 mm at every location. A limited variation is allowed because elastomer molding, shrinkage, material behavior, and inspection conditions affect the final dimensions.

Designers must account for this variation when calculating:

• Minimum and maximum squeeze
• Groove fill
• O-ring stretch
• Diametral clearance
• Extrusion risk

Replacing an ISO O-ring with one having approximately the same outside diameter is not recommended. Even a small difference in cross-section can substantially change compression and groove fill.

4. Dimensional Tolerances and Size Classes

Dimensional tolerances are essential because sealing performance depends on the relationship between the O-ring and its groove. ISO 3601-1 establishes allowable variations for the inside diameter and cross-sectional diameter of each standardized size.

The tolerance is not necessarily the same for every O-ring. It can vary according to nominal size, dimensional class, and intended application.

Inside-diameter tolerance

Inside-diameter tolerance generally increases as the O-ring becomes larger. A small O-ring requires relatively tight dimensional control, while a very large O-ring may be permitted a greater absolute variation.

Inside-diameter variation affects how the O-ring fits over a piston, shaft, or groove. If the O-ring is too small, it may be overstretched during installation. Excessive stretch reduces its cross-sectional diameter and may create insufficient squeeze.

If the inside diameter is too large, the O-ring may fit loosely in the groove. It can twist, pinch, or fall out during assembly. In dynamic applications, a loose O-ring may also roll within the groove and suffer spiral failure.

Cross-sectional tolerance

Cross-sectional diameter has a direct effect on compression. An O-ring at the upper end of its cross-sectional tolerance receives more squeeze in a fixed-depth groove. An O-ring at the lower end receives less squeeze.

Excessive squeeze can cause:

• High assembly forces
• Increased friction
• Rapid heat generation
• Accelerated compression set
• Excessive wear in dynamic service
• Insufficient space for thermal expansion

Insufficient squeeze can result in:

• Leakage at low pressure
• Poor sealing during pressure fluctuations
• Loss of contact after compression set
• Reduced sealing reliability under vibration

The groove must therefore be designed using the worst-case dimensional conditions rather than nominal dimensions alone.

Class A and Class B tolerances

Class A covers O-rings commonly used in general industrial and aerospace applications. These sizes are generally associated with tighter inside-diameter tolerances.

Class B is intended mainly for metric-dimensioned hardware. Its dimensional arrangement is suitable for components such as metric cylinder bores, pistons, and rods.

Class A and Class B should not be confused with ISO 3601-3 quality grades. A dimensional class controls size and tolerance, while a quality grade controls allowable surface imperfections.

Measuring an O-ring

Small O-rings can be measured using an optical comparator, measuring cone, or other suitable non-contact equipment. Large O-rings may be measured using circumference tapes or specialized fixtures.

Ordinary calipers must be used carefully because excessive contact pressure can deform the elastomer and produce an inaccurate result. The O-ring should be:

• Clean and undamaged
• Free from tension or compression
• Allowed to stabilize after storage or shipment
• Measured at the specified temperature
• Inspected without being twisted

The cross-section should be checked at several positions because molding variation may not be uniform around the entire circumference.

5. O-Ring Housing and Groove Design According to ISO 3601-2

ISO 3601-2 specifies housing dimensions for O-rings used in general hydraulic and pneumatic applications. It covers Class A O-rings and selected Class B metric O-rings, with provisions for installations using backup rings.

A properly selected O-ring can still fail if its groove is incorrectly designed. Important design parameters include groove depth, width, clearance, compression, fill, stretch, surface finish, and edge geometry.

Static radial seals

A static radial seal is installed between concentric cylindrical surfaces that remain stationary after assembly. The O-ring may be located in a groove on an internal piston or an external housing.

Radial compression occurs when the available sealing space is smaller than the free cross-sectional diameter of the O-ring. The amount of squeeze must be sufficient to establish initial contact without excessively deforming the elastomer.

Static radial seals are commonly used in:

• Hydraulic plugs
• Valve cartridges
• Tube fittings
• Manifold passages
• Cylinder heads
• Stationary sleeves

Static axial seals

An axial seal, also called a face seal, is compressed between two surfaces perpendicular to the axis of the assembly. It is commonly used between flanges, covers, end caps, and manifold faces.

The groove should be positioned so that system pressure helps push the O-ring toward a supporting groove wall. The designer must determine whether pressure acts from the inside or outside of the sealed diameter.

Reversing the pressure direction without reviewing the groove arrangement may allow the O-ring to move away from its intended support.

Dynamic seals

Dynamic applications involve movement between the O-ring and the mating surface. Common arrangements include:

• Reciprocating piston seals
• Reciprocating rod seals
• Slowly rotating or oscillating shafts

Dynamic grooves usually require lower compression than static grooves to limit friction, heat, and wear. Surface finish, lubrication, alignment, and operating speed become particularly important.

Although O-rings can be used dynamically, they may not be the best choice for high-speed rotary service. Specialized rotary seals are often more appropriate.

O-ring squeeze

O-ring squeeze is the reduction in cross-sectional height after installation. For a radial arrangement, it can be expressed approximately as:

The final percentage must be checked against the application type, O-ring tolerances, groove tolerances, material behavior, temperature, and manufacturer recommendations.

Groove width and fill

The groove must be wide enough to accommodate the compressed O-ring. Elastomers are almost incompressible by volume, so squeezing an O-ring changes its shape rather than significantly reducing its total volume.

Additional groove space is required for:

• Thermal expansion
• Fluid-induced swelling
• Dimensional tolerances
• O-ring movement
• Lubrication
• Backup rings, when required

Excessive groove fill may prevent the O-ring from deforming properly and can generate high contact stresses. A groove that is too wide, however, may allow undesirable movement in dynamic service.

Stretch and compression

An O-ring installed in an external groove is normally stretched slightly to remain seated. Excessive stretch reduces the cross-sectional diameter and can shorten service life.

An O-ring fitted inside a bore may experience circumferential compression. Too much compression can cause the seal to buckle or wrinkle inside the groove.

Stretch and compression must therefore be included in the squeeze and groove-fill calculations.

Clearance and extrusion

Under high pressure, the O-ring can be forced into the clearance gap between mating components. Repeated pressure cycles may cut or nibble the elastomer, producing extrusion damage.

Extrusion risk increases with:

• Higher pressure
• Larger clearance gaps
• Higher temperature
• Softer elastomer compounds
• Pressure cycling
• Reduced material strength

The risk can be reduced by minimizing clearance, selecting a harder or more extrusion-resistant compound, or installing backup rings in accordance with ISO 3601-4.

Surface finish and installation geometry

The sealing surface must be smooth enough to prevent leakage and wear but appropriate for retaining the required lubrication film. Dynamic surfaces generally require more carefully controlled finishes than static surfaces.

Sharp edges, threads, splines, drilled ports, and burrs can damage an O-ring during installation. Suitable lead-in chamfers, rounded edges, clean grooves, and installation tools help prevent cutting or shaving.

Correct groove design must always be combined with suitable material selection, lubrication, cleanliness, and installation practice. ISO 3601-2 provides standardized housing dimensions, but the complete application conditions must still be evaluated before the design is finalized.

6. ISO 3601 Quality Grades and Acceptance Criteria

An O-ring can meet the required dimensional tolerances and still be unsuitable for service if it contains cuts, cracks, excessive flash, voids, or other surface defects. ISO 3601-3 establishes quality acceptance criteria for finished O-rings and defines limits for manufacturing imperfections.

The standard does not require every O-ring surface to be perfectly free of visible marks. Instead, it classifies imperfections according to their type, location, and allowable size. Acceptance limits depend on the O-ring dimensions and specified quality grade.

ISO 3601-3 defines three primary quality grades:

Grade N is sufficient for many standard industrial applications. Grade S applies tighter limits to surface imperfections and is commonly selected when improved reliability is required. Grade CS imposes highly restrictive acceptance criteria for critical sealing services.

A higher grade does not automatically provide better material compatibility, temperature resistance, or dimensional accuracy. The grade primarily controls surface quality. Material type, compound properties, hardness, and dimensional class must be specified separately.

Common O-ring surface imperfections

O-rings are normally manufactured by compression, transfer, or injection molding. The molding and finishing processes can produce several types of imperfections.

Common examples include:

• Flash: Thin excess material at the mold parting line
• Parting-line projection: A raised section where the mold halves meet
• Offset: Misalignment between the two halves of the molded O-ring
• Flow marks: Irregular patterns produced as the elastomer flows through the mold
• Non-fill: An area where the mold cavity was not completely filled
• Pits and voids: Small depressions or missing material on the surface
• Foreign-material inclusions: Contaminants embedded in the elastomer
• Cuts or indentations: Mechanical damage caused during trimming, handling, or installation
• Excessive trimming: Removal of too much material from the parting line

The effect of a defect depends strongly on its location. A cut extending across the sealing surface is generally more serious than a small mark located away from the primary contact area.

Visual inspection

Inspection may be completed using suitable lighting, magnification, measuring equipment, and reference samples. The O-ring should be examined over its entire circumference, including the mold parting line.

Inspectors should evaluate:

• Type of imperfection
• Length, width, and depth
• Number of imperfections
• Distance between adjacent defects
• Location relative to the sealing surface
• Applicable acceptance grade

O-rings should not be stretched excessively during inspection because stretching can distort defects or create misleading measurements.

Selecting the appropriate quality grade

Specifying the strictest grade for every application may unnecessarily increase manufacturing and inspection costs. The selected grade should reflect the consequences of leakage, system pressure, fluid type, maintenance accessibility, and overall equipment criticality.

Grade N is normally appropriate for noncritical industrial equipment. Grade S or CS may be justified for aerospace systems, hazardous fluids, high-purity service, difficult-to-access equipment, or applications where seal failure could create significant safety or production consequences.

The required grade should be included in purchasing documents. If no quality grade is stated, the supplier and purchaser may apply different acceptance criteria even when they agree on the O-ring size and material.

7. O-Ring Materials and ISO 3601-5 Compatibility

ISO 3601-5 specifies selected elastomeric materials for O-rings used in general industrial applications. Material selection is essential because an O-ring with the correct dimensions and groove design can still fail rapidly if its compound is incompatible with the fluid or operating temperature.

An elastomer should be selected according to:

• Working fluid or gas
• Minimum and maximum temperature
• Operating and peak pressure
• Static or dynamic service
• Required hardness
• Compression-set resistance
• Exposure to ozone, sunlight, or weather
• Decompression conditions
• Cleanliness and regulatory requirements

The generic polymer name is only the beginning of the selection process. Two compounds described as NBR, for example, may perform differently because of their formulation, hardness, fillers, curing system, and additives.

Nitrile rubber—NBR

NBR is one of the most widely used O-ring materials in hydraulic and pneumatic equipment. It generally offers good resistance to mineral-based hydraulic oils, lubricants, and many petroleum fluids.

Its advantages include:

• Good abrasion resistance
• Good mechanical properties
• Relatively low cost
• Wide availability
• Reliable performance in general hydraulic service

Standard NBR is generally unsuitable for phosphate-ester fluids, strong ozone exposure, and some polar solvents. Its low- and high-temperature capabilities depend on the specific compound.

Fluorocarbon rubber—FKM

FKM is selected where higher temperature and chemical resistance are required. It is commonly used with fuels, mineral oils, synthetic lubricants, and many aggressive chemicals.

FKM offers:

• Good high-temperature capability
• Excellent resistance to many oils and fuels
• Low gas permeability
• Good resistance to ozone and weathering
• Good compression-set resistance at elevated temperatures

However, standard FKM compounds may not be suitable for hot water, steam, certain amines, or some low-temperature applications. Specialized FKM compounds can extend performance in particular environments.

Ethylene propylene rubber—EPDM

EPDM is commonly used with hot water, steam, glycol-based fluids, brake fluids, and many water-based chemicals. It also provides good resistance to ozone, sunlight, and outdoor weathering.

EPDM is generally not compatible with mineral oils, petroleum fuels, or hydrocarbon lubricants. Installing an EPDM O-ring in a mineral-oil hydraulic system can cause excessive swelling, loss of strength, and leakage.

Hydrogenated nitrile rubber—HNBR

HNBR is a modified form of nitrile rubber that provides improved temperature, wear, and chemical resistance. It is used in demanding hydraulic, automotive, and oil-and-gas applications.

Compared with conventional NBR, HNBR generally provides:

• Better heat resistance
• Improved mechanical strength
• Greater resistance to ozone and aging
• Better wear performance
• Good resistance to many petroleum-based fluids

Compatibility must still be confirmed for the actual chemical mixture and temperature.

Silicone rubber—VMQ

Silicone O-rings offer a wide operating-temperature range and remain flexible at very low temperatures. They are used in aerospace, food-related equipment, electronics, and static sealing applications.

Silicone generally has lower tear strength and abrasion resistance than NBR or HNBR. It is therefore more suitable for static service than for demanding reciprocating applications.

Perfluoroelastomer—FFKM

FFKM provides exceptional chemical and high-temperature resistance. It is used in semiconductor processing, chemical service, pharmaceutical equipment, and other severe applications.

Its advantages are accompanied by a much higher cost. FFKM should normally be selected only when less expensive elastomers cannot meet the service requirements.

Hardness selection

O-ring hardness is commonly expressed in Shore A units. A nominal hardness of approximately 70 Shore A is widely used for general service.

Softer compounds can conform more easily to minor surface irregularities and may seal effectively at lower pressure. Harder compounds offer greater resistance to extrusion but may require higher assembly force and better surface control.

Typical selections include:

• Approximately 70 Shore A for general service
• Approximately 80 Shore A for moderately high pressure
• Approximately 90 Shore A for high-pressure or extrusion-resistant applications

These values are only general guidelines. Pressure, clearance, motion, temperature, and compound properties must be evaluated together.

Fluid compatibility and temperature

Fluid compatibility charts are useful for preliminary screening, but they should not be treated as absolute approval. Actual performance can be influenced by fluid concentration, additives, contamination, temperature, pressure, and exposure time.

An incompatible elastomer may experience:

• Excessive swelling
• Shrinkage
• Hardening
• Softening
• Cracking
• Loss of tensile strength
• Increased compression set
• Chemical decomposition

Where service conditions are uncertain, the equipment manufacturer or seal supplier should confirm compatibility using application data or laboratory immersion testing.

8. Backup Rings and High-Pressure Sealing

Backup rings, also called anti-extrusion rings, are installed beside an O-ring to prevent the elastomer from entering the clearance gap between mating components. ISO 3601-4 standardizes backup-ring dimensions and tolerances for use with selected ISO 3601 O-ring sizes and housings.

A backup ring does not normally create the primary seal. Its main function is to provide mechanical support to the O-ring on the low-pressure side of the groove.

How O-ring extrusion occurs

When pressure is applied, the O-ring moves toward the low-pressure side of the groove. If the clearance gap is too large, the elastomer can be forced into it.

During repeated pressure cycles, the extruded material may be cut or torn by the edges of the mating components. The resulting damage is frequently described as nibbling or extrusion failure.

Typical evidence includes:

• Ragged or chewed edges
• Material loss on the low-pressure side
• Repeated small cuts around the circumference
• Flattening near the clearance gap
• Leakage after pressure cycling

Extrusion risk increases as pressure and temperature rise because the elastomer is subjected to greater force and may become softer.

Factors affecting extrusion resistance

The likelihood of extrusion depends on several interacting conditions:

• Maximum and peak pressure
• Size of the clearance gap
• Elastomer hardness
• Operating temperature
• Pressure cycling frequency
• O-ring material strength
• Component deflection under load
• Groove geometry
• Direction of pressure

A clearance that appears acceptable at atmospheric conditions may increase when a cylinder, housing, or piston deforms under working pressure.

Backup-ring arrangements

For pressure acting in one direction, one backup ring is normally installed on the low-pressure side of the O-ring. This is the side toward which the pressure pushes the elastomer.

For pressure that can alternate between two directions, backup rings may be required on both sides of the O-ring. This arrangement supports the seal regardless of the direction of the pressure differential.

The basic configurations are:

Installing a single backup ring on the wrong side leaves the O-ring unsupported when pressure is applied.

Backup-ring types

ISO 3601-4 covers several configurations, including spiral, angle-cut, solid, angle-cut concave, and solid concave backup rings.

Split or spiral designs can be installed into closed grooves more easily. Solid backup rings provide continuous support but may require an open or separable housing for installation. Concave backup rings have a profile designed to support part of the O-ring surface.

The appropriate type depends on groove accessibility, pressure, movement, temperature, and the selected O-ring size.

Backup-ring materials

Polytetrafluoroethylene, commonly known as PTFE, is widely used because it offers:

• Low friction
• Broad chemical resistance
• Good temperature capability
• Resistance to permanent compression
• Compatibility with many hydraulic fluids

Other materials may include filled PTFE, thermoplastic polyester, polyamide, PEEK, or specialized high-performance polymers. The material must be compatible with both the operating fluid and the temperature range.

When is a backup ring required?

A backup ring should be considered when:

• System pressure is high
• Extrusion clearance cannot be reduced
• The elastomer is relatively soft
• Temperature reduces the compound’s mechanical strength
• Pressure changes direction
• Components may deflect under load
• The system experiences rapid pressure cycling
• Previous seals show extrusion damage

A backup ring does not correct an excessively large or uncontrolled clearance gap. Good component design, suitable tolerances, correct O-ring hardness, and proper groove dimensions remain necessary.

For demanding hydraulic service, the final arrangement should be checked against the pressure, temperature, extrusion gap, O-ring compound, and housing requirements of ISO 3601-2 and ISO 3601-4.

9. ISO 3601 Compared with AS568 and Other O-Ring Standards

ISO 3601 is not the only standard used to define O-ring sizes. Depending on the equipment’s country of origin and industry, engineers may also encounter SAE AS568, DIN 3771, BS 1806, BS 4518, JIS B 2401, and manufacturer-specific size systems.

O-rings from different standards may appear nearly identical, but this does not mean they are automatically interchangeable. Small differences in inside diameter, cross-section, or tolerance can change the installed squeeze, stretch, and groove fill.

ISO 3601 vs. SAE AS568

SAE AS568 is one of the most widely used O-ring size standards in North America. It defines standardized inch-based O-ring dimensions commonly identified by dash numbers, such as:

• AS568-006
• AS568-012
• AS568-214
• AS568-325

Many Class A dimensions in ISO 3601 are coordinated with established inch-based O-ring sizes. This creates substantial dimensional overlap between ISO 3601 Class A and SAE AS568.

However, the two standards should not be assumed to be completely identical. Differences can exist in:

• Size designation systems
• Dimensional tolerances
• Application scope
• Quality requirements
• Material specifications
• Referenced inspection methods

ISO 3601 uses its own size codes and separates its requirements across five parts. AS568 primarily focuses on O-ring sizes and dimensional tolerances, while other SAE or ASTM documents may be required to define material and quality requirements.

When replacing an AS568 O-ring with an ISO 3601 O-ring, the engineer should compare the actual d1d_1 and d2d_2 dimensions, tolerances, material, hardness, and groove requirements rather than matching only the apparent size code.

ISO 3601 vs. DIN 3771

DIN 3771 is a German standard historically used for O-ring dimensions, materials, identification, and quality requirements. Many metric O-rings associated with DIN 3771 have dimensions that are also available under ISO 3601.

For new international projects, ISO 3601 is generally the more globally recognized reference. However, legacy European equipment and supplier documentation may continue to specify DIN sizes.

A DIN-designated O-ring should be compared dimensionally with the corresponding ISO size before substitution. The engineer must also verify whether the original specification includes material, hardness, inspection, or application requirements not covered by the nominal dimensions alone.

ISO 3601 vs. JIS B 2401

JIS B 2401 is commonly used for O-rings in Japanese industrial equipment. Its size series includes O-rings intended for pistons, rods, general applications, vacuum flanges, and other specific uses.

Some JIS sizes have close ISO equivalents, but many do not directly correspond to ISO 3601 dimensions. Selecting the nearest available ISO size may produce incorrect stretch or squeeze, even if the difference appears small.

This issue is particularly important when maintaining:

• Japanese hydraulic machinery
• Machine tools
• Pneumatic equipment
• Automotive components
• Vacuum systems
• Imported production equipment

The original groove dimensions should be measured or obtained from the manufacturer before changing from JIS to ISO O-rings.

ISO 3601 vs. BS O-ring standards

BS 1806 historically covered inch-size O-rings, while BS 4518 covered metric sizes. Many traditional BS dimensions overlap with sizes found in modern international standards.

Nevertheless, older equipment drawings may use British size references without clearly identifying tolerances or updated equivalent standards. Direct dimensional comparison is necessary before selecting a replacement.

Why approximately matching dimensions is unsafe

An O-ring that can physically enter a groove is not necessarily suitable for that groove. A slightly oversized cross-section may create excessive squeeze and groove fill, while an undersized cross-section may fail to develop adequate sealing contact.

An incorrect inside diameter may also cause:

• Excessive installation stretch
• Reduction of the installed cross-section
• Loose fit in the groove
• Twisting during assembly
• Buckling in an internal groove
• Incorrect positioning of the parting line
• Premature wear or leakage

Interchangeability should therefore be evaluated using minimum and maximum dimensions rather than nominal values alone.

Information required when ordering an ISO 3601 O-ring

A reliable O-ring purchase specification should include more than a size code. Depending on the application, the following information may be required:

Specifying only “ISO 3601 O-ring” is incomplete because the standard contains many dimensions, materials, and quality levels.

Selecting the correct standard

The best choice is normally the standard specified by the equipment designer. For new equipment, the selected standard should match the intended market, available components, groove design, and customer requirements.

When replacing an existing O-ring:

1. Identify the original standard if possible.
2. Determine the nominal inside diameter and cross-section.
3. Inspect and measure the groove dimensions.
4. Confirm the working fluid and temperature.
5. Check pressure and extrusion clearance.
6. Select the required elastomer and hardness.
7. Verify dimensional tolerances and quality grade.
8. Conduct functional testing when substituting between standards.

The original O-ring should not be measured as the only source of dimensional information because used elastomers may have swollen, shrunk, flattened, or taken a permanent compression set.

Conclusion

ISO 3601 provides an internationally recognized system for specifying O-rings and the components that support their sealing performance. Rather than covering only nominal O-ring sizes, the series addresses dimensions, tolerances, designation codes, housing design, surface quality, backup rings, and elastomeric material specifications.

The five parts of the series perform different functions:

• ISO 3601-1 defines inside diameters, cross-sections, tolerances, and designation codes.
• ISO 3601-2 provides housing and groove dimensions for general applications.
• ISO 3601-3 establishes quality acceptance criteria for surface imperfections.
• ISO 3601-4 standardizes anti-extrusion or backup rings.
• ISO 3601-5 specifies selected elastomeric materials for industrial applications.

Successful O-ring selection requires all these factors to be considered as part of one sealing system. The O-ring must have the correct dimensions, but it must also receive suitable squeeze, fit within the available groove volume, resist extrusion, and remain compatible with the operating fluid and temperature.

Engineers should avoid selecting replacement seals based only on color, outside diameter, or an approximate physical fit. The original standard, dimensional class, material, hardness, quality grade, pressure, temperature, and application type should be confirmed before installation.

ISO 3601 and SAE AS568 include many closely related sizes, but their designation systems and requirements are not universally interchangeable. The actual dimensions and tolerances must be compared whenever an O-ring is substituted between standards.

By applying ISO 3601 correctly, equipment manufacturers and maintenance teams can improve seal interchangeability, reduce leakage, extend component life, and create clearer purchasing and inspection requirements for hydraulic and pneumatic systems.

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