O-Rings sizes.

Overview

Understanding O-Ring Dimensions

International O-Ring Size Standards

Non- Standard O-Ring Sizes

O-rings are essential components in various sealing applications across industries. They are defined by two primary dimensions: Inside Diameter (ID) and Cross Section (CS). Understanding the correct size and standard is crucial for ensuring optimal performance and compatibility

View o-ring size list

Inside Diameter (ID): The distance across the inner circle of the O-ring.

Cross Section (CS): The thickness of the O-ring from the inner to the outer edge.​

These dimensions are specified in various international standards to ensure consistency and interchangeability..

AS568 (USA)

  • Issued by: Society of Automotive Engineers (SAE).

  • Units: Inches.

  • Identification: Dash numbers (e.g., -001 to -932).

  • Applications: Widely used in aerospace, automotive, and industrial sectors.​

ISO 3601 (International)

  • Issued by: International Organization for Standardization.

  • Units: Metric.

  • Classes:

    • Class A: Aligns with AS568 dimensions.

    • Class B: Includes additional metric sizes.

  • Applications: Global standard for O-ring dimensions and tolerances.​

BS 1806 (UK)

  • Issued by: British Standards Institution.

  • Units: Inches.

  • Note: Superseded by ISO 3601 but still referenced in certain industries.​

BS 4518 (UK)

  • Issued by: British Standards Institution.

  • Units: Metric.

  • Identification: Four-digit number indicating ID in tenths of millimeters, followed by a hyphen and two digits indicating CS.

  • Applications: Common in UK-based engineering applications.​

DIN 3771 (Germany)

  • Issued by: Deutsches Institut für Normung (DIN).

  • Units: Metric.

  • Identification: ID x CS, with optional quality grades (e.g., N for normal, S for special).

  • Applications: Widely used in European mechanical engineering

SMS 1586 (Sweden)

  • Issued by: Sveriges Mekanstandardisering.

  • Units: Metric.

  • Identification: ID x CS.

  • Classification:

    • D: For dynamic and static applications.

    • S: For static applications.​

JIS B 2401 (Japan)

  • Issued by: Japanese Industrial Standards.

  • Units: Metric.

  • Series:

    • P: Dynamic applications.

    • G: Static applications.

    • S: Special sizes.

    • V: Vacuum flange applications.​

NF T 47-501 (France)

  • Issued by: Association Française de Normalisation (AFNOR).

  • Units: Metric.

  • Note: Similar to ISO 3601, with some unique dimensions.​Global O-Ring and Seal

ISO 6149

  • Issued by: International Organization for Standardization.

  • Units: Metric.

  • Applications: Specifies O-rings for metric tube fittings, including 13 different sizes

Non-Standard O-Ring Sizes

In many industries, standard-size O-rings cover the majority of sealing requirements. However, there are countless situations where a standard part won’t do — due to legacy equipment, unusual groove geometries, or specific performance conditions. At orings.co.uk, we support engineers and procurement teams by supplying non-standard O-rings for unique applications, with short lead times and material flexibility.

What Are Non-Standard O-Rings?

Non-standard O-rings are defined by their custom inside diameter (ID), cross section (CS), or both. They do not conform to major international standards like AS568, ISO 3601, BS4518, or JIS B2401.

Typical examples include:

  • Oversized or undersized cross sections

  • Unusual diameters not supported by existing tooling

  • Lengths for joined (vulcanised) O-rings

  • Custom tolerances or surface finishes

  • Static seals for cast or legacy parts with irregular dimensions

These parts are often required in low to medium volumes, for use in sectors like defence, specialist manufacturing, scientific instrumentation, and refurbishment of older equipment.

How Are Non-Standard O-Rings Manufactured?

Depending on material and quantity, non-standard O-rings can be produced by:

  1. Compression or Injection Moulding

    • Best for repeatable, medium-volume production

    • Tooling costs apply (may be amortised on large orders)

  2. Lathe-Cut O-Rings

    • Machined from extruded tube stock

    • Ideal for low-volume or large-diameter parts

    • No tooling required, faster turnaround

  3. Vulcanised (Spliced and Joined)

    • Cut from cord stock and bonded into a loop

    • Suited for very large O-rings or non-critical dynamic applications

    • Typically made from NBR, FKM, or EPDM cord

When Do You Need a Non-Standard O-Ring?

You might require a non-standard O-ring if:

  • The groove dimensions do not match any known standard

  • A sealing solution must work in extreme environments (pressure, temperature, or media)

  • You are restoring or maintaining obsolete equipment with no technical documentation

  • Tighter (or looser) tolerances are needed to improve seal compression or reduce friction

  • You are developing a prototype or proof-of-concept requiring exact dimensional control

Materials Available for Custom Sizes

We can manufacture or supply non-standard sizes in all common elastomers, including:

  • Nitrile (NBR) – Oil-resistant, general-purpose

  • Fluorocarbon (FKM / Viton®) – High temperature and chemical resistance

  • EPDM – Excellent resistance to steam, water, and ozone

  • Silicone (VMQ) – Flexible, suitable for food and pharmaceutical use

  • Perfluoroelastomer (FFKM) – Maximum chemical resistance for high-end applications

Supporting You Through Specification

We know that non-standard parts can create friction — sourcing delays, incorrect tolerances, or material mismatches can hold up entire projects. That’s why we offer:

  • Sample supply for fit-checks or validation

  • 3D drawings on request to support your technical documentation

  • Advice on groove design and compression rates

  • Rapid quote turnaround from a real person, with no gatekeeping

Need Help Identifying or Replacing a Non-Standard Size?

If you're unsure what you need, send us the part, measurements, or even a photo. We’ll help identify the right replacement — whether it’s from existing tooling or needs to be custom-made.