What Are Standard O-Ring Sizes?

Standard O-ring sizes are determined by globally recognized specifications and regional standards, which define dimensions, tolerances, and material compatibility. This guide provides a comprehensive overview of key standards, size ranges, and practical considerations for selecting O-rings.

1. Core International Standards

O-ring sizing conventions vary by region and application. Below are the primary standards governing their dimensions:

A. ISO 3601-1 (Metric Standard)

• Size Format:
  • Dimensions are specified as inner diameter (ID) × cross-sectional diameter (CS) in millimeters (e.g., 24.4×3.1 mm).
  • Example: An O-ring labeled *7.5×1.8-G-N* under ISO/GB/T3452.1-2005 has an ID of 7.5 mm and CS of 1.8 mm.
• Cross-Section Groups:
  • Classified into groups A–E based on nominal CS diameters:
    • A: 1.5 mm
    • B: 1.78 mm
    • C: 2.00 mm
    • D: 2.50 mm
    • E: 3.53 mm
• ID Range:
  • Spans from 1.28 mm to 260 mm, designated by a four-digit code (e.g., B0128 = CS 1.78 mm, ID 12.8 mm).

B. SAE AS568 (Imperial Standard)

• Size Format:
  • Uses a dash numbering system (e.g., –214) to denote combinations of ID (inches) and CS (inches).
  • Example: 0.864 ID × 0.070 WD corresponds to an inner diameter of 0.864 inches and cross-section of 0.070 inches.
• Cross-Section Series:
  • 000-series: 0.070 in (1.78 mm)
  • 100-series: 0.103 in (2.62 mm)
  • 200-series: 0.139 in (3.53 mm)
  • 300-series: 0.210 in (5.33 mm)
  • 400-series: 0.275 in (6.99 mm)
  • 900-series: Tube-fitting sizes (1.42–3.00 mm).
• ID Range:
  • Varies by series:
    • 000-series: 0.029 in → 5.239 in
    • 100-series: 0.049 in → 9.737 in
    • 200-series: 0.171 in → 17.955 in
    • 300-series: 0.412 in → 25.940 in
    • 400-series: 1.350 in → 25.940 in.

C. Regional Variations

• Old Chinese Standard (GB1235-76):
  • Specifies outer diameter (OD) × CS (e.g., *20×2.4* = OD 20 mm, CS 2.4 mm).
• Japanese JIS B2401:
  • Includes large-diameter O-rings (ID up to 2000 mm) for hydraulic systems.

2. Common Size Ranges

O-rings are available in diverse dimensions to suit industrial, automotive, and aerospace applications:

A. Small to Medium Sizes

• ID: 1.78–133.07 mm
• CS: 1.78–6.99 mm
• Examples:
  • Metric: 0.8–2000 mm ID, 0.5–15 mm CS.
  • JIS B2401 P3 series: 2.8×1.9 mm.

B. Large-Diameter O-Rings

• ID: 300–2000 mm
• CS: 3.1–20 mm
• Applications: Hydraulic systems, heavy machinery.

3. Material-Specific Standards

Material choice impacts O-ring performance and sizing tolerances:

4. Tolerance Guidelines

Tolerances ensure fit and functionality across standards:

• Metric O-Rings (ISO/JIS):
  • JIS B2401: ±0.13–±11.7 mm for IDs 300–2000 mm.
• AS568 (Imperial):
  • Tighter tolerances (e.g., ±0.07 mm CS) for precision instruments.

5. How to Specify O-Ring Sizes

1. For Imperial (AS568): Use dash numbers (e.g., –214 = ID 0.683 in, CS 0.139 in).
2. For Metric (ISO 3601-1): Use alphanumeric codes (e.g., B0128 = CS 1.78 mm, ID 12.8 mm).
3. Key Steps:
   • Confirm groove dimensions (ID and CS).
   • Match to the closest standard size.
   • Verify material compatibility for the application.

6. Summary

Standard O-ring sizes are governed by ISO 3601-1 (metric), AS568 (imperial), and regional standards like JIS or GB. Critical parameters include:

• Inner diameter (ID) and cross-sectional thickness (CS).
• Tolerances based on material and application.
• Material compatibility (e.g., NBR, FKM, FFKM).