What Do O-Ring Dash Numbers Mean? A Simple Guide
By Mutual Screw & Supply | Published: 2026-10-17
Category: How-to Guides
Learn how AS568 dash numbers define o-ring size, how to read them, and why they matter for sealing applications. Practical guide for choosing the right o-ring.
Quick Answer
An o-ring dash number (like AS568-901) is a standardized size code from the AS568 specification. It tells you the inside diameter and cross-section of the o-ring. The dash number increases with size, but you need a cross-reference chart to match the number to actual dimensions.
How the AS568 Standard Works
The AS568 standard, published by SAE International, assigns a unique dash number to each o-ring size. The number ranges from 001 to 475 and beyond, covering inside diameters from about 0.029 inches to over 25 inches. Each dash number corresponds to a specific inside diameter (ID) and cross-section (CS) combination. For example, AS568-901 has a specific ID and CS that you can look up in a standard chart. The system is consistent across manufacturers, so a dash 901 from any reputable supplier will have the same dimensions.
Dash numbers are not sequential in size order in a simple way. For instance, dash 001 is very small, but dash 010 is not ten times larger. Always consult a size chart rather than guessing. The standard also defines tolerances for each dimension, which is critical for sealing performance.
Reading a Dash Number: Example
Take the Viton FKM O-Ring AS568-901 as an example. The '901' is the dash number. To find its dimensions, you would look up AS568-901 in a standard chart. That chart will tell you the inside diameter (ID) and cross-section (CS). The 75 durometer rating refers to the hardness of the material, not the size. Durometer affects how the o-ring compresses and seals. A 75 Shore A is a medium-hard compound suitable for many dynamic and static applications.
Why Dash Numbers Matter for Your Seal
Using the correct dash number ensures the o-ring fits properly in the gland or groove. An o-ring that is too small may not seal, while one that is too large can extrude or cause excessive friction. The dash number also helps you order the right replacement part quickly without measuring each time. If you are replacing an o-ring, check the existing one for a dash number printed on the package or the part itself.
Frequently Asked Questions
- What does the dash number on an o-ring mean?
- The dash number is part of the AS568 standard. It identifies the inside diameter and cross-section of the o-ring. For example, AS568-901 refers to a specific size defined in the standard. Always use a size chart to decode a dash number.
- How do I find the right dash number for my application?
- Measure the inside diameter and cross-section of your current o-ring, or check the part number on the package. Then look up the dash number in an AS568 chart that matches those dimensions. Measuring is the most reliable method if the number is unknown.
- Does durometer affect the dash number?
- No. Durometer (hardness) is a material property, not a size property. Dash numbers only define dimensions. You can have the same dash number in different durometers, like 70 or 75 Shore A. Choose durometer based on sealing pressure and temperature, not size.
- Are all dash numbers the same across brands?
- Yes, the AS568 standard ensures dimensional consistency across manufacturers. A dash 901 from any brand should have the same ID and CS within standard tolerances. You can mix brands as long as the dash number and material are compatible.
Product Facts
- Viton FKM O-Rings | AS568-901 | Dash 901 | 75 Durometer | Brown (400 / Pkg.): Viton FKM O-Rings in AS568-901, 75 Durometer, Brown, 400 per package. Service range -15°F to +400°F. This specific dash number (901) is a standard size. The 75 durometer and Viton material make it suitable for high-temperature and chemical-resistant sealing applications. The 400-count package is economical for maintenance or production runs.
