How to Select the Right O-Ring Size and Material
By Mutual Screw & Supply | Published: 2026-10-19
Category: How-to Guides
Learn to pick the correct o-ring size and material for your application. This guide covers AS568 dash numbers, durometer, and square profile options.
Quick answer
Start by measuring the groove dimensions, then match the cross-section and inner diameter to an AS568 dash size. Choose a material based on temperature and chemical exposure—Buna-N nitrile works for most petroleum applications from -40°F to +250°F.
Why size and material both matter
An o-ring that fits the groove but can't handle the fluid or temperature will fail early. Conversely, the right material won't seal if the cross-section is too thin or the inner diameter too tight. You need to balance both dimensions and operating conditions.
Square profile vs. standard round profile
Most o-rings have a round cross-section, but square profile o-rings offer a wider sealing surface for static applications. They resist extrusion better in high-pressure static seals but require more precise groove dimensions. If your groove is designed for a round profile, switching to square may cause installation issues.
How to select the right o-ring size and material
- 1. Measure the groove or shaft: Use calipers to get the groove width, depth, and diameter. For a piston seal, measure the bore diameter; for a rod seal, measure the shaft diameter. The o-ring cross-section should be slightly larger than the groove depth (typically 10-25% squeeze). A common mistake is measuring the old o-ring instead of the groove—old o-rings can be compressed or stretched, giving false dimensions.
- 2. Match to an AS568 dash size: Look up the measured cross-section and inner diameter in an AS568 chart. For example, a 0.103" cross-section and 0.924" inner diameter corresponds to dash size 119. The dash number standardizes dimensions so you can order replacements confidently. Double-check that both cross-section and inner diameter match the chart—mixing up dash numbers is a frequent error.
- 3. Select the material based on temperature and fluid: For petroleum oils, fuels, and hydraulic fluids from -40°F to +250°F, Buna-N (NBR) is a reliable choice. For higher temperatures or aggressive chemicals, consider fluorocarbon (Viton) or silicone. Check the material data sheet for exact compatibility. Never assume all rubber handles the same fluids—Buna-N swells in brake fluid and degrades in ozone, so verify your application first.
- 4. Choose the durometer (hardness): 70 Shore A is the most common for general sealing. Use 90 Shore A for high-pressure or extrusion resistance, and 50 Shore A for low-pressure or soft sealing. Harder o-rings require higher squeeze force to seal. A 70 durometer o-ring is a safe starting point for most industrial applications, but check if your groove has sharp edges that could cut a softer compound.
- 5. Verify the profile type: If your application is static (flange, face seal), a square profile o-ring can improve sealing and reduce extrusion. For dynamic applications (reciprocating rod or rotating shaft), stick with a standard round profile to reduce friction and wear. Square profile o-rings are not interchangeable with round ones without adjusting the groove design—verify groove dimensions before switching.
Product facts
- 0.103" CS X 0.924" ID Buna-N Nitrile O-Rings | AS568-119 | Dash 119 | Square Profile | 70 Durometer (500 / Pkg.): Buna-N Nitrile o-rings in AS568-119, 0.924" inner diameter by 0.103" cross-section, 70 Shore A durometer, square profile. The square profile provides a wider sealing surface ideal for static applications, and the 70 durometer offers a good balance between compression set and sealing force for most industrial fluids.
0.103" CS X 0.924" ID Buna-N Nitrile O-Rings | AS568-119 | Dash 119 | Square Profile | 70 Durometer (500 / Pkg.)
