How to Choose the Right O-Ring Durometer

Quick Answer

Choose a 70 Shore A durometer for general-purpose sealing and easier installation, or a 75 Shore A durometer for higher pressure resistance and better extrusion resistance. Match the hardness to your pressure, temperature, and installation needs.

Step-by-Step Durometer Selection

1. Check the Operating Pressure: Higher pressures tend to push the o-ring into the gap between mating parts. If pressure exceeds 1,500 psi, choose a harder durometer (75 Shore A or higher) to resist extrusion. For low-pressure or vacuum applications, a softer 70 Shore A is usually sufficient. If unsure, start with 70 Shore A for pressures below 1,500 psi.
2. Assess the Temperature Range: Temperature affects hardness. At low temperatures, o-rings become stiffer; at high temperatures, they soften. Check the material's service range and choose a durometer that maintains sealing at your operating extremes. For example, 70 Shore A Buna-N works to -40°F, while 75 Shore A Viton handles up to 400°F. Verify the material's temperature range, not just the durometer.
3. Evaluate the Gland Design and Gap: Measure the extrusion gap between the mating surfaces. Larger gaps require harder o-rings to prevent extrusion. For standard SAE glands with minimal gaps, 70 Shore A is common. For dynamic applications or larger gaps, consider 75 Shore A or harder. When in doubt, use a harder durometer for larger extrusion gaps.
4. Consider Installation and Assembly: Softer o-rings are easier to stretch over flanges and less likely to be damaged during installation. Harder o-rings require more care and may need lubricants. If your assembly process is manual and the o-ring must fit into a tight groove, 70 Shore A is more forgiving. If installation is tricky, choose a softer durometer to reduce assembly issues.
5. Match the Material to the Fluid: Durometer is only one factor; the elastomer must be compatible with the fluid. Buna-N (NBR) suits petroleum oils and water, while Viton (FKM) resists high temperatures and many chemicals. Always verify chemical compatibility before finalizing your choice. Durometer selection cannot override material compatibility.

O-Ring Durometer Examples

• 0.21" CS X 7.475" ID Buna-N Nitrile O-Rings | AS568-361 | Dash 361 | 70 Durometer | Black (40 / Pkg.): Buna-N Nitrile o-rings with a 70 Shore A durometer, service range -40°F to +250°F. This hardness offers a balance of flexibility and sealing for general-purpose applications, especially at low temperatures.

  

• Viton FKM O-Rings | AS568-445 | Dash 445 | 75 Durometer | Black (4 / Pkg.): Viton FKM o-rings with a 75 Shore A durometer, service range -15°F to +400°F. The harder compound provides better extrusion resistance for higher-pressure or higher-temperature environments.