O-ring hardness affects how easily a seal compresses, how it installs, and how well it resists extrusion under pressure. The most common general-purpose range is Shore A hardness, and buyers often compare 70A O-rings with 80A O-rings when replacing industrial rubber seals.
Hardness does not replace size selection. Before changing durometer, confirm the inside diameter, cross section, and groove dimensions with the O-ring size chart and the guide on how to measure an O-ring.
What O-Ring Hardness Means
O-ring hardness is usually measured with a Shore A durometer. A lower number is softer and easier to compress. A higher number is harder and can resist deformation better in some pressure conditions.
For many general sealing jobs, 70A is the common starting point. 80A is often considered when the seal needs more extrusion resistance, a tighter fit, or better shape stability.
70A O-Rings
A 70A O-ring is flexible enough for many static sealing applications. It is easier to install than harder rubber, can conform to small surface variations, and is widely available in common materials such as NBR O-rings.
- Common general-purpose hardness
- Usually easier to install than 80A
- Good starting point for many static seals
- Useful when the groove and pressure are not extreme
80A O-Rings
An 80A O-ring is harder. It may be chosen when extrusion, pressure, or repeated assembly makes a softer seal less reliable. The tradeoff is that 80A can be less forgiving during installation and may need more careful groove confirmation.
- Harder and more shape-stable than 70A
- Can improve extrusion resistance in some applications
- May need more installation force
- Less forgiving if the groove size is wrong
70A vs 80A O-Rings
| Factor | 70A O-Ring | 80A O-Ring |
|---|---|---|
| Compression feel | Softer and easier to squeeze | Harder and more resistant |
| Installation | Usually easier | May require more care |
| Extrusion resistance | Good for many moderate applications | Often better when gaps or pressure are higher |
| Best starting point | General static sealing | Higher pressure or tighter support needs |
When to Choose 70A
Choose 70A when the original seal was general-purpose rubber, the application is mostly static, pressure is moderate, and installation needs to be straightforward. For maintenance teams, 70A assortments are often useful because they cover many common repair needs.
If your team keeps multiple sizes on hand, the guide to O-ring kits for maintenance teams can help plan practical assortments. A product such as the AS568 Small O-Ring Kit may also be useful for trial fitting common inch sizes.
When to Choose 80A
Choose 80A when the seal has to resist extrusion, hold shape more firmly, or work in a groove where a softer seal tends to roll, pinch, or fail. This is not automatic. The groove, pressure, backup support, material, and fluid still need to be checked.
Do Not Change Hardness Without Checking the Application
Changing from 70A to 80A can solve some problems, but it can also create new ones if the seal no longer compresses correctly. A harder O-ring may reduce extrusion risk, but it can also make installation harder or reduce sealing contact in a low-compression groove.
Always compare hardness with the material guide and the equipment requirements before ordering a large quantity.
Buyer Checklist
- Confirm the original hardness if possible
- Measure ID, cross section, and groove dimensions
- Check pressure, temperature, and fluid exposure
- Confirm whether the seal is static or dynamic
- Test a small quantity before buying many sizes
- For repeat maintenance, request a bulk quote with size, material, and hardness details
FAQ
Is 70A or 80A better for O-rings?
Neither is always better. 70A is a common general-purpose starting point, while 80A may be chosen for more extrusion resistance or firmer shape control.
Can I replace a 70A O-ring with 80A?
Sometimes, but only after checking groove compression, installation force, pressure, and sealing contact. A harder seal is not automatically an upgrade.
What does Shore A mean?
Shore A is a hardness scale used for flexible rubber and elastomers. A higher Shore A number means the rubber is harder.
What hardness should I choose for NBR O-rings?
NBR 70A is common for many general oil-related applications. NBR 80A may be considered when extrusion resistance matters, but the final choice depends on the equipment and groove design.
Final Tip
Start with the original seal specification whenever possible. If that is not available, choose hardness from the application backward: groove fit, pressure, movement, material, and fluid exposure first, then 70A or 80A.
