Confused about whether to choose a 70 or 90 O-ring for your project? Using the wrong hardness can lead to leaks, extrusion, or premature failure.
The main difference between 70 and 90 O-rings is hardness. A 70-durometer O-ring is softer and more flexible, while a 90-durometer O-ring is harder and more resistant to extrusion under pressure.
Let’s explore how these hardness levels affect sealing performance, durability, and ideal use cases.
What Does “Durometer” Mean in O-Rings?
Not sure what the numbers 70 or 90 actually refer to?
Durometer measures the hardness of rubber on the Shore A scale. A higher number means a harder material. 70A is standard for most O-rings, while 90A is for high-pressure or high-wear environments.
✅ Key Shore A Hardness Reference:
| Durometer1 | Feel (Touch) | Flexibility2 | Typical Use |
|---|---|---|---|
| 40A | Very soft (gel-like) | Very flexible | Low-pressure static seals |
| 70A | Medium firmness ✅ | Good flexibility | Standard O-ring3 for most uses |
| 90A | Very firm ✅ | Low flexibility | High-pressure hydraulic/pneumatics |
70 Durometer O-Rings: Why They're Most Common
Looking for a general-purpose seal?
70A durometer O-rings are the industry standard for most sealing applications due to their balance of flexibility, compression set resistance, and media compatibility.
✅ Benefits of 70A O-Rings:
- Easy to compress into sealing grooves
- Good sealing with low to moderate pressure4
- Ideal for static and slow-moving dynamic seals5
- Cost-effective and widely available6
- Available in materials like NBR, FKM, EPDM, HNBR, Silicone
✅ Ideal For:
- Plumbing fittings
- HVAC systems
- Automotive fuel and oil lines
- Water-resistant enclosures
- General-purpose hydraulic seals
90 Durometer O-Rings: When to Choose Harder Rubber
Need a seal that won’t get extruded under high pressure?
90A durometer O-rings are stiffer and stronger, making them suitable for high-pressure environments where extrusion or deformation is a concern.
✅ Benefits of 90A O-Rings:
- High resistance to extrusion7
- Better wear and abrasion resistance
- Good dimensional stability under load8
- Less prone to rolling or distortion in dynamic applications
✅ Ideal For:
- Hydraulic cylinders and pistons
- High-pressure valve stems
- Pneumatic actuators
- Heavy machinery and industrial systems
⚠️ Note: They require more compression force and are less forgiving in poorly machined grooves.
70 vs 90 O-Rings: Performance Comparison
Here’s a quick look at how the two durometers differ across key performance factors.
| Property | 70 Durometer ✅ | 90 Durometer ✅ |
|---|---|---|
| Hardness (Shore A) | Medium | High |
| Flexibility | Good ✅ | Low ⚠️ |
| Extrusion Resistance | Moderate ⚠️ | Excellent ✅ |
| Compression Force Needed | Low ✅ | High ⚠️ |
| Best For | General sealing | High-pressure sealing9 |
| Groove Fill/Conformity | Better ✅ | Stiffer ⚠️ |
Which Durometer Should You Choose?
Not sure which is right for your application?
✅ Choose 70 Durometer10 When:
- Dealing with static or slow-moving parts
- Groove dimensions may vary
- Sealing low to moderate pressure fluids
- You need a flexible, easy-to-install seal
✅ Choose 90 Durometer11 When:
- Operating in high-pressure hydraulic or pneumatic systems
- Extrusion risk is high (wide gaps, sharp edges)
- Wear resistance is critical
- The seal is exposed to repetitive movement or pressure spikes
Conclusion
70-durometer O-rings are the best choice for most standard sealing needs, offering flexibility and sealing efficiency. 90-durometer O-rings are ideal for high-pressure, high-wear environments where extrusion resistance matters most. Always match hardness with pressure, fluid, and application conditions.
🔧 Need Help Choosing the Right O-Ring Hardness?
Contact Julong Rubber or Request a Quote for custom durometer O-rings made to fit your specifications.
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