You’re scrolling through O-ring listings and there it is again: 70A. Or 90A. Maybe 75. What is that number, and why does it keep showing up?
That number is durometer. It just means hardness. Once you understand it, picking the right O-ring gets a lot less frustrating.
What Durometer Really Measures
Grab a rubber band. It’s soft, right? Now think about a car tire. Same basic material idea, but way harder. Durometer is how we put a number on that difference.
The scale used for most O-rings is called Shore A. It runs from 0 to 100. A rubber band lands around 25. A tire is about 70. A hard plastic-like wheel is up near 95.
So 70A means 70 on the Shore A scale. Medium-firm. 90A is noticeably stiffer.
Why You Should Care
Soft O-rings squish more. Hard O-rings resist squishing. That single difference decides where each one works.
70A: The One You’ll Use Most
A 70 durometer O-ring is the standard for most sealing jobs. It’s flexible enough to squeeze into a groove and seal tight, but strong enough to hold up under normal pressure.
You’ll find 70A in tons of everyday applications:
- Plumbing fixtures and faucets
- Hydraulic and pneumatic fittings
- Pumps and valves
- Compressors and air tools
- Food and beverage equipment
- General industrial sealing
If you’re doing a normal repair and nothing unusual is going on, 70A is usually the safe bet. It’s also the durometer you’ll see most often on the shelf.
90A: Built for High Pressure
A 90 durometer O-ring is stiffer. It doesn’t flex much, and that’s the point. Stiffness gives you better extrusion resistance. That’s the term for an O-ring getting forced out of its groove when pressure gets high. A soft O-ring can squish its way into a gap and fail. A hard one holds its shape.
Parker’s documentation is pretty clear on this: reach for 90 durometer O-rings when pressure hits 1500 psi or higher. You’ll spot them in:
- High-pressure hydraulic systems
- Fuel injection and high-pressure fuel lines
- Industrial valves and manifolds
- Heavy equipment fittings
- Anywhere running serious pressure
There’s a trade-off, though. 90A O-rings need more force to seat. They’re less forgiving if the groove isn’t cut right or the surfaces are rough or worn. A soft O-ring can fill in small imperfections. A hard one can’t.
The In-Between Options
You’ll run into 75A and 80A too. These split the difference. A 75A FKM O-ring might be the pick when you want a bit more stiffness than 70A but don’t need the full rigidity of 90A.
What Actually Decides It
Durometer doesn’t work alone. Three things matter together:
- Pressure. Higher pressure pushes you toward 90A. Lower pressure is fine with 70A.
- Temperature. Long exposure to heat can soften rubber, so a harder compound may hold up better in hot spots.
- The groove and surfaces. Rough or worn sealing surfaces pair better with a softer O-ring that can conform.
The Mistake People Make
Here’s where folks go wrong. They see 90A and figure harder must mean better. It doesn’t work that way. Put a 90A O-ring in a low-pressure spot with a rough surface and it might leak worse than a 70A would. Too hard means it can’t deform enough to fill the gap.
O-ring material selection isn’t about picking the toughest number. It’s about matching hardness, material, and size to the actual job.
Where Stradian Direct Fits In
At Stradian Direct, we carry O-rings in the materials, sizes, and durometers that real-world jobs call for. If you’re not sure which hardness fits your application, reach out and we’ll help you sort it out before you order.
The Short Version
70A covers most everyday O-ring jobs. 90A is for high pressure and extrusion resistance. Everything between is a judgment call based on pressure, temperature, and the condition of your sealing surfaces. Pick the durometer that fits the job, not the one that sounds strongest.