Hey there, if you’ve ever shopped for O-rings, you’ve probably seen that random number on the packaging or product spec sheet—like 70A, 90 Shore A, or even something lower. As a More O Rings supplier, I get it; most folks ask me way more about size or material than they do about hardness, until something goes wrong. Then suddenly, “Wait, why’d this O-ring crush so fast?” or “Why’s this one leaking when it’s the same size as the last?” That’s almost always tied right to hardness—so today let’s break down what More O Rings hardness actually means, why it matters, and how we (and you) pick the right one for the job. More O Ring
First off, let’s cut through the jargon. When we talk about O-ring hardness, we’re measuring how much the material deforms under pressure—simple as that. The test we use is called the Shore durometer test, and 99% of the time for O-rings, it’s Shore A. There’s Shore D for super-hard plastics, but that’s way too rigid for seals; no one uses that for More O Rings. The scale runs from 0 (super squishy, like a rubber band stretched thin) to 100 (rock hard, like a hockey puck). Most More O Rings we stock land between 60A and 90A, and that range isn’t random—each number serves a purpose.
Let’s start with the soft end: 60A to 70A. These are your go-to when you need a seal that can squeeze into tiny gaps or conform to rough, uneven surfaces. Think about hydraulic systems where the groove has a few nicks and scratches, or plumbing fittings that aren’t perfectly machined. A soft O-ring will squish into those imperfections, creating a tight seal without forcing you to tighten bolts so much you warp the fitting. I’ve seen a lot of first-time customers reach for a 70A out of habit, but when they tell me their seal’s failing on a worn pump shaft, swapping to a 65A More O Ring fixed it right up. The tradeoff here? They don’t hold up to heavy loads or high pressure as well as harder O-rings. If you’re running 3000 PSI of hydraulic pressure every day, a soft O-ring’s gonna flatten out fast, and that’s a leak waiting to happen.
Then we’ve got the middle ground, which is the most popular: 75A to 80A. This is the sweet spot for 90% of the applications we deal with—general industrial equipment, automotive components, home appliances, even some aerospace trim seals. It’s firm enough to resist extrusion (that’s when O-ring material gets squeezed into the gaps between mating parts, like a cylinder and its piston) but still pliable enough to install easily. Installing a too-hard O-ring? You can’t stretch it enough to fit over a shaft without nicking it, and if you do force it on, it’ll sit funny in the groove and leak. I had a customer last month who tried to use a 90A More O Ring on a small engine’s valve cover gasket—he snapped two during installation, and the third leaked within a week because it couldn’t conform to the uneven flange surface. Swapping to an 80A fixed it immediately. That’s the thing about the middle range—it’s balanced. If you’re not sure where to start, 75A is our default More O Ring, and we have it in every material (nitrile, EPDM, silicone, Viton) you could need.
Now the hard end: 85A to 90A. These are for heavy-duty, high-pressure jobs. Think construction equipment hydraulic cylinders, deep well pumps, heavy machinery that runs nonstop 12 hours a day. When you have 5000 PSI pushing on a seal, you need something that won’t squish out of the groove or deform under load. A harder More O Ring here will hold its shape, last longer, and maintain the seal even when temperatures swing wildly or the system has small, sudden pressure spikes. The downside? They’re less forgiving. You can’t use them on rough surfaces, and they’re way more prone to cracking if you stretch them too far during installation. I always make sure to warn customers if their application leans toward 90A—if there’s any chance the groove has sharp edges or the mating parts are a little uneven, we’ll suggest adding a backup ring too, just to be safe.
Wait, but here’s a thing I see a lot of people mix up: hardness vs. material. Like, a 70A nitrile More O Ring is way different than a 70A silicone O-ring, right? Material affects temperature resistance, chemical compatibility, flexibility at cold temperatures, everything. Hardness is just one variable. For example, silicone More O Rings are naturally softer—most silicone O-rings top out at 80A, because silicone can’t get much harder and still seal. Nitrile (Buna-N) can go all the way up to 90A, which is why it’s so popular for high-pressure hydraulic work. Viton (FKM) is similar to nitrile, great for chemicals and high temperatures, so we stock it from 70A to 90A too. EPDM for water and UV resistance is usually 70A to 80A, since it’s not as rigid as nitrile. So hardness doesn’t tell you what material you have, and material doesn’t tell you the hardness—they’re two separate specs, but they work together.
Another common question: does hardness change over time? Short answer, yes. That’s called compression set, and it’s a big deal for More O Rings (and all O-rings). If you leave an O-ring compressed in a groove for years, it’ll deform and lose its ability to bounce back—so it gets effectively harder than it was new. That’s why old equipment with original O-rings often leaks, even if you didn’t touch it. We see this all the time with vintage machinery, or even household appliances that haven’t been serviced in a decade. When a customer comes to us with an old dishwasher that’s leaking from the door seal, we ask if they remember the last time they replaced it—9 times out of 10, the seal is just compressed and gone hard, so a new More O Ring in the right hardness fixes it.
What about temperature? Does heat or cold affect hardness? Definitely. When it’s cold, rubber gets stiffer—so a 75A More O Ring that’s soft and pliable at room temp might be 80A or 85A in freezing weather. If you’re running a system in a freezer or a remote cold storage facility, you don’t want to pick an O-ring that gets too hard at low temps—because then it won’t seal when it needs to. On the flip side, high heat softens rubber. A 90A More O Ring that’s firm at room temp might get squishy and deform at 300°F, which ruins its pressure resistance. That’s why when we help customers pick hardness, we always ask what the operating temperature is, too—we factor that in along with pressure, surface roughness, and application type.
Let me give you a quick real example, since I always find stories help. Last year, a landscaping equipment dealer came to me because they were having a problem with their new skid steer loaders’ hydraulic cylinders. The O-rings we’d been supplying were 80A, and they were failing within 6 months—leaking, then the cylinder wouldn’t extend. We checked their spec, and turns out their new skid steers run at 5500 PSI, not the 4000 PSI their old models did. The 80A was too soft to hold that pressure long-term, so we swapped them to 90A More O Rings made from nitrile, and now they’re lasting 18+ months, no leaks. That’s exactly why hardness matters—too soft, and it can’t handle the job; too hard, and you’re wasting money on something that’s overkill and harder to install.
I get that hardness can seem like a tiny, boring detail when you’re ordering O-rings, but it’s one of the biggest factors in whether your seal works for a week or a year. As a More O Rings supplier, my job isn’t just to send you a box of O-rings—it’s to make sure you get the right ones for your specific job. If you’re not sure what hardness you need, just shoot me a note. Tell me what application it’s for, pressure, temperature, what the mating surfaces look like, and I’ll help you pick the perfect More O Ring. No jargon, no hidden fees, just a recommendation that works.
At the end of the day, O-rings are simple parts, but they make everything from your car to your factory to your dishwasher work. Hardness isn’t just a number on a spec sheet—it’s what keeps your fluid from leaking, your pressure holding, your equipment running. So next time you order O-rings, don’t just look at the size—ask about the hardness, and make sure it matches what you’re doing.
If you have questions about More O Rings hardness, need a custom size, or just want to place an order, feel free to reach out to us to connect and discuss your requirements. We’re here to help you get the right seal, every time.
Rod/Piston Seal References:
- O-Ring Handbook. Parker Hannifin Corporation, 2021.
- Rubber Durometry Basics. ASTM International, 2019.
- Sealing Technology: Principles and Applications. CRC Press, 2020.
- More O Rings Product Specification Guide. More Sealing Solutions, 2022.
Zhejiang Jiacheng New Material Co., Ltd.
Zhejiang Jiacheng New Material Co., Ltd. is one of the most professional more o ring manufacturers and suppliers in China, specialized in providing high quality customized products. We warmly welcome you to buy or wholesale more o ring in stock here from our factory. Contact us for quotation and free sample.
Address: #64 Anjiang Road, Jianshan New Zone, Huangwan Town, Haining City, Jiaxing City, Zhejiang Province
E-mail: jcseal_zhu@126.com
WebSite: https://www.sjkseal.com/
