O-Ring Basics
Straight answers to the questions every seal conversation starts with, from Darrell Hansen and the crew at The O-Ring Store, backed by our factory partners at Freudenberg-NOK Sealing Technologies and others.
What is an O-Ring?
A doughnut-shaped ring (a torus) with a round cross-section, usually molded from rubber. Special jobs also get PTFE and other plastics, but the rubber O-Ring is the most widely used seal in the world because it is simple, compact, and it works.
Compare the compounds →What is an O-Ring seal, exactly?
Two parts working together: the rubber ring and the gland, the groove machined into the harder part that contains and supports it. The ring alone is not the seal; the ring squeezed inside a properly sized gland is.
Groove design →How does an O-Ring actually work?
Treat the rubber as an incompressible, very thick fluid with high surface tension. Squeezed in its groove, it flows into every microscopic scratch and closes the leak path. System pressure then pushes it tighter against the gland, so within its limits, more pressure seals better, not worse.
Sealing principles →Why use an O-Ring instead of a flat gasket?
No critical bolt torque, because the joint closes metal to metal. It seals across wide swings of pressure, temperature, and tolerance, needs almost no room, installs by hand with no special tools, can often be reused, and costs less than any other seal this dependable.
Applications →How much pressure can an O-Ring handle?
Static seals hold to 5,000 psi and beyond with the right gland. The limit is extrusion: pressure forcing rubber into the clearance gap. A standard 70-durometer ring starts extruding around 1,500 psi with a typical gap; a harder compound, a tighter gap, or a back-up ring buys the rest.
Back-up rings →Can O-Rings seal moving parts?
Yes. Reciprocating pistons and rods are everyday duty, and rotary shafts work at low surface speeds. Keep the surfaces polished, keep them lubricated, and never run a pressurized ring across a port edge; a trace of weep in dynamic service is normal and healthy.
Seal types →What if pressure comes from both sides?
One O-Ring seals pressure from either direction, alternating included. For severe duty, arrange the design so each seal sees pressure one way. And know that seals in series do not share load: the first seal takes all of it, the second is a backup.
Gland design →What temperatures can O-Rings take?
It is set by the compound. Everyday nitrile runs -40°F to +250°F, silicone -75°F to +400°F, and FFKM reaches +617°F. Cold stiffening is temporary and harmless once warmed; overheating hardens rubber permanently.
Temperature ranges →How do O-Rings fail?
Gradually, and that is a feature: leakage creeps up and warns you before letdown. The usual causes are extrusion at pressure, abrasion in dynamic service, and chemical attack from the wrong fluid, and each one leaves a signature you can read.
Failure gallery →Quick Answers
Short, factual answers to the questions we field most.
What temperature can Nitrile (Buna-N) O-Rings handle?
Standard Nitrile (Buna-N) O-Rings work from -40°F to +250°F (-40°C to +121°C). For more heat, HNBR reaches +300°F and FKM (Viton®) reaches +400°F.
What is the AS568 O-Ring size standard?
AS568 is the SAE Aerospace standard that defines inch O-Ring sizes. Each size has a dash number (such as -214) that encodes the cross-section and inside diameter.
Which O-Ring compound is best for water and steam?
EPDM is the standard choice for water, steam, and glycol brake fluid, rated -65°F to +300°F. Never use EPDM with petroleum oils, fuels, or greases.
What O-Ring material works with fuels and high heat?
FKM (Viton®) resists fuels, oils, and many chemicals up to +400°F. Avoid it with ketones, esters, amines, and hot water or steam.
How do I measure an O-Ring?
Measure two dimensions: the inside diameter (ID) and the cross-section (CS, the cord thickness). When possible, measure the groove rather than a used ring, which can swell or take a set. And it is now affordable to skip the guesswork of calipers and PI tapes entirely: Olypsys® O-Ring Measurement Tools pair a phone vision app with calibrated A4 and A3 grids to measure ID and cross-section in seconds, including worn, encapsulated, large, and vulcanized O-Rings, with repeatable results and traceable records for repair shops, OEM production, MRO, and quality labs. See how it works.
What makes O-Rings fail?
Most early failures are extrusion from too large a clearance gap, compression set from heat or over-squeeze, or installation damage from sharp edges and no lubrication.
A reference built by people who live O-Rings all day
Straight answers on compounds, sizes, and installation. No fluff.
What's inside the handbook?
The handbook collects the questions our customers ask most: which compound survives their fluid and temperature, what dash number they actually need, and how to install a seal so it lasts. Start with the section that matches your problem, or read the short articles for quick tips.
Latest articles
Freudenberg N888: Low-Temp Aerospace Nitrile O-Ring (70 Duro NBR)
Freudenberg N888 is a 70 durometer low-temperature nitrile (NBR/Buna-N) O-Ring for aerospace: flexible to -65°F, qualified to MIL-G-5514 and SAE AS5440, built to AS28775, MS28775, and MS28784.
O-Ring Certificates of Conformance: House CoC and Manufacturer's CoC
What certification comes with your O-Ring order: The O-Ring Store's House Certificate of Conformance on every order, the Manufacturer's CoC for aerospace and mil-spec, PPAP, and how to request each.
HNBR vs Buna-N Nitrile O-Rings: Same Fit, Longer Life
HNBR (Highly Saturated Nitrile) O-Rings drop into the same sizes as standard Buna-N and add higher heat, better wear, and longer service life. Full side-by-side.
How to Measure an O-Ring With Your Phone: Inside Olypsys® Measurement Accuracy
How Olypsys® measures O-Rings with a smartphone, what actually drives its accuracy, and why gasket measurement is right around the corner.
Know your compound and size? Order the exact O-Rings you need.
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The O-Ring Store LLC
1847 Wilma Dr, Clarkston, WA 99403
(208) 413-6377
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