What NBR (nitrile) o-rings are —
and where they seal best
Black, springy, cost-effective and very widely used — nitrile is the rubber that quietly seals most of the machines around you. Here is what it actually is, the jobs it does well, and the handful of places where it will let you down.
One of the most widely used o-ring elastomers
NBR — nitrile butadiene rubber, or just “nitrile” — has been the workhorse o-ring elastomer in industry for decades. If a fitting has a plain black o-ring and no specialist compound has been specified, NBR is a likely candidate — though colour alone is never a safe way to identify the material. It is inexpensive, easy to mould, mechanically tough, and it does the one thing most machines need above all else: it resists oil.
Chemically, nitrile is a copolymer of two ingredients — acrylonitrile (ACN) and butadiene — and the ratio between them is a big part of the story. More acrylonitrile means better resistance to oils and fuels, but a stiffer rubber that struggles in the cold. Less acrylonitrile stays supple at low temperature but gives up some oil resistance. Most general-purpose NBR sits around a third ACN: a deliberate compromise that covers the widest range of everyday jobs. That single number is why two “NBR” o-rings can behave quite differently.
Where NBR performs well
The headline property is oil and fuel resistance. NBR resists petroleum oils, diesel, mineral hydraulic fluids and greases well, and copes with many fuels — which is exactly why it lives in engines, gearboxes, hydraulic rams, fuel systems and pneumatics. Just how well it handles a given fuel still depends on the formulation, additives and temperature. It handles water and compressed air without drama in most everyday duties, has good mechanical strength and abrasion resistance, and holds its shape under normal loads. For the majority of static seals in machinery, plumbing hardware and general industry, NBR is simply the sensible default.
Rule of thumb: if the o-ring lives in contact with petroleum oil, grease or mineral hydraulic fluid, NBR is usually a strong first candidate. For fuels, check the exact blend and compound data.
The numbers that matter
Temperature. Most general-purpose NBR grades work from about −30 °C to +100 °C, with some compounds reaching −40 °C or +120 °C — the exact limit depends on the compound, hardness and medium. Outside that window nitrile either stiffens in the cold or hardens in the heat.
Hardness. 70 Shore A is the everyday workhorse. Softer grades conform to rougher surfaces; harder grades resist being pushed into the gap at higher pressure.
Media. Petroleum oils, many mineral hydraulic fluids, greases, plenty of compressed-air duties, and water or fuel duties where temperature, additives and approvals are compatible — the bread-and-butter of most machinery.
Where NBR is the wrong choice — and what to use instead
Plenty of o-ring failures are not about the o-ring at all — they are the right rubber in the wrong place. NBR has three classic weak spots, and knowing them saves a lot of callbacks:
Sunlight, ozone and weather. Leave nitrile outdoors and it crazes and cracks. That perished o-ring on a garden tap or an exposed fitting is almost always NBR that should have been EPDM.
Hot water, steam and sustained heat. Above its temperature limit NBR hardens, loses its squeeze and weeps. For hot water, EPDM is usually the stronger starting point; steam and higher temperatures need their own material check against temperature and pressure.
Aggressive chemicals. Brake fluid, ketones such as acetone and MEK, esters, strong acids and polar solvents swell or attack nitrile. Depending on the exact medium, these belong to EPDM, FKM (sold under trade names such as Viton) or PTFE, checked against the actual fluid.
Two of the most common ways to misuse NBR are outdoor exposure and unsuitable hot-water service — both usually belong to EPDM. Match the rubber to the environment, not just to the size.
NBR or EPDM — the one-line rule
Oil, fuel and hydraulics → NBR. Weather, hot water and ozone → EPDM. The two are almost mirror images: nitrile loves oil and hates sunlight; EPDM loves sunlight and hates oil. Get that one decision right and you remove one major cause of o-ring failure — though size, squeeze and groove condition still have to be correct. If you are choosing between them, the EPDM vs NBR comparison walks through it side by side.
Specifying NBR the right way
Three things decide whether an NBR o-ring actually seals: the right material for the medium, the right size, and the right hardness. Confirm the medium and temperature first — that is what rules NBR in or out. Then size by inside diameter and cross-section, not by eye and not from an old ring that may already have swelled or set. 70 Shore A suits most jobs; go harder only when pressure is trying to push the ring into the gap.
Whatever the job, confirm the material against the actual medium, temperature, pressure and the compound data — this guide is a starting point, not a substitute for that check.
Frequently asked questions
Is NBR good for fuel and oil?
Yes — oil, fuel and hydraulic resistance is nitrile’s defining strength, which is why NBR is widely used in engines, many fuel-system duties, gearboxes and mineral-oil hydraulics. If the seal lives in contact with petroleum oil or grease, NBR is usually a strong first candidate. For fuels it is widely used too, but check the exact fuel blend, additives and temperature, since these change compatibility.
Can I use NBR o-rings outdoors?
Not ideally. NBR has poor resistance to ozone, UV and weathering and tends to crack outdoors over time. For outdoor taps, hoses and exposed fittings, EPDM is usually the better starting point.
What temperature can NBR handle?
Typical general-purpose NBR runs from about −30 °C to +100 °C, with some compounds rated to −40 °C or +120 °C. Beyond that range it stiffens in the cold or hardens and cracks in the heat. The exact limit depends on the compound, hardness and medium.
Nitrile earns its place as one of the most widely used o-ring materials by being strong where machines need it most — in petroleum oil and general duty.
Respect its main limits — ozone and outdoor exposure, and sustained high temperature — and NBR will quietly handle a large share of everyday sealing work. When the job moves outdoors, into hot water, or into aggressive chemistry, that is your cue to reach for EPDM or a specialist grade instead.