Next to the size question, the one we hear most often is: NBR or FKM? Both are black elastomer rings, both resist oil, and both work in most hydraulic fittings. The difference is that FKM (often sold under the brand name Viton) typically costs five to fifteen times as much per ring. Choosing correctly saves real money on a 50,000-piece order; choosing wrongly costs a teardown.
| Property | NBR (Nitrile) | FKM (Fluorocarbon / Viton) |
|---|---|---|
| Continuous temperature | −30°C to +100°C (special grades −40°C / +120°C) | −15°C to +200°C (low-temperature grades to −40°C) |
| Mineral oils & hydraulic fluids | Excellent | Excellent |
| Aromatic/oxygenated fuels, sour gas | Limited | Excellent |
| Ozone, weathering, UV | Poor — keep indoors or protected | Excellent |
| Hot water / steam | Good | Poor for general grades (use peroxide-cured or EPDM) |
| Ketones (MEK, acetone), amines, Skydrol | Not suitable | Not suitable — use EPDM or FFKM |
| Compression set at high temperature | Moderate; hardens over time | Excellent — recovers even hot |
| Relative cost | 1× (baseline) | Roughly 5–15× |
Nitrile is a copolymer of butadiene and acrylonitrile (ACN). The ACN content tunes fuel and oil resistance: higher ACN means better oil resistance but less low-temperature flexibility. For the vast majority of industrial hydraulics, gearboxes, pneumatic systems and fuel-contact parts running at ordinary temperatures, a standard 70 Shore A NBR compound delivers everything the application needs at the lowest possible piece price. This is why NBR is the default compound in most AS568 O-ring kits.
FKM replaces many of the hydrogen atoms in the polymer chain with fluorine. That chemistry delivers three things nitrile cannot: continuous service near 200°C, outstanding resistance to aggressive fuels and chemicals, and near-immunity to ozone and aging. An FKM ring installed in an engine-bay or chemical-plant fitting can outlast several NBR replacements — so even at 10× the unit price, it can be the cheaper choice when labor and downtime are counted.
The right compound covers your three operating parameters — media, temperature, pressure — with margin. Margin beyond that is money you are paying twice.
Need help deciding? Send us the media, temperature range and duty cycle and our engineers will recommend the lowest-cost compound that still has safety margin. For a broader comparison that also covers EPDM, silicone and FFKM, see our elastomer materials guide or the introductory material selection article.