Buna-N vs Viton O-Rings: Durometer and Material Trade-Offs

By Mutual Screw & Supply | Published: 2026-11-01

Category: Comparisons

Compare Buna-N and Viton FKM O-rings by temperature, durometer, and dash size. Use AS568-137 and Dash 904 examples to pick the right seal for your application.

Quick answer

Choose Buna-N when you need a cost-effective, oil-resistant seal for temperatures from -40°F to +250°F and can use a 90 durometer for better extrusion resistance. Choose Viton FKM when temperatures reach up to +400°F or you need broader chemical resistance, accepting a higher material cost and a 75 durometer that is softer but still suitable for many static seals.

Temperature range is the first filter

The most decisive difference between these two materials is their service temperature. Buna-N (nitrile) handles -40°F to +250°F, which covers most hydraulic, pneumatic, and general industrial sealing tasks. Viton FKM extends that range to -15°F to +400°F, making it the practical choice for higher-temperature environments such as engine compartments or heated process lines. If your application regularly exceeds 250°F, Buna-N is outside its rated range and Viton becomes the safer default.

Durometer affects installation and extrusion resistance

Durometer measures hardness, and the two products here differ: the Buna-N O-ring is 90 Shore A, while the Viton FKM O-ring is 75 Shore A. A 90 durometer O-ring is harder and resists extrusion through gaps better under pressure, but it is less forgiving during installation and may not conform as well to rough surfaces. A 75 durometer O-ring is softer, easier to seat, and seals well on smoother surfaces at lower pressures. Neither hardness is universally better—match it to your groove design and pressure.

Chemical exposure and cost trade-offs

Buna-N is well known for resistance to petroleum-based oils and greases, which is why it is common in hydraulic and automotive applications. Viton FKM offers broader resistance to many aggressive chemicals and solvents, but it is not a universal chemical seal and can be attacked by certain fluids such as ketones and some esters. Cost also differs: the Buna-N package here contains 400 pieces at a lower per-piece cost than the Viton package, so for high-volume, moderate-temperature jobs Buna-N is often the more economical choice.

Size and dash number must match your groove

Material and durometer only matter if the O-ring fits. The Buna-N example is AS568-137, with a 2.049-inch inner diameter and 0.103-inch cross-section. The Viton example is Dash 904, a different size entirely. Always confirm the dash number and cross-section against your gland or groove dimensions; a material upgrade cannot compensate for an incorrect fit. If you are unsure of the dash size, measure the groove and compare against an AS568 size chart before ordering.

Key trade-offs at a glance

Temperature capability: Buna-N tops out at +250°F; Viton FKM reaches +400°F. If your operating temperature is above 250°F, choose Viton. Check the maximum continuous temperature, not just the peak.
Durometer and sealing behavior: 90 durometer Buna-N resists extrusion better but is harder to install; 75 durometer Viton is more conformable but may not suit high-pressure gaps. Match durometer to groove clearance and pressure, not just temperature.
Chemical resistance: Buna-N excels with petroleum oils; Viton FKM handles many aggressive chemicals but not all. Verify compatibility with your specific fluid. When in doubt, test a sample or consult a chemical compatibility chart.
Cost per seal: The Buna-N package offers 400 pieces at a lower per-piece cost than the 325-piece Viton package, making Buna-N more economical for high-volume, moderate-temperature use. Balance seal life and failure risk against unit cost.

Product facts

0.103" CS X 2.049" ID Buna-N Nitrile O-Rings | AS568-137 | Dash 137 | 90 Durometer (400 / Pkg.): Buna-N Nitrile O-ring, AS568-137, 2.049" ID x 0.103" CS, 90 durometer, -40°F to +250°F. A hard, oil-resistant seal for standard temperature ranges where extrusion resistance is important.
0.103" CS X 2.049" ID Buna-N Nitrile O-Rings | AS568-137 | Dash 137 | 90 Durometer (400 / Pkg.)
Viton FKM O-Rings | AS568-904 | Dash 904 | 75 Durometer | Brown (325 / Pkg.): Viton FKM O-ring, AS568-904, 75 durometer, -15°F to +400°F, brown. A softer, high-temperature seal for applications that exceed the limits of nitrile.
Viton FKM O-Rings | AS568-904 | Dash 904 | 75 Durometer | Brown (325 / Pkg.)

Which O-ring should you choose?

Choose Buna-N if:

Your application stays between -40°F and +250°F and involves petroleum-based oils or greases.
You need a harder 90 durometer O-ring for better extrusion resistance in high-pressure static seals.
You are sealing a high-volume production run where per-piece cost matters.

Choose Viton FKM instead if:

Temperatures regularly exceed 250°F or reach up to 400°F.
You need broader chemical resistance than nitrile offers.
You prefer a softer 75 durometer O-ring for easier installation on smooth surfaces.