Nitrile vs Fluorocarbon (FKM) O-Rings

Buna-N and Viton® overlap heavily: both seal petroleum oils and fuels well, which is why the question is almost always temperature, chemistry at the edges, and budget. On the Parker O-Ring Handbook (ORD 5700) ranges, standard nitrile runs -34 C to 100 C (-29 F to 212 F) with short-term service to 121 C, and standard FKM runs -26 C to 204 C (-15 F to 400 F). Nitrile is the industrial default; Viton is its high-temperature, chemical-resistant upgrade.

Buna-N and Viton compared

Which material for which application?

Where Buna-N is the usual callout

Standard oil or hydraulic service under about 100 C, indoors, at volume price. It remains the industrial default, and a standard AS568 size in nitrile needs no tooling. The Buna-N guide covers the hydraulic and fuel-system specifications.

Where Viton is the usual callout

Continuous temperatures push past nitrile territory, the fuel blend is aggressive, the media includes aromatics or chlorinated solvents, or the seal sees ozone and weathering. The Viton guide covers the specifications.

The middle option: HNBR

Hydrogenated nitrile keeps nitrile's oil and fuel resistance, adds ozone resistance, and is rated -32 C to 149 C in the Parker table. Where the only problem with nitrile is sunlight or a ceiling a few tens of degrees too low, HNBR is often the answer before FKM. The temperature range guide lists all three.

What the print might say

Hydraulic and fuel-system prints name documents rather than trade names. Statuses are read from the DLA ASSIST and SAE document records; most of the military originals are inactive or cancelled while the part numbers stay in daily use.

How each material fails, and what it looks like

The Parker handbook's failure-mode table makes the two easy to tell apart after the fact. A nitrile ring that cracked with no chemical exposure met ozone or sunlight; one that came out swollen met an aromatic fuel, a chlorinated solvent or a ketone; one that hardened and took a flat-sided set ran too hot for too long. A Viton ring almost never cracks from ozone; it fails by swelling in ketones or amines, by hardening in steam, or by leaking at cold start-up before it warms. Both extrude at high pressure with too much clearance, which is where the 90 A grades and backup rings come in; the groove design guide and the compression set guide cover the mechanics.

Gland and installation notes for the pair

Both materials use the same AS568 sizes and the same gland rules from the Parker handbook, and both assemble with a petroleum-based grease. Where the print owner approves a change from one to the other, two things change. Extrusion resistance at a given hardness is similar, so a 70 A nitrile that was extruding is not fixed by a 75 A Viton; go to 90 A or add a backup ring, per the handbook's pressure-versus-hardness curves. And cold start-up behavior reverses: a low-temperature nitrile stays flexible where a standard Viton stiffens, so a change on outdoor or aircraft hydraulics calls for a low-temperature FKM grade named on the print, not a standard one. The squeeze guide has the numbers.

Viton is a trademark of The Chemours Company. No affiliation or endorsement is implied.

Sources

1. Parker O-Ring Handbook ORD 5700, section 2.2, read 2026-09-14
2. AMS-P-5315
3. AMS-P-5510
4. AMS-P-83461
5. AMS7259
6. AMS7276
7. AS28775
8. AS3208
9. AS3209
10. AS3581
11. AS568
12. AS83248
13. ASTM D1418
14. DLA ASSIST
15. MIL-H-5606
16. MIL-P-25732
17. MIL-P-5315
18. MIL-P-83461
19. MIL-PRF-25732
20. MIL-R-83248
21. MS28775
22. MS28778
23. MS29512
24. MS29513

Reviewed by Grant Midstokke, Conquest Seal Corporation. Last updated September 2026. Spec values in this guide are drawn from the sources named in the sentence where each appears; the compound datasheet and your application conditions govern the final choice.