Nitrile vs Fluorocarbon (FKM) O-Rings

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; Parker is a trademark of Parker Hannifin Corporation. No affiliation or endorsement is implied.

Sources

1. Parker O-Ring Handbook ORD 5700, section 2.2, read 2026-09-14
2. Chemours, Viton Fluoroelastomers Selection Guide C-11021 (9/19), page 1: about 3,000 hours at 232 C (450 F), a time-limited service limit, read 2026-09-27
3. Parker O-Ring Handbook ORD 5700, pages 2-10 and 3-6 (hardness guidance, stock kits) and Tables 8-4 and 8-5, read 2026-09-24 and 2026-09-25
4. AMS-P-5315
5. AMS-P-5510
6. AMS-P-83461
7. AMS7259
8. AMS7276
9. AS28775
10. AS3208
11. AS3209
12. AS3581
13. AS568
14. AS83248
15. ASTM D1418
16. DLA ASSIST
17. MIL-H-5606
18. MIL-P-25732
19. MIL-P-5315
20. MIL-P-83461
21. MIL-PRF-25732
22. MIL-R-83248
23. MS28775
24. MS28778
25. MS29512
26. MS29513
27. SAE Mobilus records for AS28775A and AMS-P-5510B, read 2026-09-25
28. DLA ASSIST, MIL-P-25732C Notice 1 (15 November 1989), notice of inactivation for new design, read 2026-09-25
29. SAE Mobilus current-revision records for AMS-P-5315C and AMS-P-25732A, read 2026-09-27
30. Parker O-Ring Handbook ORD 5700, section VII fluid compatibility ratings (Gasoline, page 7-25; Diesel Oil, page 7-19) and section 2.2.1 (nitrile resistance list), read 2026-09-25
31. SAE Mobilus current-revision record for AS83461/1C (revised October 2016, reaffirmed November 2021), read 2026-09-27