Comparison guide
Fluorocarbon (FKM) vs Silicone O-Rings
For dry heat with no oil, the Parker handbook points to silicone; for hot oil, fuel or aggressive chemicals, it points to Viton® (FKM). On the Parker O-Ring Handbook (ORD 5700) ranges, standard FKM runs -26 C to 204 C and standard silicone (VMQ) runs -54 C to 204 C, so the top end is a tie and the choice comes down to what the seal touches, how cold it gets and whether the seal moves.
Viton and silicone compared
| Property | Viton (FKM) | Silicone (VMQ) | Source |
|---|---|---|---|
| Continuous temperature range | -26 C to 204 C (-15 F to 400 F) | -54 C to 204 C (-65 F to 400 F) | Parker ORD 5700 |
| Special compounds | Low-temperature grades to -46 C (-51 F); short excursions above 204 C, special grades to 232 C | To -115 C and to 260 C | Parker ORD 5700 |
| Fuels and petroleum oils | Excellent | Poor; swells | Parker ORD 5700 section 2.2 |
| Water and steam | Limited in standard grades; peroxide-cured grades exist | Water good; steam degrades it | Parker ORD 5700 |
| Ketones and amines | Not recommended in standard grades | Not recommended | Parker ORD 5700; both lose to EPDM or FFKM |
| Low-temperature flexibility | Limited; standard grades stiffen below -26 C | Excellent | Parker ORD 5700 |
| Dynamic service, tear strength | Good | Poor; static seals only | Parker ORD 5700 |
| Ozone and weather | Excellent | Excellent | Parker ORD 5700 |
| Gas permeability | Low | High | Parker ORD 5700 |
| Hardness range supplied | 60 to 90 Shore A; 75 A default | 50 to 70 Shore A on Conquest's former compound chart; 70 A default | Conquest's former compound chart |
| Food and medical listings | Specific compounds FDA compliant for fats and oils | The usual compliant family; platinum-cured grades for USP Class VI | Compound datasheets |
Where the handbook points, by application
| Application | Handbook points to | Why |
|---|---|---|
| Fuel system, carburetor, injector seal | Viton | Fuel swells silicone |
| Engine or gearbox oil seal | Viton | Oil swells silicone |
| Oven door, heater, lighting fixture (dry heat, static) | Silicone | Dry heat with no oil; wide range |
| Cold-weather equipment below about -30 C | Silicone | Standard FKM stiffens; silicone stays flexible |
| Food, beverage, medical device (static) | Silicone | The usual compliant family; platinum-cured grades for USP Class VI |
| Fryer or oil-contact food line at temperature | Viton | Compliant FKM grades handle fats and oils |
| Chemical processing with solvents | Viton (or FFKM) | Silicone has little chemical resistance beyond water and weak solutions |
| Any rod, piston or rotating seal | Viton | Silicone tears and abrades; static service only |
These are published manufacturer guidelines, not Conquest Seal recommendations; Conquest quotes the material your print names.
When the handbook points to FKM
The seal sees fuels, oils or aggressive chemicals, especially combined with heat, or the seal moves. Fuel systems, engine seals and chemical processing are Viton territory; the Viton (FKM) O-ring page carries the grades, the specifications that call it out and the sizes.
When the handbook points to silicone
The application is static and clean: temperature extremes, dry heat, food, beverage or medical service, or cold-weather equipment. The silicone O-ring page and the silicone guide cover the compliant grades and the aerospace AMS series.
The third option: fluorosilicone
Where the job is fuel at silicone temperatures, especially in the cold, fluorosilicone (FVMQ) combines silicone's range (-73 C to 177 C on the Parker table) with fuel resistance. It is the aerospace fuel-system material behind AMS-R-25988 and is static-only like silicone. The fluorosilicone page and the temperature range guide cover all three families.
What the print might say
Aerospace and OEM drawings name a material document rather than a trade name.
| Callout | Material | Part standard | Note |
|---|---|---|---|
| AMS7276 | FKM, 70 to 80 A | AS3209, AS3208, AS83248/1 | The current aerospace fluorocarbon O-ring; see the AMS7276 guide |
| M83248/1 | FKM, 75 A | Legacy MIL part family | Supplied to AMS7276; see the M83248 guide |
| AMS3304 (AMS3302 for the 45 to 55 grade) | Silicone | AS3582 | General-purpose aerospace silicone |
| AMS7267 | Silicone | MS9385, MS9386 | Heat-resistant, low-set silicone |
| AMS-R-25988 | Fluorosilicone, by class and grade | M25988/1 to /4 | Fuel-resistant silicone |
| FDA 21 CFR 177.2600, USP Class VI | Compliant compounds of either family | None | Compliance belongs to the compound; see the FDA guide |
How each material fails, and what it looks like
The Parker handbook's failure-mode notes make the two materials easy to tell apart after the fact. A Viton ring that met a ketone or hot steam comes out swollen and soft, or hardened and cracked after prolonged heat; a Viton ring run too cold leaks at start-up and recovers as it warms. A silicone ring in a moving seal shows one-sided flat wear and nicks from its low tear strength; a silicone ring in fuel or oil comes out swollen and gummy; and a silicone ring in a high-pressure gland extrudes early, which is why the handbook tightens its allowable clearance. The compression set guide covers the flat-sided ring both materials produce when over-squeezed at temperature, and the squeeze guide and groove design guide carry the gland numbers.
Viton is a trademark of The Chemours Company; no affiliation or endorsement is implied.
Sources
- Parker O-Ring Handbook ORD 5700 (2018), section 2.2, read 2026-09-14
- ASTM D1418
- AS568
- AMS7276, AMS7259, AMS-R-25988, AMS7267, AMS3304, AS3208, AS3209, AS3581, AS3582, AS83248/1 (titles only)
- 21 CFR 177.2600
- USP Class VI
- Conquest's former compound chart (hardness ranges only)
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.
Frequently asked questions
Which one takes more heat?
On the Parker ranges both standard families reach 204 C; special silicone compounds reach 260 C and FKM tolerates short excursions above 204 C. Of these two, only FKM survives hot oil; in hot dry air silicone is the usual callout.
Can I use silicone in a fuel line?
No. Fuel swells silicone. Fluorosilicone or Viton is the material.
Is Viton food grade?
Specific FKM compounds are offered FDA compliant, typically for fats, oils and higher temperature. Silicone is the more common food-contact choice.
Which one for a cold room or freezer?
Silicone. Standard FKM stiffens below about -26 C; even the low-temperature FKM grades stop at about -46 C on the Parker handbook range, where silicone is still flexible.
Do both use the same groove?
Same AS568 sizes and gland dimensions, with one rule from the Parker handbook: silicone gets a tighter extrusion clearance because of its low tear strength. See the groove design guide.
Which SAE documents specify silicone O-rings?
AMS7267 stands behind the MS9385 and MS9386 part standards and AMS3304 behind AS3582; fluorocarbon rings are specified through AMS7276 and AMS7259, so the print tells you which family it wants.
What is the middle option when fuel meets cold?
Fluorosilicone: silicone's cold flexibility with the fuel and oil resistance silicone lacks, on the Parker range from -73 C to 177 C, carrying the M25988 callouts under AMS-R-25988.
Can a silicone O-ring replace a Viton one in a moving seal?
No. Silicone is a static-only material on the Parker notes; a rod, piston or rotating seal in silicone wears flat and nicks. Keep Viton, or move to a dynamic-rated compound the print allows.
Related guides
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