O-rings made of nitrile

Nitrile (Buna-N) vs Viton® (FKM) O-Rings for a Gold Wing Carburetor

We’ve been getting questions asking if our carb kits o-rings are made of Viton®, Dupont’s registered trademark for their fluorocarbon (FKM). The answer is no because we’re simply not convinced of the need to use this expensive material and pass the costs on to our customers.  While both Nitrile and Viton® are in the class of elastomers, there is a recent trend towards using Viton® in the automotive industry due to its alleged superiority to Nitrile relating to heat tolerance, compression, permeation, as well as its defense against biofuels particularly ethanol.

Unfortunately, we find contradicting data on the comparisons of Nitile and Viton® ethanol compatibility leading us to believe the promotion of Viton® superiority is marketing propaganda. Does anyone remember the R12 refrigerant (Freon®) scare when R134A came on the market? Owners of pre-1994 cars were scared into converting their R12 systems to handle the new ozone safe R134A Freon to avoid corrosion of their rubber and plastic components. History proved the scare was unfounded.


Similarly, what of the scare about the effects of Nitrile exposure to ethanol, which is nothing more than grain or ethyl alcohol? Besides being intoxicating alcohol that dates back centuries, ethanol has a long history of being the fuel for heat and light. Its use in automotive combustion engines is nothing new. The original 1908 Model T Ford could be modified to run on either gasoline or ethanol because Henry Ford was a supporter of homegrown renewable fuels.

  • Ethanol was used to fuel cars well into the 1920s and 1930s as several efforts were made to sustain a U.S. ethanol program. Standard Oil marketed a 25-percent ethanol by volume gasoline in the 1920s in the Baltimore area. Ford and others continued to promote the use of ethanol, and by 1938 an alcohol plant in Atchison, Kansas, was producing 18 million gallons of ethanol a year, supplying more than 2,000 service stations in the Midwest. By the 1940s, however, efforts to sustain the U.S. ethanol program had failed. After World War II, there was little interest in the use of agricultural crops to produce liquid fuels. Fuels from petroleum and natural gas became available in large quantities at low cost, eliminating the economic incentives for production of liquid fuels from crops. Federal officials quickly lost interest in alcohol fuel production, and many of the wartime distilleries were dismantled. Others were converted to beverage alcohol plants. “Outlook for Biomass Ethanol Production and Demand” (PDF) by Joseph DiPardo. United States Department of Energy

Ethanol used over a century ago apparently posed no particular problem to the early engines all of which used nitrile gaskets and o-rings. Otherwise, there would’ve been a wide search for a better replacement material. The gasoline sold in our area has 10% ethanol. We placed nitrile o-rings in 40% ethanol and have seen no measureable difference in the ring over a week’s time.

Heat Tolerance

What about the need for increased heat tolerance in carburetor o-rings? The fuel of a spark-ignition engine is mixed with air within its flammable limits and heated above its flash point before ignition by the spark plug. A chemical’s flash point is the lowest temperature where fluid evaporates to form a combustible concentration of gas. The flash point of ethanol or ethyl alcohol is 55 F and gasoline or petrol is –45 F. Nitrile’s temperature ranges as low as -70 F and up to 275 F. Viton®’s range is to –40 F to 450 F. I found data that stated 75 Duro Viton® only ranges down to –15 F which is far from gasoline’s flashpoint of –45F.

If higher heat tolerance is due to concern about the effects of engine overheating, safeguarding your carburetor o-rings should be the least of your worries. How about a cracked or blown head gasket allowing seepage of coolant into the cylinders and warping the heads? A burst radiator and hoses? A burned piston or engine seizure?  And worst of all, your engine catching fire if a carburetor happens to be flooded at that time.

Should your engine ever catch fire, keep in mind that Viton® puts off hydrogen fluoride gas when it burns which turns into hydrofluoric acid (HF) in the presence of water. Breathing in hydrogen fluoride can cause severe lung damage. When approaching a toasted engine, always use adequate protection against handling HF because of the possibility of Viton® being used in other automotive engine gaskets and seals.

Compression Set

Elastomer compression set is the measurement of how fast it returns to its original shape when compression is removed. 100% compression is bad. The lower the number, the less likely the elastomer will “set” and lose sealing pressure under compression. The compression set of fluorocarbons (Viton®) is 11%, which is greater than Nitrile at 4% but both are considered to be in the very low range when you compare against 100%.


Permeation is the rate a gas or liquid can be transmitted through the elastomer. Nitrile has a rating of 8 while fluorocarbon is rated 12.7, which is not a significant difference and not of any consequence by the use of either material.


Viton® is an excellent product in situations requiring resistance to a broader range of chemicals that apparently are not found in every day gasoline (petrol). Higher heat tolerances are necessary in turbine and jet engines but not the liquid-cooled combustion engine of the Gold Wing. Customers who bought our carb kits over the past decade know this is a non-issue.

How do you know you truly have Viton® o-rings instead of Nitrile? Both Viton® and Nitrile have a poor resistance to esters, ethers, ketones, amines and brake fluid but Viton® excels in its resistance to halogenated solvents most of which are deadly. One easy to acquire and common such solvent is chloroform.


  1. Thanks for demystifying the ethanol thing. I had no clue it goes so far back. The flashpoint of gas is -45F and Viton only goes down to -40F. Does it get brittle or what. Ok so what about bikes stored in cold garages.

  2. i always buy your carb kits, never a problem, whats new aint always better.

  3. Your the man. Gotta give you credit since you been at it so long. I’ve rebuilt 2 GL1000 carburetors with your kits. No o-rings ever swelled! Ill never for get when you competitor came out with his version of GL1200 slide diaphragms. Talk about cutting corners.

  4. Your the man. Gotta give you credit since you been at it so long. I’ve rebuilt 2 GL1000 carburetors with your kits. no o-rings ever swelled!

  5. I asked my mechanic. He says he’s never had problems with nitrile o-rings and seals but he has had problems with Dexcool eating up gaskets. He told me he argued with them years then GM finally sued them. Wonder how long they got away with it.

  6. Author

    We declared our position because of questions raised by potential customers NOT because of Randakk’s position on this matter which is inconsequential to us. After all, he is not the only one including Viton o-rings in his carburetor gasket kit and we are not the only ones selling carb rebuild kits with nitrile o-rings. We conducted our own research and tests and simply decided to publicly explain why we can’t justify using the more expensive Viton. It’s too bad that he’s taken this personally. Is he also going to take on NAPCO, K&L Supply, Keyster and Honda?

  7. Like this makes so much sense. Now explain why any carb expert would keep selling viton orings? Because there’s a sucker born every minute? We need ya Saber Cycle. Tell the truth.

  8. What about for E 85?

  9. From a polymer chemist, nicely explained!

  10. So from all this, Can we assume, Original Honda Carb kits for CB’s and Goldwings are Buna/Nitrile material? Or were they something else?
    As long as we are getting that quality or better, Im on board. Let me know Thanks

  11. Author

    To Curtis Thomas: E85 is 85% ethanol which is grain or ethyl alcohol which we already explained does not affect Nitrile. That set of nitrile manifold o-rings that we placed in E40 when we wrote this blog is still sitting in that solution 2 years later unaffected.

    To SWISSPUP: Nitrile has been the material of choice for decades.

  12. Viton becomes hard and inflexible below 5 Deg. F. I had some O-Ring kits which I was unsure of the material so I placed an unknown one next to a Buna-N one (of a similar size) in the freezer for 10 minutes and then squeezed them between my fingers, the Viton are very hard, comparatively. Huge difference. Sterlitech Compatibility charts give Nitrile an “A” (satisfactory) for ethyl alcohol, and they give Viton a “C” (Severe Effects) for ethyl alcohol.

Got something to say?

This site uses Akismet to reduce spam. Learn how your comment data is processed.