Fuel-resistant O-rings retain useful sealing properties in a specified fuel. Nitrile rubber (NBR or Buna-N) and fluorocarbon rubber (FKM) are common choices, but they solve different problems.
NBR or FKM? Start here.
NBR: A practical starting point for conventional petroleum fuels when temperature and compatibility requirements are met.
Fuel-suitable FKM: Shortlist it when greater heat resistance or better swelling control is needed.
FFKM: Consider a specific grade when available FKM options cannot meet a defined requirement.
The economical choice is the least costly compound that passes the service requirements. A premium material earns its place by solving a defined problem—not simply by having a higher temperature rating.
Define the Fuel and Operating Conditions
Before comparing materials, record six inputs:
Fuel: specification, additives and cleaning fluids.
Blend: ethanol or biodiesel percentage and the expected concentration range.
Temperature: minimum startup, normal operation and brief peaks at the seal.
Pressure: differential pressure and cycling.
Motion: static joint or moving seal; include speed for dynamic service.
Exposure: immersion, splash, vapor, wet/dry cycles and required life.
Use compatibility data that identifies the supplied compound, fuel mixture, exposure time and temperature. Confirm performance in the assembled component before approving a substitution. The comparisons below help narrow that work to a sensible shortlist.
NBR vs FKM vs FFKM: Key Differences for Fuel Service
Nitrile, NBR and Buna-N refer to the same polymer family. Viton™ is a brand covering several fluoroelastomer grades; FKM is the generic material designation. Kalrez® is a brand of FFKM perfluoroelastomer parts.
Nitrile vs Viton O-rings: practical comparison and verification
Selection factor | NBR / Buna-N | FKM / Viton™ grades | FFKM / Kalrez® grades |
|---|---|---|---|
Best reason to choose | Economical sealing in conventional petroleum fuels. | Higher heat capability and strong resistance to hydrocarbons, including many aromatic fuels. | Broad chemical resistance and specialty high-temperature duty. |
Compound identity | The cited LILY parts identify NBR / Buna-N and hardness; a specific formulation code is not provided in the article’s source records. | The cited solid part identifies Viton™ fluoroelastomer; its specific FKM formulation is not identified. Treat the FEP-encapsulated example separately. | The cited LILY part identifies Kalrez® 4079. Confirm that this named compound is required for the proposed service. |
Fuel-compatibility evidence and its scope | The ORNL gasoline/alcohol study and NREL biodiesel guide provide application context. Neither qualifies the cited LILY NBR parts for a particular fuel. | The Chemours selection guide explains grade differences; ORNL reports results for tested fluorocarbons. Neither establishes fuel qualification for the cited LILY part. | No fuel-exposure result for the cited LILY 4079 part is presented here. A compound designation and catalog temperature range are not fuel-test data. |
Catalog temperature example | Cited LILY NBR parts: approximately −29 to +121°C (−20 to +250°F). | Cited LILY solid FKM part: approximately −26 to +204°C (−15 to +400°F). | Cited LILY Kalrez® 4079 part: approximately −1 to +316°C (+30 to +600°F). |
Cold flexibility | Lower-acrylonitrile grades generally favor cold flexibility; fuel resistance trades off against it. | Standard grades can limit cold-start performance. Low-temperature grades are a separate selection. | Highly grade-dependent; a high upper limit says little about cold starting. |
Relative cost tendency | Usually the lower-cost starting point. | Generally costs more than NBR. | Usually a substantial premium over FKM. |
Compression set | No comparable result is presented for the cited part. Record test method, temperature, duration and compression. | No comparable result is presented for the cited part. Compare results under matched test conditions. | No comparable result is presented for the cited 4079 part. Do not infer a numerical result from the compound name. |
LILY supply route | Oil-resistant range; listings include 70A and 90A examples. | Chemical-resistant range; solid and FEP-encapsulated constructions must be distinguished. | Chemical-resistant range; a named 4079 compound is listed. |
Decision boundary | Reassess after a fuel change, excessive swelling or thermal aging. | Select the grade for alcohol content and cold conditions; FKM is not one universal formulation. | Use only when its additional capability is needed; it is not an automatic upgrade for every fuel. |
The temperature figures are examples from the specific LILY listings below, not universal fuel-service limits. Cost tendencies assume otherwise comparable parts; no fixed price multiplier applies.
Compression set is the permanent deformation left after a seal has been compressed and allowed to recover. Lower set generally indicates better recovery under the stated test. Comparing percentages from different temperatures or durations can be misleading.
Are nitrile O-rings fuel resistant?
Yes—suitable NBR compounds resist many petroleum fuels. Keep an established NBR compound when the fuel is unchanged and service performance is satisfactory. A change to a more demanding blend, higher temperature or longer exposure is a reason to reassess it.
What makes one FKM grade different from another?
A higher fluorine level generally improves fluid resistance but reduces cold flexibility within conventional FKM families. Specialty grades change that balance. The Chemours Viton™ selection guide shows why a grade chosen for hot fuel service may differ from one chosen for cold starting.
Are FKM or FFKM O-rings more fuel resistant?
Choose FKM first when it satisfies the fuel and temperature requirements. Consider FFKM when a particular chemical mixture or thermal condition rules out the available FKM grades. The polymer family alone does not predict which finished compound will retain sealing force longer.
Are HNBR O-rings gasoline resistant?
Some HNBR grades are candidates. HNBR offers useful mechanical and heat-aging properties, so it is worth evaluating when those properties drive the decision. Gasoline and alcohol-blend resistance still depend on formulation. Include the required HNBR grade and fuel conditions when discussing availability.
LILY product examples: compare the actual part
Browse the LILY O-Rings catalog, then use the oil-resistant NBR range or chemical-resistant FKM and FFKM range to identify candidates. The following parts illustrate the fields to compare; their sizes differ and they are not a replacement set.
Selected LILY catalog examples
Part and construction | Actual ID × cross-section; hardness | Listed temperature range | Selection point |
|---|---|---|---|
JCGCKDCD | 101 × 3 mm | ≈ −29 to +121°C | 70A NBR example for conventional fuel-service screening. |
BCEHNBII | 18 × 2.5 mm | ≈ −29 to +121°C | 90A hardness changes mechanical behavior; it does not establish better fuel resistance. |
BCIINCJB | ≈ 266.294 × 3.531 mm | ≈ −26 to +204°C | Solid FKM construction; check the grade against the fuel blend. |
BCHINBJ | 15 × 2.5 mm | ≈ −1 to +316°C | Use the listed 4079 part’s temperature boundaries for screening. |
JDEEFKDCI | 46.99 × 5.334 mm | ≈ −26 to +204°C | FEP casing and Viton™ core require a separate fit and installation review. |
Dimensions converted from the original units are rounded. Shore A and Shore D are different indentation-hardness scales. These catalog examples establish product identity and initial limits; fuel qualification remains a separate requirement.
Choosing O-Rings for Gasoline, Diesel and Jet Fuel
Fuel-based shortlisting guide—not a compatibility approval
Fuel | NBR starting point | FKM starting point | Specialty-material question |
|---|---|---|---|
Petroleum gasoline / E10 | Consider an established fuel-service compound with relevant blend history. | Prioritize when heat or swelling makes the NBR option unsuitable. | FFKM needs a specific justification; HNBR may be assessed for mechanical demands. |
E85 / high-ethanol gasoline | Do not carry over an E10 qualification. | Start with grades intended for oxygenated fuels and review cold behavior. | Choose by mixture data, not a generic FFKM or HNBR label. |
Petroleum diesel | A common candidate in established service. | Consider for higher thermal demands or other fluid requirements. | Check whether added capability solves an actual problem. |
B20 | An approved NBR installation may remain suitable; check OEM guidance and fuel quality. | Assess where the existing compound or service conditions require it. | Higher biodiesel fractions and oxidized fuel require a separate review. |
Jet fuel | Use a compound qualified to the component specification. | Use a compound qualified for both fuel and cold conditions. | Fluorosilicone may enter cold-fuel applications; aviation approval controls the selection. |
Gasoline, E10 and E85
Gasoline is a mixture. Aromatics, alcohols and additives can change swelling even when the fuel keeps the same familiar name. Treat a switch in fuel formulation as a material-review trigger.
E10 contains approximately 10% ethanol by volume. The U.S. Department of Energy’s Alternative Fuels Data Center describes E85 as containing 51–83% ethanol, depending on season and geography. That is a substantial composition change, not a minor extension of E10 service.
For high-ethanol gasoline, a fuel-suitable FKM grade is a sensible first shortlist. The next decision is which grade balances mixture resistance with the minimum startup temperature. Avoid treating separately listed gasoline and neat-ethanol ratings as a prediction for their mixture.
Diesel, B20 and higher biodiesel blends
NBR and FKM are both candidates for petroleum diesel. B20 conventionally denotes 20% biodiesel by volume. FAME biodiesel and hydrotreated renewable diesel, often called HVO, are chemically different fuels.
The NREL Biodiesel Handling and Use Guide, Sixth Edition reports favorable B20 materials-compatibility experience in diesel ground-vehicle fuel systems, including nitrile. It also identifies some nitrile compounds as vulnerable to B100. Its B20 discussion concerns blends made with specification-grade biodiesel, not every industrial sealing application.
The practical implication: do not replace a service-proven, OEM-approved NBR seal solely because the fuel is B20. For higher blends, deteriorated fuel or a new industrial assembly, review the compound and service conditions again.
Which O-rings can handle jet fuel exposure?
Qualified NBR, FKM and fluorosilicone compounds can be used in jet-fuel applications. Cold-soaked seals and the specified fuel composition matter alongside hot-service capability. Fluorosilicone is relevant where cold flexibility and fuel resistance are important, but its relatively poor abrasion and tear resistance limits demanding dynamic use.
For aircraft components, the applicable part number and approved material specification govern replacement. Match the maintenance or design documentation; a generic polymer name or catalog temperature cannot establish an aviation substitution.
What to Check Before Replacing an O-Ring
First match the drawing: inside diameter, cross-section, tolerances, construction and hardness. The O-Ring Size Chart covers size identification. Here, size is only the starting point for a material change.
Fit: review groove space and extrusion clearance with the candidate’s expected swelling. A harder ring changes contact and extrusion behavior.
Cold and motion: check recovery at startup. Moving seals also need suitable lubrication, surface finish and wear performance.
Service life: Define the required operating hours, cycles or replacement interval, along with acceptable leakage. Include wet/dry periods and maintenance exposure.
Qualification: combine relevant liquid-exposure tests with compression recovery, leak testing and service cycling. ASTM D471 addresses liquid effects; compression-set testing addresses a different property.
Required documents: List the material specification, traceability records and application-specific compliance or approval documents required by the drawing, customer or governing specification. Confirm that each document applies to the supplied part.
Why wet and dry measurements both matter
In a 2014 ORNL study of gasoline/alcohol test fuels, specimens were immersed for 4 weeks at 60°C (140°F), then assessed again after 20 hours of drying at 60°C. The fuels included aggressive gasoline formulations with 10% and 17% ethanol by volume.
The tested fluorocarbons swelled least among the tested material types, although they still expanded. The tested NBRs shrank and became more brittle after drying as plasticizers were extracted. Plasticizers are ingredients that help rubber remain flexible.
This matters in equipment that alternates between filled and empty periods: a ring can change in service and change again during shutdown. The study’s temperatures and durations are research conditions, not a universal acceptance test or a seal-life forecast.
Set acceptance limits for the component. A low swelling value is useful, but retained sealing force and leakage performance decide whether the replacement works.
When to Reassess Your O-Ring Material
A soft, swollen ring points toward fluid absorption. Flattening suggests lost recovery; cuts or extruded edges direct attention to installation and clearance. These are investigation clues, not a diagnosis from appearance alone.
Keep the failed ring and an unused sample if available. Record whether leakage began after a fuel change, at cold startup or after the equipment stood empty. Inspect the groove and mating surfaces before ordering a more expensive polymer.
Replace a damaged fuel-exposed ring rather than reinstalling it because drying restored its appearance. Its mechanical properties may have changed even if its dimensions look familiar.
Requesting a Quote for Fuel-Service O-Rings
For a new selection or proposed substitute, include:
Medium and blend: Fuel specification, additives, ethanol or biodiesel concentration, and cleaning fluids.
Temperature: Minimum startup, normal operating and peak seal temperatures.
Pressure: Differential pressure, cycling and any relevant transients.
Size and groove: Current part number or drawing, inside diameter, cross-section, tolerances, groove dimensions and extrusion clearance.
Motion and exposure: Static or dynamic service, speed if relevant, contact form and wet/dry cycles.
Material and compound: Current polymer, formulation code if known, hardness and solid or encapsulated construction.
Quantity and life: Required quantity, target operating life or replacement interval, and acceptable leakage.
Documentation: Required material records, traceability, compliance documents and component approvals. Include failed samples, photographs or service history where available.
For a proposed substitute, ask for the supplied compound identity and relevant test conditions. Compare quotations for the same size, construction, quantity and documentation requirements; otherwise a material-price comparison can be misleading.
Discuss your fuel-service requirements
Use the oil-resistant NBR range or chemical-resistant FKM and FFKM range to identify a starting candidate. Send the checklist above with your inquiry to review the specific compound and part.
Frequently Asked Questions
Does a harder O-ring resist fuel better?
Hardness and chemical resistance answer different questions. Shore A hardness measures indentation resistance. A harder ring may help a particular extrusion problem, but fuel resistance comes from the formulation. Choose the chemistry first, then check hardness against the seal design.
Can a fuel-resistant O-ring also handle the cleaning fluid?
Assess both exposures. An O-ring that works in fuel may face a very different solvent, temperature or concentration during cleaning. Include the cleaning sequence and rinse conditions in the material selection rather than treating them as occasional exceptions.
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