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Materials comparison

FKM vs Nitrile O-Rings for Aerospace Fluid Systems

Choose nitrile (NBR) to AMS-P-83461 or AMS-P-5315 for petroleum hydraulics and jet fuel where the seal must flex at -65°F and stays below about 250°F. Choose fluorocarbon (FKM) to AMS7276 for hot fuel, oil and engine bay service up to about 400°F, accepting stiffness below roughly -15°F and a higher part cost.

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FKM and nitrile side by side

The two families split cleanly on temperature: nitrile owns the cold end, fluorocarbon owns the hot end, and both resist petroleum fuels and hydraulic oils. Neither one handles aircraft phosphate ester hydraulic fluid. The numbers below are typical family or compound values from the named producer documents, not specification minimums, and every aerospace drawing should still call a specification.

PropertyNitrile (NBR)Fluorocarbon (FKM)Source
Approximate service range, general grades-30 to 250°F (general service); -65 to 225°F (low temperature grades)-15 to 400°FParker O-Ring Handbook ORD 5700 (2018), service temperature table, typical
Long term heat limitUp to 212°F, shorter life at 250°FUp to 400°F; Chemours lists 3,000 h at 450°F, 1,000 h at 500°F, 240 h at 550°F and 48 h at 601°F for VitonParker ORD 5700 section 2.2; Chemours Viton Selection Guide C-11021 (9/19), typical
Cold flexibility-30 to -70°F depending on compoundAbout -15°F static for standard grades, some compounds to -50°F; about 5 to 0°F dynamicParker ORD 5700 section 2.2, typical
TR-10 low temperature retractionCompound specific, not tabulated in our sourcesViton A -17°C, B -13°C, F -6°C, GLT-S -30°C, GFLT-S -24°CChemours Selection Guide Table 1, nominal values for a 75 durometer test compound, not for specifications
Compression setWorsens as acrylonitrile content rises to improve fuel resistanceDescribed by Chemours as exceptionally good, even at high temperature; AMS7276 is titled for very low compression setParker ORD 5700 sections 2.2.1 and 8; Chemours Selection Guide; qualitative
Petroleum hydraulic fluidDefault choice; AMS-P-83461 names MIL-H-5606 and MIL-H-83282 type fluidsCompatible; Parker lists V1164-75 and V1226-75 FKM for hydrocarbon base hydraulic fluidsSAE AMS-P-83461 record; Parker ORD 5700 Table 3-15
Jet fuelYes, for kerosene type fuels; not for high aromatic fuels without a special compoundYes, including aromatic hydrocarbons that dissolve other rubbersParker ORD 5700 section 2.2.1; Chemours Selection Guide
Aircraft phosphate ester (alkyl type) hydraulic fluidNoNo; Parker recommends ethylene propylene to NAS1613Parker ORD 5700 Table 3-15
Ozone and weatheringNot resistantExcellentParker ORD 5700 section 2.2
Ketones (acetone, MEK)Not compatibleStandard grades not resistant; Chemours reports over 200 percent swell in MEKParker ORD 5700; Chemours Selection Guide Table 1
Amines and strong causticsNot listed as compatibleAttacked, except ETP type gradesChemours Selection Guide
Gas permeabilityCompound dependentVery low, similar to butylParker ORD 5700 section 2.2.7
Shelf life under AS531615 yearsUnlimited, under the storage conditions in AS5316Global O-Ring and Seal summary of AS5316

Viton is a trademark of The Chemours Company FC, LLC and is one brand of FKM polymer. Drawings and POs should name the fluorocarbon specification, not the brand, because many qualified compounds are built on other FKM polymers. Chemours itself notes Viton was introduced in 1957 for aerospace seal needs.

Choose nitrile when, choose FKM when

Start from the fluid list and the coldest and hottest temperature the seal sees while it must hold pressure, then pick the family. Cost is a tiebreaker, not a starting point.

Choose nitrile when

  • The system runs MIL-PRF-5606 or MIL-PRF-83282 type petroleum hydraulic fluid and the seal must work cold. AMS-P-83461 nitrile covers -65 to 275°F, a range no standard FKM matches at the low end.
  • The seal sits in an airframe fuel system with kerosene type jet fuel and must flex at altitude temperatures.
  • The seal is in a dynamic rod or piston gland that must move at low temperature, where FKM loses resilience first.
  • The equipment is a legacy design whose drawing calls an MS29512 or MS29513 fuel ring or an M83461/1 hydraulic ring. Buy what the drawing calls unless engineering approves a change.
  • Quantity is high, the environment is benign and the lower part cost matters.

Choose FKM when

  • Continuous temperature exceeds what nitrile tolerates: engine bay fuel and oil seals, hot lubrication systems, high temperature test stand plumbing.
  • The fluid contains aromatics, or a mix of fuels and lubricants that would swell nitrile.
  • The seal is exposed to sun and ozone, as on outdoor ground support equipment at a coastal launch site, and nitrile would crack.
  • Low permeability matters, for example a gas seal or a vacuum chamber port. Chemours reports Viton seals against vacuum as low as 10 to the minus 9 mm Hg; check outgassing separately for spacecraft use.
  • Compression set at temperature drives leak life, as in static seals that must hold after long hot soaks.

If the answer is both cold and hot, look at low temperature FKM grades or fluorosilicone before compromising. The aerospace O-rings page covers fluorosilicone and EPDM, which are outside this comparison.

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Temperature limits: what the numbers actually mean

A temperature range printed beside a specification is often a test condition, not a promise of service life. Parker's military specification table says outright that its listed temperatures are limits for particular tests required by the specifications and do not necessarily represent operating limits.

Three practical consequences follow:

  • Nitrile high end. Parker gives 212°F as the general long term limit with shorter life at 250°F, while AMS-P-83461 is titled for improved performance at 275°F. A hydraulic seal that sees 275°F only during brief excursions is a fair nitrile application; one that soaks there is not.
  • FKM high end. Chemours publishes time at temperature rather than a single limit: 3,000 hours at 450°F falls to 48 hours at 601°F. Fluid exposure at those temperatures shortens life further, so treat 400°F as the planning figure and test above it.
  • FKM low end. Parker puts standard FKM at about -15°F static and between 5 and 0°F dynamic. A static face seal can tolerate a colder start than a reciprocating rod seal in the same compound.

Write the actual operating envelope on the drawing note or the seal specification sheet, including transient peaks and how long they last. That gives the seal supplier something to check the compound against instead of a family name.

Fluid compatibility: fuels, hydraulic fluids, phosphate esters and oxygen

Both families handle petroleum fuels and hydraulic oils; neither handles aircraft phosphate ester fluids; and oxygen service depends on test data for the exact compound, not on the family.

Jet fuels

Nitrile with medium acrylonitrile content is the classic fuel seal. Parker explains the trade: more acrylonitrile improves oil and fuel resistance but hurts elasticity and compression set, and it lists fuels of high aromatic content as incompatible with standard nitrile. FKM resists aliphatic and aromatic hydrocarbons, which is why it is chosen where fuel blends vary or temperatures are high. AMS7276 FKM is written for fuel and specific engine oil systems, but its revisions carry a warning about high temperature stabilized engine oils, so check each oil, as noted on our O-rings page.

MIL-PRF-5606 and MIL-PRF-83282 hydraulic fluids

Both petroleum based hydraulic fluids are the home ground of AMS-P-83461 nitrile, whose scope names them. Parker also lists FKM compounds for hydrocarbon base hydraulic fluids from -15 to 400°F, so FKM works where a hot hydraulic seal does not need cold flexibility. Do not mix seal families inside one gland family without engineering approval, because maintenance staff will replace like with like from the parts list.

Phosphate ester fluids

For aircraft (alkyl) phosphate ester fluids, Parker recommends ethylene propylene to NAS1613, not nitrile or FKM. Parker lists FKM only for industrial (aryl) phosphate esters and phosphate ester and petroleum blends, which are not the fluids in airliner hydraulic systems. If a drawing pairs FKM or nitrile with a phosphate ester system, raise it with the design authority.

Oxygen

Parker credits FKM with excellent resistance to oxygen and ozone in the aging sense, which is not the same as ignition resistance in pressurized oxygen. For LOX and GOX hardware, compatibility comes from tests on the specific compound, such as the NASA-STD-6001 mechanical impact methods and ASTM G86, at your pressure and configuration. Name the program approved compound, require cleaning to the stated level and keep hydrocarbon lubricants away; the precision and oxygen cleaning reference covers the cleaning side, and PTFE vs PCTFE covers the seat materials that usually sit beside the O-ring in oxygen valves.

Low temperature performance

Nitrile wins at low temperature unless you buy a specialty fluorocarbon. Parker places nitrile cold flexibility between -30 and -70°F depending on compound, while standard FKM stops at about -15°F static.

The reason FKM is stiff when cold is the same reason it resists fluids. Chemours explains that fluid resistance improves as fluorine content rises, while low temperature flexibility falls, so its A, B and F types run from about 66 to 70 percent fluorine with TR-10 values moving from -17 to -6°C. Specialty grades with a fluorinated vinyl ether monomer break the trade: GLT-S reaches a TR-10 of -30°C, and Chemours states service as low as -24°F in dynamic seals and -49°F in static seals with GLT-S compounds. Those are producer nominal values for a test compound; the qualified part still has to pass its specification.

On the specification side, SAE AMS-R-83485 covers fluorocarbon rubber with improved performance at low temperatures, and Parker notes it supersedes MIL-R-83485. Confirm the current revision on ASSIST or SAE before calling it. If a design needs FKM fluid resistance below about -20°F, call the low temperature FKM specification explicitly; a supplier quoting AMS7276 will deliver a standard grade.

Compression set and seal life

Compression set is the permanent flattening an O-ring takes after being squeezed at temperature, and it is the usual reason a static seal starts to weep after long service. FKM generally holds its shape better at heat; nitrile compounds vary widely.

Parker notes that compression set appears as stiffening in highly filled compounds held above their normal limits, and that exceeding a compound's maximum temperature always reduces service life. In nitrile, the acrylonitrile content raised for fuel resistance makes compression set worse, so a high fuel resistance nitrile is not automatically a long life static seal. Fluorocarbon's reputation rests on compression set: Chemours describes it as exceptionally good even at high temperature, and Parker's specification table describes both AMS7276 and AMS7259 as very low compression set FKM.

Compression set also depends on cure system. Chemours rates bisphenol cured FKM good and peroxide cured FKM excellent for compression set, with diamine cures poor. A buyer cannot see this on a cert, which is another reason to call the specification that sets the compression set test, rather than "Viton, 75 durometer".

Aerospace compound specifications and their status

Aerospace O-rings are bought to an SAE material or part standard, and several older military documents on legacy drawings have SAE successors. The table shows what we could verify on SAE records, Parker's specification tables and DLA notices.

DocumentFamily and scopeStatus as verified
SAE AMS7276FKM, high temperature fluid resistant, very low compression set, 70 to 80 Shore A, for molded rings and seals in fuel and specific engine oil systemsRevision J, revised May 2020
SAE AS3209, AS83248/1O-ring part standards molded from AMS7276 material, sizes per AS568AS83248/1A reaffirmed November 2021; DLA points cancelled MIL-R-83248/1 buys to AS3209
MIL-R-83248Military FKM, high temperature fluid and compression set resistantCancelled; Parker notes AMS-R-83248 supersedes it, and DLA named AMS documents including AMS7276 as replacements
SAE AMS-R-83485FKM with improved low temperature performanceSupersedes MIL-R-83485 per Parker; confirm the current revision on ASSIST or SAE
SAE AMS-P-83461Nitrile packing for petroleum hydraulic fluids (MIL-H-5606, MIL-H-83282 types), -65 to 275°F, better at high temperature than MIL-P-25732 materialIssued April 1998; part numbers retained under SAE AS83461/1, adopted by DoD in January 2010
SAE AMS-P-5315Nitrile, hydrocarbon fuel resistant O-ring packings in the MS29512 and MS29513 sizes, one gradeRevision C, dated 5 August 2022, per SAE's revision list; supersedes MIL-P-5315 per Parker
MIL-P-5315BMilitary fuel resistant nitrile packingSuperseded by AMS-P-5315; Parker lists its specification test range as -65 to 160°F
MIL-P-25732Nitrile packing, petroleum hydraulic fluid, limited performanceInactive for new design, valid for existing equipment
SAE AS5316Storage, packaging and recording of elastomer sealsIssued November 2017, reaffirmed November 2022

One caution on the fuel ring: the MS29513 temperature range published in distributor listings, -65 to 200°F, is wider than Parker's -65 to 160°F specification test range for MIL-P-5315B. That gap is exactly the test limit versus operating limit point above. If 200°F fuel temperatures matter, ask the design authority which figure governs.

Shelf life, storage and receiving

Under SAE AS5316, as summarized by Global O-Ring and Seal, nitrile carries a 15 year shelf life and fluorocarbon is unlimited, both assuming proper storage. The clock starts at the cure date, commonly taken as the last day of the cure quarter.

The storage conditions behind those figures are 59 to 100°F, relative humidity below 75 percent unless parts are in sealed moisture proof bags, no direct sunlight or UV rich light, no ozone generating equipment, parts free of strain and separated from other elastomers. Nitrile's sensitivity to ozone is the main reason its shelf life is finite, so do not store nitrile seals beside electric motors or high voltage test gear.

AS5316 is a recommended practice rather than a binding specification, and a program limit overrides it. If your program sets a maximum age at receipt, put it on the PO. Our shelf life control page covers how to write dates and receive dated items.

At receiving, color is not identification. Black is the common color for both families and some FKM compounds are brown; a mixed bin cannot be sorted by eye. Match the bag label (part number, compound, lot, cure date) to the certificate, and quarantine anything loose. A Shore A check screens gross errors only.

Cost and availability drivers

FKM parts cost more than nitrile parts of the same size, and specification grade parts cost more than commercial ones in either family. We do not publish prices; the drivers are worth knowing when you compare quotes.

  • Polymer grade. The base fluoroelastomer is the main reason an FKM ring costs more than a nitrile ring, and specialty low temperature or high fluorine FKM grades add to that.
  • Specification testing. Parker notes extra charges may apply for testing and documentation on parts ordered to military, AMS or NAS specifications. Lot testing and certification cost the same whether you buy ten rings or ten thousand, so small lots carry most of it.
  • Size and tooling. Standard AS568 sizes run on existing molds. Nonstandard sizes need tooling, which Parker describes as prohibitive for small quantities.
  • Qualified source count. Common sizes in AMS-P-83461 and AMS7276 have several qualified molders and distributor stock. Odd sizes in legacy specifications, or a single approved compound on an oxygen drawing, may have one source and a molding lead time.
  • Cure date. A tight maximum age at receipt rules out older distributor stock and can force a fresh molding run, particularly for nitrile.

The seal is rarely the cost that matters. A leak on a test stand or a scrubbed operation costs far more than the difference between compounds, so choose on service conditions and let cost decide only between equally suitable options.

Processing and installation notes

Both families install the same way; the differences are in what touches them.

  • Lubricants. Use an assembly lubricant compatible with both the seal and the system fluid. Petroleum grease suits nitrile in a petroleum system but must stay out of oxygen hardware and away from EPDM phosphate ester seals. See greases and lubricants.
  • Stretch. Parker recommends installed inside diameter stretch of no more than 5 percent, and installation expansion within 50 percent of the compound's ultimate elongation, so check elongation on the compound data sheet before stretching a ring over a large fitting.
  • Cleaning solvents and swell. Ketones such as acetone and MEK attack both families. Parker treats 25 to 30 percent volume change as tolerable in static seals and about 10 percent as the dynamic limit, useful screening figures when a new fluid or cleaner is introduced.

Callout examples for the drawing or PO

Call the part standard and its revision, then the controls. A family name alone invites a commercial ring.

  • Hot fuel or oil, FKM: "PACKING, PREFORMED, AS3209-012 (AMS7276 FKM). Cure date by quarter and year, lot and compound on each bag per SAE AS5316. C of C to AS3209 at the revision stated on the PO."
  • Petroleum hydraulic, nitrile: "PACKING, PREFORMED, M83461/1 dash number per drawing, material per SAE AMS-P-83461, procured under SAE AS83461/1. Maximum age at receipt per program requirement."
  • Jet fuel, legacy nitrile: "O-RING, MS29513 dash number per drawing, material per SAE AMS-P-5315, revision C or later as approved by engineering."
  • Cold FKM: "Fluorocarbon per SAE AMS-R-83485, revision per PO; AMS7276 material not acceptable."
  • Oxygen service: name the program approved compound by manufacturer and compound number, the cleaning specification and level, and "no hydrocarbon lubricant".

For every line, require the qualified manufacturer's name on the cert, one lot per bag, and DFARS or QPL requirements where the contract flows them down. Receiving guidance is on our receiving inspection page, and suspect part controls are on the counterfeit parts avoidance page. Related gland seals are covered under gaskets and packings, and other fluid parts under seals and fluid parts.

Get FKM or nitrile seals sourced

Send the part standard, dash numbers, quantities, fluid, temperature envelope and any cure date or oxygen cleaning limit through the fluid systems quote form. We source O-rings through the qualified manufacturer or authorized and franchised distributors, flow down the specification and packaging requirements, check certs and bag labels against the PO, and can put seals on the same PO as the manifold, valve body or tube assembly they go into. We do not hold inventory and will tell you if a qualified source cannot be found. A person replies within one business day. For discontinued compounds, see obsolete aerospace parts; more material comparisons are on the materials hub.

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Questions

Is Viton the same as FKM?

Viton is a trademark of The Chemours Company FC, LLC for its fluoroelastomer polymers, and FKM is the generic family name. Seal makers compound O-rings from Viton or from other makers' FKM polymers. For aerospace parts, call the material or part specification, such as AMS7276 or AS3209, instead of the brand. A qualified compound meets the specification whatever polymer brand it uses, and a brand name alone does not guarantee any aerospace qualification.

Can I replace a nitrile O-ring with a Viton one?

Not without engineering approval. FKM handles higher temperatures and more aggressive fuels, but standard grades stiffen near -15°F static and around 0°F in dynamic seals, while AMS-P-83461 nitrile is tested to -65°F. A swap that fixes a hot leak can create a cold start leak. It also changes the part number and the parts list. Ask the design authority to approve the substitute and record it on the drawing or an approved alternate list.

Which O-ring material is used with MIL-PRF-83282 hydraulic fluid?

Nitrile to SAE AMS-P-83461, bought under AS83461/1 part numbers, is the usual choice; its scope names MIL-H-5606 and MIL-H-83282 type petroleum fluids and covers -65 to 275°F. Parker also lists FKM compounds for hydrocarbon hydraulic fluids from -15 to 400°F, useful where heat matters more than cold flexibility. Follow the component drawing or maintenance manual, since it governs which seal the hardware was qualified with.

Are FKM or nitrile O-rings compatible with Skydrol?

No. Aircraft phosphate ester hydraulic fluids call for ethylene propylene seals, commonly to NAS1613, and Parker's hydraulic fluid table recommends EPDM for aircraft alkyl phosphate esters. Parker lists FKM only for industrial aryl phosphate esters and phosphate ester and petroleum blends. EPDM in turn must be kept away from petroleum oils and greases, so check the assembly lubricant as well as the seal.

What is the shelf life of nitrile and FKM O-rings?

As summarized by Global O-Ring and Seal from SAE AS5316, nitrile has a 15 year shelf life and fluorocarbon is unlimited, assuming storage at 59 to 100°F, low humidity or sealed bags, and no UV light or ozone sources. The clock usually starts at the end of the cure quarter. AS5316 is a recommended practice, so any program limit on age at receipt overrides it and belongs on the purchase order.

What replaced MIL-P-5315 fuel O-rings?

Parker's specification table states that AMS-P-5315 supersedes MIL-P-5315. SAE's record describes AMS-P-5315 as hydrocarbon fuel resistant O-ring packings in the MS29512 and MS29513 sizes, one grade, with revision C dated August 2022. Legacy drawings that call MS29513 part numbers can usually be bought to AMS-P-5315, but confirm the accepted revision with your design authority and state it on the PO.

Which is better for low temperature, FKM or nitrile?

Nitrile, unless you specify a low temperature fluorocarbon. Parker puts nitrile cold flexibility at -30 to -70°F depending on compound and standard FKM at about -15°F static. Specialty FKM grades such as Viton GLT-S reach a nominal TR-10 of -30°C, and SAE AMS-R-83485 covers fluorocarbon with improved low temperature performance. Call that specification explicitly, because AMS7276 parts are standard grades.

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