AUGUST 2005

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alve seats seem to be afairly simple engine com-ponent but they play a

critical role in sealing compres-sion and cooling the valves.When a seat becomes worn, itmay leak compression and allowthe valve to run hotter than nor-mal. The same thing can happenif the seat is out-of-round or has lost its con-centricity with res- pectto the valve.

Any mismatch bet-ween the valve and seat,therefore, will have neg-ative consequences onsealing, the operatingtemperature of thevalves and valve lon-gevity.

The key to achieving along lasting valve seat isto match the seat alloywith the application andreplace same with sameor better. As a rule, mostexperts recommendreplacing OEM valveseats with ones that areof a similar material – except incases where extra durability isrequired because of a change infuels (converting to propane ornatural gas, for example), or anengine is being built for racing.

Replacement valve seats areavailable in a wide variety ofalloys and types, from the morecommon standard cast iron

alloys to harder nickel alloys toexotic alloys like copper berylli-um for racing engines with titani-um valves. And there’s powdermetal, too.

Suppliers of cast alloy valveseats typically offer a variety ofdifferent alloys, so the bestadvice is to follow their recom-

mendations as to which materialworks best in a particular appli-cation. One supplier may notagree with another as to whichalloy they would recommend fora particular application, but onepoint they all agree on is to usethe same alloy or type of seatwithin the engine. In otherwords, don’t just replace one or

two seats with an alloy that dif-fers from the OEM alloy andleave the rest alone. Replace allthe seats with the same alloy.

What happens if you mix andmatch? You’ll probably run intotrouble down the road. If areplacement seat is harder thanthe original seat, it will hold up

better than the remainingoriginal seats. Sooner orlater one of the otherseats will burn a valveand the engine will beback. So if one exhaustseat is bad, replace allthe exhaust seats.

Different seat materi-als can be used for theintake and exhaustvalves because theexhaust valves run somuch hotter than theintakes. For a racingapplication, you mightonly need to upgrade theexhaust seats to handlethe extra heat.

The valve seat materialmust also be compatible

with the type of valves that areused. For racing engines withtitanium valves, copper berylli-um or nickel aluminum bronzeseats are usually recommended.These materials can also be usedwith stainless steel and stellitevalves, too. Again, follow yoursuppliers’ recommendations forwhich type of seats they recom-

Valve seats, though seemingly simple, are an importantaspect of any cylinder head reconditioning job. A key toa long lasting seat is to replace same with same or better.

V

Circle 221 for more information

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mend for certain valves.

Different Applications,Different ApproachesWhen the cylinder heads on highmileage engines are rebuilt, it’sespecially important to pay closeattention to the condition of thevalve seats. In the case of cast ironheads with integral valve seats, aslong as the seats are undamaged,free from cracks and have notreceded into the head, they can berecut or reground torestore the sealing sur-face. If the seats aredamaged or too badlyworn to be remachined,the old seats can be cutout and replaced withinserts if the head isworth the effort torepair and has enoughcasting thickness toaccept seat inserts.

With nonintegralseats in aluminumheads, it’s pretty muchthe same story with acouple of exceptions.As long as the originalseats are in relativelygood condition (notloose, cracked or heavily worn),they can usually be touched upwith a cutter or grinding stones torestore the surface. But sometimesthe seats are loose or have hairlinecracks or other damage. In thesecases, the original seat must beremoved and replaced with a newone – preferably a seat that ismade of the same or better materi-al than the original.

Many late model domestic andimport engines have seats that aremade of powder metal. Thesetypes of seats are very hard anddurable, so they typically show lit-tle wear at high mileages.

Consequently, the seats may needlittle work when the cylinder headis rebuilt.

One difference between castalloy seats and powder metal seatsis the way the seats are manufac-tured. Cast alloy seats are made bymelting and mixing different met-als together so they combinechemically. The molten soup isthen poured into a mold and cast toshape. The rate of cooling and sub-sequent heat treatment of the metal

determines its microstructure,hardness, strength and other phys-ical properties.

Powder metal seats, by compar-ison, are made by mixing togethervarious dry metal powders such asiron, tungsten carbide, molybde-num, chromium, vanadium, nick-el, manganese, silicon, copper,etc.), pressing the mixed powdersinto a die, then subjecting the dieto high heat and pressure (aprocess called “sintering”). Thiscauses the powders to bondtogether and form a solid compos-ite matrix with very uniform andconsistent properties.

One of the advantages of pow-der metal sintering is that materi-als that are difficult or impossibleto mix together in a molten statecan be blended together and bond-ed to create totally unique materi-als. For example, in powder metalbushings and ball joints, graphiteis combined with steel to make thematerial “self-lubricating.” Thiseliminates the need for grease andperiodic maintenance.

Another advantage of the pow-der metal process isthat parts can bemanufactured veryclose to final toler-ances, reducing theamount of machiningthat’s needed to fin-ish the part to size.This lowers produc-tion costs and boostsmanufacturing pro-ductivity.

The main reasonwhy vehicle manu-facturers haveswitched from castalloy seats to powdermetal seat inserts isto extend durability.Most late model

engines have to be emissions-cer-tified to 150,000 miles or higherdepending on the application andmodel year. If the valve seats can’tgo the distance during durabilitytesting, the vehicle manufacturercan’t certify the engine.

Powder metal seats are verygood at handling thermal stress aswell as impact stress, and typicallyshow minimal wear after tens ofthousands of miles of use. Thehomogeneous consistency of apowder metal seat also improvesheat transfer, which is good for thevalves, too. Powder metal seatsalso tend to experience less micro-

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Different materials can be used for the intake and exhaust seatsbecause exhaust valves run much hotter than intakes.

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welding between the seat and valve even at high com-bustion temperatures, which helps extend the life ofboth components.

Even so, some powder metal seats tend to be veryhard (up to Rockwell 40 to 50) and can be difficult tomachine. As long as you have equipment that can cuthard powder metal seats, remachining the seatsshould be no problem. But if you don’t haveequipment that is designed for this kind of work,you may be better off replacing the seats with newones to restore the proper finish and seal.

RacingWe can’t make any sweeping generalizationsabout what kind of seats work best in a perform-ance engine application because “performance”covers a lot of territory, everything from hot streetengines with stainless steel valves to top fuel dragracing and NASCAR engines with titaniumvalves. Seat requirements vary depending on theapplication, what kind of valves are in the engine(stainless steel or titanium), how much money isgoing into the engine, and what kind of longevitythe engine is expected to deliver. The valve seatsin a street engine may remain in use for moremiles or years than the seats in a drag engine, butthey are unlikely to experience the same severityof service.

There are a number of different valve seat mate-rials from which engine builders can choose.Many of these materials will work in a wide vari-ety of performance applications while others aredesigned primarily for special applications suchas industrial engines that run dry fuels likepropane or natural gas. The only consensus is thatdifferent valve seat materials can be used success-fully in most performance engines.

What kind of materials are we talking about?Everything from nodular/ductile iron alloys andpowder metal steel seats to hard aluminum-copperand bronze alloys, and beryllium copper alloys.Many valve seat suppliers have their own propri-etary alloys while others use industry standardalloys. But you don’t have to be a metallurgist toappreciate the differences between some of thesematerials.

A valve seat must do several things. It must sup-port and seal the valve when the valve closes, itmust cool the valve, and it must resist wear andrecession. Consequently, a performance valve seatmaterial should provide a certain amount of

dampening to help cushion the valve when it closes athigh rpm. Very hard materials, especially on theintake side, are not the best choice here becauseintake valves tend to be larger, heavier and close atfaster rates than exhaust valves. The wilder the camprofile, the more pounding the valve and seat under-go at high rpm.

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Circle 223 for more information

Cooling is more of an issue withexhaust valves because exhaustvalves run much hotter than intakevalves. Cooling is provided byheat transfer from the valve to seatduring the time which the valve isclosed, and by conduction upthrough the valve stem and intothe valve guide and head.Titanium valves do not shed heatas quickly as stainless steel valves,so the tradeoff for switching fromsteel to titanium to saveweight is often hotterrunning valves. Thehigher the temperature ofthe exhaust valve, thegreater the risk of thevalve causing a preigni-tion or detonation prob-lem. There is alsoincreased risk of thevalve burning. That’swhy many suppliers oftitanium valves recom-mend seat materials suchas beryllium copper.

Beryllium copperseats are often used indrag racing, NASCAR,Formula 1 and Indy rac-ing because the material workswell with titanium valves and hasa higher thermal conductivity thansteel alloy seats. The main reasonwhy racers use beryllium copperis for cooling the valves.

For racing applications usingeither stainless steel or titaniumvalves exhaust valves, some sup-pliers recommend a sintered valveseat insert, which includes a blendof finely dispersed tungsten car-bide in a matrix of tempered M22tool steel and special alloy ironparticles. These powder metalseats have a very uniformmicrostructure, and are highlymachinable. Because powdermetal seats work harden as theyage, they don’t have to be as hard

initially to provide good long termdurability, and the self-lubricatingqualities of the material allows itto handle a wide variety of fuels,including unleaded and leadedgasoline, straight alcohol, nitrousoxide and nitro methane. A shot ofnitrous will cause combustiontemperatures to soar, but the doseusually doesn’t last long enough tohave any detrimental effect on theseats.

The next step up is a high alloyseat material, for applicationswhere high heat resistance isrequired, such as a propane ornatural gas fired stationary enginebut also for high performanceengines, heavy-duty and extremeduty engines where longevity is amust. seats are made out of a highspeed tungsten carbide tool steel,which gives it ceramic-like char-acteristics for extreme tempera-ture resistance.

Because they tend to run muchcooler than exhaust valves, lowalloy seats work well with intakevalves in performance applica-tions, even in such extreme casesas offshore racing boats that runfor hours on end..

For dry fuel (propane and natu-ral gas) and high load (diesel andracing) applications, harder seatmaterials are almost always rec-ommended. These include toolsteel tungsten carbide seats andhigh nickel alloy seats.

Take A Seat, PleaseThe first step in replacing an inte-gral valve seat is getting the seat outof the head. Any number of tech-

niques can be used here,ranging from pulling andprying to heat shrinking,cutting and machining.Sometimes thermalcleaning alone will dothe trick. When theheads get hot, the seatsfall out. Those that don’tcan be extracted bywhatever means worksbest for you.

One way to removestubborn cast alloy seatsthat works well is toweld a small bead on theinside diameter of theseat. When the weldcools and contracts, it

shrinks the seat and pulls it loose.Unfortunately, this techniquedoes not work with powder metalseats.

If an old seat has loosened upand is moving, the counterbore inthe head should be enlarged toaccept an oversize replacementseat. Simply pressing in a newseat probably won’t work becausethe interference fit has been lost.Using some anaerobic sealer tohold a new seat in place is noguarantee it will stay put if thereis not enough interference to lockit in place.

If the original seat was loose, ifthe counterbore is flared more than.001˝ (wider at the top than thebottom), or if the difference

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between the counterbore’s insidediameter (ID) and a standard seat’soutside diameter (OD) isn’tenough to provide the desiredinterference fit, then machiningwill be necessary.

Most seat suppliers say staking,peening and/or the use of anaero-bic sealer should not be necessaryif a seat is installed with the properamount of interference fit in thecounterbore. The amount of inter-ference required to lock a seat inplace depends on the diameter ofthe seat (the larger theseat, the greater theinterference that’srequired), the type ofhead (aluminum orcast iron), the applica-tion (hotter runningapplications typicallyrequire more interfer-ence to keep the seatsfrom falling out), andin some cases the typeof material used in theseat itself (hard seatscan’t take as muchinterference as softerseats).

If the OEM pub-lishes the interferencespecifications fortheir seats, follow their guide-lines. If no specs are available,follow the recommendations ofyour seat supplier.

Some experts recommend usinga minimum of .003˝ interference ina cast iron head up to a maximumof .006˝, and .005˝ for aluminumheads up to a maximum of about.008˝. There are always exceptions,and no two experts seem to agreeon this subject. One machinist maysay .005˝ to .006˝ of interference fitis all he ever uses on aluminumand cast iron heads while anothersays cast iron heads need moreinterference than aluminum heads

to keep the seats in place.Something else to keep in mind

when replacing seats is that anyhead straightening, crack repairs orwelding should be completedbefore you cut the counterboresand install new seats. The processof straightening a head can oftenpush seats out of round and createmisalignment between the seatsand guides.

Before installing a seat, makesure the counterbore is clean andhas a smooth surface finish. The

seat should be placed with theradius or chamfer side down andlubricated (ATF works fine) priorto being pressed or driven in with apiloted driver (recommended toprevent cocking).

If the replacement seat has asharp edge, it should be chamferedor rounded so it won’t scrape metaloff the head as it is being driveninto place. If metal gets under theseat, it will create a gap that formsa heat barrier. This, in turn, willinterfere with the seat’s ability tocool the valve and premature valvefailure will likely result. One sup-plier of aftermarket powder metal

seats puts a smooth radius on theoutside of its seats to make themeasier to press into place.

Preheating the head and/or chill-ing the seats with dry ice or carbondioxide are also tricks that canmake installation easier and lessenthe danger of broaching the coun-terbore as the seat is beinginstalled.

Seat MachiningValve seats must be as concentricas possible to maximize valve seal-

ing and cooling. Agood number to aimfor here is less than.001˝ of runout perinch of seat diameter.Less is always best.

The best way tocheck concentricity iswith a runout gauge.Pulling vacuum onthe valve port withthe valve in place isanother method forchecking the matingof the seat and valve.But the ability to holdvacuum is no guaran-tee of concentricity initself. That’s whyboth methods should

be used to check the quality ofyour work.

Refinishing powder metal seatsrequires a slightly different touchthan cast alloy seats as a rule. Ifgrinding, you typically need harderstones (ruby, nickel-chrome or stel-lite). If cutting, you need a goodsharp carbide cutter and to slow itdown a bit.

The one thing you want to avoidwhen cutting powder metal is anychatter on the seat surface. Powdermetal seats can accept a high qual-ity finish, but the finish is only asgood as the tools that are used tocut them. EB

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Beryllium copper valve seats such as these are used in per-formance applications to cool titanium valves. When machining valve seats you want them to be as concentric as

possible. It is recommended that you check the seats after machiningwith a runout gauge and vacuum tester. You should aim for .001˝runout per inch of seat diameter or less.