Valvestem seals play a critical role in controlling valve lubricationas well as oil consumption. If the seals do not fit properly orare not installed correctly, the guides may be either starvedfor lubrication or flooded with oil. Either way, the engine isgoing to have problems – and you’re going to have an unhappy customerand perhaps a warranty claim.
Seal longevity is another issue thatshould be considered when choosing replacement valve stem seals.The material from which the seals are made must be capable ofwithstanding the harsh operating environment inside the enginefor an extended period of time (not just the warranty period).Some materials are longer lived than others, which is usuallyreflected in the material’s price.
High operating temperaturescause lower grade materials such as nitrile to harden and becomebrittle over time. Eventually, this can lead to cracking, lossof oil control and seal failure. When a valve stem seal losesits ability to control the amount of oil that enters the guide,it can cause a variety of problems.
Spark plug fouling may occuras oil ash builds up on the plug’s electrodes. The accumulationof heavy, oily carbon deposits on the backs of the intake valvesmay cause hesitation and performance problems in some fuel injectedengines. As carbon deposits build up in the combustion chamber,compression may increase to the point where it causes engine-damagingdetonation and/or preignition problems.
Increased oil consumptiondue to worn or leaky valve stem seals will also increase hydrocarbon(HC) emissions in the exhaust – which may cause a vehicle to failan emissions test. Oil burning can also damage the catalytic converterbecause phosphorus in motor oil contaminates the catalyst. Ifoil is fouling the spark plugs, misfiring can cause HC emissionsto soar as unburned fuel passes into the exhaust. This may damagethe converter because unburned fuel in the exhaust makes the converter’soperating temperature soar.
The converter may overheat to thepoint where the substrate breaks down or melts creating a restrictionor blockage in the exhaust.
Debris from deteriorating seals isanother concern that can cause additional problems inside an engine.Pieces of the seal may clog oil passages starving lifters or rockersfor lubrication. Debris may also end up in the crankcase whereit may be sucked into the oil pickup screen creating an obstructionthat causes a loss of oil pressure – and you know what that means!
Depending on the application and the designof the seal, the material used may be nitrile, polyacrylate, fluoroelastomer(Viton), silicone, nylon or Teflon¨. Nitrile is one of theleast expensive materials, and has been used for many years inumbrella or deflector type seals for older pushrod engines. Nitrile’stemperature range is -40º to 250º F. It can withstandintermittent operating temperatures of up to 300º F, whichis usually good enough for intake valve seals but not exhaustvalve seals.
A step up from nitrile is polyacrylate. Polyacrylateis about twice the cost of nitrile and has a temperature rangeof -30º to 350º F; it is a good step up from nitrilefor umbrella seals.
It is also used for some positive seals aswell.Some engines such as older big block Chevy V8s have positiveseals made of nylon. Nylon is a hard material with a temperaturerating of -40º to 300º F. Nylon is impervious to oil,but it can melt if the engine overheats.
A higher grade seal materialis silicone, which is rated from -60º to over 400º F,depending on the grade of the material. Some silicone seals canoperate at 330º F continuously and handle up to 400ºF intermittently, while others can take 375º and go as highas 450º to 500º F intermittently without damage. Siliconeis a good high-temperature material, but costs four to five timesas much as nitrile.
In the mid-1980s, positive valve stem sealsmade of fluoroelastomer materials (FKM and Viton) began to appearin import and domestic overhead cam engines. Fluoroelastomer sealscost roughly 12 times as much as nitrile, but have a temperaturerange of -5º to 450º F, making them one of the besthigh-temperature seals available.
Viton has good flexibility likenitrile, which means it can handle some runout between the valvestem and guide. It is also considered to be a more durable materialthan silicone. Viton also has better wear resistance than mostother seal materials, making it a good choice for applicationswhere long term durability is a must.
The highest rated positiveseal material is Teflon, with a range of -5º to 600ºF. Like nylon, Teflon is a hard material so it cannot handle asmuch runout between the stem and guide as more flexible seal materialscan. What’s more, Teflon is expensive – costing 20 to 25 timesas much as nitrile.
It’s importantto know what type of material the valve stem seals are made ofwhen rebuilding an engine so you can replace same with same, orbetter. Upgrading to a better grade of material should certainlybe considered if the original seals are badly deteriorated andyou have a choice as to the type of seal material that’s availablefor the engine. Upgrading from nitrile to polyacrylate, siliconeor Viton, for example, would provide better durability and longevityif the original nitrile seals were found to be hardened or fallingapart.
Identifying seal material
How can you tell one typeof seal material from another? Color is not necessarily an accurateguide because the same material may come in several differentcolors. Nitrile seals may be black, green or blue. Polyacrylateis usually black, while Viton may be brown, orange or black. Nylonhas a translucent appearance while Teflon is white. Silicone isusually black.
Replacement seals may not be the same color as theOEM seals even if the materials are identical, while others maybe the same color but made of a different material. The coloridentification information contained in some OEM service manualsis also inaccurate. So going by color alone is not a very goodway to tell what type of material is in a valve stem seal.
Someengines may also have two different types of seal materials whichmay be color coded to distinguish the intake and exhaust valveguide seals (a higher temperature material being used for theexhaust valves). AERA has published a technical bulletin (September1997, TB 1488) identifying the seals used in 1984 to ’96 Chrysler/Jeep2.5L and 4.0L engines. On this application, black seals (polyacrylate)are used on the intake valves and brown seals (Viton) are usedon the exhaust valves.One way to identify an unknown seal materialis with a burn test:
- Nitrile will burn easily and produce thick black smoke that smells like burning rubber.
- Polyacrylate will also burn easily producing a less dense black smoke that smells like burning plastic.
- Silicone will turn white when burned, regardless of the original color of the seal, producing smoke that has little color and no odor.
- Viton/fluoroelastomer seals will be difficult to burn and produce white smoke with no odor. The seal color will either remain the same or turn black.
Choosing the "right" seal
Most aftermarket suppliers of valve stem seals use the sametype of seal material as that used by the original equipment enginemanufacturer. That’s because many aftermarket suppliers sourcetheir seals directly from the OEM supplier rather than make theseals themselves. Others who do manufacture some of their ownseals may use the same or a higher grade of material in theirseals.
Some suppliers substitute silicone or Viton for nitrileto provide better, higher temperature performance for extendeddurability. But there are also aftermarket suppliers who caterto those who are looking for the least expensive seals they canbuy. Such suppliers typically use the least expensive grade ofseal material (nitrile) to reduce cost.
The "right" sealmaterial for any given engine application will depend on the designof the engine and OEM seal, the "normal" engine operatingtemperature, how the engine will be used (normal service or heavy-dutyuse), whether or not the OEM seals performed adequately in thenew engine application, and how important seal longevity is toyou and your customer.
A lower grade seal material such as nitrilemay be okay in a low-priced rebuilt engine for normal everydaydriving, but may not be adequate in a more demanding application.Chuck Wible of Anderson Automotive, Louisville, TX, is an exampleof a rebuilder who says he usually likes to go up a step whenreplacing seals in his rebuilt engines. "If the originalseals are nitrile, I usually replace them with polyacrylate orsilicone. I prefer silicone for umbrella seals and Viton for positiveseals," he said.
Valve stem oil seals comein two basic types – umbrella seals and positive seals. Used mostlyon older pushrod engines, umbrella or deflector style seals (whichalso include O-rings) are installed on the valve stem and ridethe stem up and down as the valve opens and closes.
An umbrellaseal controls the amount of lubrication the valve guide receivesby deflecting oil splash away from the guide. An O-ring does thesame thing by preventing oil from flowing down the valve steminto the guide. Umbrella seals are a simple and effective design,and are easy to install. But they do not provide the same degreeof oil control as positive seals.
Positive seals are used on mostlate model engines for two reasons: emissions control and oilcontrol. A positive valve stem seal provides a tighter seal whichreduces the amount of oil that enters the guides. This minimizesoil consumption and hydrocarbon emissions, and also helps to keepintake vacuum high for better idle quality (air being sucked pastworn valve guides and seals can cause lean misfire and a roughidle).
A positive seal is also needed in most overhead cam enginesto prevent oil from flooding the guides. An umbrella seal cannothandle the amount of oil that’s found in most OHC heads.
Unlikean umbrella seal, a positive seal does not move. It is pressedin place on the end of the valve guide and wipes the oil off thevalve stem as the stem moves up and down. The seal does not actuallymake direct contact with the stem but rides on a thin film ofoil creating a hydrodynamic seal. This allows a small amount ofoil to slip past the seal to lubricate the guide. For this reason,a precise fit is extremely important with a positive seal to getaccurate oil metering.
If a positive seal fits too loose aroundthe valve stem, too much oil will get past the seal and floodthe guide. Oil consumption will go up along with all the problemsthat go with too much oil in the combustion chamber. If a positiveseal fits the stem too tight, the hydrodynamic seal may be lostas the oil film is scraped off the stem. This will starve theguide for lubrication causing increased valve stem and guide wear(seal wear, too), and may even cause the valve stem to overheat,gall and stick.
Subtle differences in the design of the sealinglip and the wire or spring around the neck area of the seal playa big role in the seal’s ability to do its job. The wire or springin the neck area helps support the seal so it can conform to thevalve stem. Design differences here and in the design of the lipdetermine how much deviation in valve stem diameter the seal canhandle.
Most positive seals can’t tolerate more than .005ýdifference in the valve stem diameter from the stock size. Ifyou’re installing new valves with oversized stems, therefore,replacement seals with a larger inside diameter (I.D.) would berequired. Likewise, if you’re reusing valves and grinding thestems, replacement seals with a smaller I.D. would be needed.
Evenso, some aftermarket positive seals are designed to handle valvestems from .005ý undersize to .015ý oversize. Sobefore choosing a seal, check with the seal supplier to find outthe range in valve stem sizes it can accommodate.
Lip abrasionof a positive seal can occur if oversized valves are used withstandard sized seals. Lip damage can also occur if the valve stemshave been reground and the finish of the stems is too rough. Butone of the most common causes of lip damage is not lubricatingthe seals and stems when the engine is assembled.
Some type oflubrication must always be used with positive seals (motor oilor assembly lube). The seal I.D. must also be protected duringinstallation by using a sleeve over the end of the valve. Thesharp edges around the keeper grooves may cut or tear a positiveseal, so that’s why some type of protection is required duringassembly.
Another thing that needs to be considered with positiveseals is concentricity. The metal jacketed-type of positive valvestem seals found on many Japanese engines and late model domesticOHC engines provide good support and help hold the seal perpendicularto the valve stem. However, they are more rigid than the nonjacketed-typeof positive seals (the same is true for Teflon positive seals).
Consequently,the outside diameter (O.D.) of the guide chimney needs to be concentricwith the inside diameter of the guide for a good seal. On mostapplications, there should be no more than about .010ýof runout. Too much runout can deform the seal lip preventingit from sealing properly resulting in increased oil consumptionand uneven seal wear.
On small block Chevy and Ford V8 engineswhere positive seals are used on cast iron guides, lack of concentricityis often a cause of oil consumption and premature seal failure.Some engines may be off as much as .030ý from the factory!The same kind of problems have been seen in Ford 6.9L and 7.3Lengines. Concentricity problems can usually be avoided by centeringoff the valve guide I.D. when machining the guide chimney O.D.
Somenewer engines such as GM’s 3.1L and 4.3L V6 and Ford’s 4.6L V8use a positive seal design that has an integral spring seat. Thiskeeps the valve spring from galling the aluminum head and alsohelps center the seal on the valve stem. On heavy-duty dieselsthis design is often used to keep the seals from blowing off theguides when the engine is under boost pressure.
Replace samewith same?
Most seal suppliers say rebuilders should stickwith the same design of seal that was originally used in an engine.In other words, replace umbrella style seals with umbrella seals,and replace positive seals with positive seals.
Older pushrod enginesusually have O-rings or umbrella style valve stem seals becausethat was the type of seal design that was in general use at thetime the engine was originally designed and built. So, in mostinstances, replacing same with same should provide the same degreeof oil control and lubrication.
Positive seals, on the other hand,are used on most late model engines and OHC engines to minimizeoil consumption and emissions. Positive seals are also requiredon most OHC engines because umbrella seals can’t handle the volumeof oil found in most OHC heads.
Some rebuilders, though, don’talways replace same with same. The reasons vary depending on theapplication. On some engines, a rebuilder may replace the originalumbrella style seals with positive seals to get better oil control.Some rebuilders are also replacing positive seals in certain pushrodengines with umbrella style seals to save money and make installationeasier.
Chuck Wible of Anderson Automotive says he’s had greatsuccess converting newer small block Chevy and Fords as well as173 Chevys from positive seals to umbrella seals. "It saveshalf the cost, and makes it easier and quicker to install theseals," said Wible. "But it only works on some engines.You have to look at the angle of the head. If there’s no riskof flooding the guide area with oil, you can probably change toan umbrella style seal. Otherwise, you should stick with a positivetype of seal. The Chevy 151, for example, has a flat head thatpuddles oil so it would not be a good choice for an umbrella seal."
|KEEPING "TABS" ON TEMPERATURE
Heat is an engine’s worst enemy. Heat can damage valve seals as well as many other engine parts, so it’s not surprising that overheating is a common cause of engine failures and warranty claims.
The most common cause of overheating is loss of coolant, often due to a failed radiator or heater hose and/or a leaky radiator. An engine can also overheat if the thermostat sticks shut (a good reason for using a "fail-safe" type of thermostat). But overheating can also occur if the cooling system is not filled properly after installing a rebuilt engine or when changing the coolant (air pockets in the block).
An engine can also run hot if there’s a blockage in the radiator, the cooling fan or fan clutch fails, there’s a blockage in the exhaust system, ignition timing is incorrect or the fuel mixture is off. Regardless of what caused the engine to overheat, it’s often hard to prove that overheating resulted in engine damage.
The telltale symptoms of severe overheating may include piston seizure and scuffing, galled valve stems, damaged valve guides, and/or a warped or cracked cylinder head. But these conditions may also be blamed on other factors such as incorrect assembly tolerances or a lack of lubrication.
Your first line of defense in such instances is proof that the engine did indeed overheat (regardless of the cause). A heat tab can provide such proof by indicating a certain temperature was exceeded in operation.
A typical heat tab for a gasoline engine has a center plug that melts out at 250º to 255º F. If the engine has gotten hot enough to melt the heat tab, any damage it suffered is likely not the rebuilder’s fault. Lower temperature heat tabs are also available for other applications such as marine (187º to 192º F) and diesel (225º to 230º F).
Heat tabs, when used properly, provide an acceptable defense against unjust warranty claims. The validity of heat tabs as a reliable and proven means for monitoring engine temperature has also held up successfully in court cases involving engine warranty claims.
Heat tabs can be mounted almost anywhere on the engine block or cylinder head. Many rebuilders will install one heat tab on the block and one on each cylinder head in a V6 or V8 engine.
The heat tab should be positioned where it will give a good indication of average head temperature, but away from exhaust ports, manifolds or pipes. The heat tab should also be located in a protected position so it isn’t accidentally damaged or knocked off during engine installation or normal use. For engine blocks, a good location is in the recess of a freeze plug. For heads, almost any exterior surface not adjacent to the exhaust ports will work.
Traditional heat tabs are small round metal buttons that are attached to the engine with high-temperature, high-strength adhesive. For a secure attachment, the mounting surface on the engine must be clean (no oil, dirt or grease).
Heat indicating labels available through Engine Rebuilders Association (AERA) can also be used to monitor temperature readings. The self-adhesive labels have a series of windows from 180º to 280º F that turn black when the indicated temperature is reached.
One very important point to keep in mind when using heat tabs or labels for warranty protection is to make sure your customer understands why the tab or label is on the engine. They should know that the engine warranty is void if the tab indicates overheating has occurred or if the tab is removed.
For added protection, some rebuilders have been known to hide an additional heat tab in a less obvious location just in case the most visible heat tab has been removed or tampered with.
Using a "personalized" heat tab with your company’s name or logo on it is also a good way to identify parts you’ve rebuilt, and to assure the heat tab on the engine is the same one you installed.
Heat tabs are relatively inexpensive. Metallic heat tabs generally cost less than about 35 cents each, and heat-sensitive labels can be bought for less than 95 cents each. Considering the potential expense of a warranty claim, heat tabs are very cheap insurance.
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