Valve Stem Seals

Valve Stem Seals

stem seals play a critical role in controlling valve lubrication
as well as oil consumption. If the seals do not fit properly or
are not installed correctly, the guides may be either starved
for lubrication or flooded with oil. Either way, the engine is
going to have problems – and you’re going to have an unhappy customer
and perhaps a warranty claim.

Seal longevity is another issue that
should be considered when choosing replacement valve stem seals.
The material from which the seals are made must be capable of
withstanding the harsh operating environment inside the engine
for an extended period of time (not just the warranty period).
Some materials are longer lived than others, which is usually
reflected in the material’s price.

High operating temperatures
cause lower grade materials such as nitrile to harden and become
brittle over time. Eventually, this can lead to cracking, loss
of oil control and seal failure. When a valve stem seal loses
its ability to control the amount of oil that enters the guide,
it can cause a variety of problems.

Spark plug fouling may occur
as oil ash builds up on the plug’s electrodes. The accumulation
of heavy, oily carbon deposits on the backs of the intake valves
may cause hesitation and performance problems in some fuel injected
engines. As carbon deposits build up in the combustion chamber,
compression may increase to the point where it causes engine-damaging
detonation and/or preignition problems.

Increased oil consumption
due to worn or leaky valve stem seals will also increase hydrocarbon
(HC) emissions in the exhaust – which may cause a vehicle to fail
an emissions test. Oil burning can also damage the catalytic converter
because phosphorus in motor oil contaminates the catalyst. If
oil is fouling the spark plugs, misfiring can cause HC emissions
to soar as unburned fuel passes into the exhaust. This may damage
the converter because unburned fuel in the exhaust makes the converter’s
operating temperature soar.

The converter may overheat to the
point where the substrate breaks down or melts creating a restriction
or blockage in the exhaust.

Debris from deteriorating seals is
another concern that can cause additional problems inside an engine.
Pieces of the seal may clog oil passages starving lifters or rockers
for lubrication. Debris may also end up in the crankcase where
it may be sucked into the oil pickup screen creating an obstruction
that causes a loss of oil pressure – and you know what that means!

material difference

Depending on the application and the design
of the seal, the material used may be nitrile, polyacrylate, fluoroelastomer
(Viton), silicone, nylon or Teflon¨. Nitrile is one of the
least expensive materials, and has been used for many years in
umbrella or deflector type seals for older pushrod engines. Nitrile’s
temperature range is -40º to 250º F. It can withstand
intermittent operating temperatures of up to 300º F, which
is usually good enough for intake valve seals but not exhaust
valve seals.

A step up from nitrile is polyacrylate. Polyacrylate
is about twice the cost of nitrile and has a temperature range
of -30º to 350º F; it is a good step up from nitrile
for umbrella seals.

It is also used for some positive seals as
well.Some engines such as older big block Chevy V8s have positive
seals made of nylon. Nylon is a hard material with a temperature
rating of -40º to 300º F. Nylon is impervious to oil,
but it can melt if the engine overheats.

A higher grade seal material
is silicone, which is rated from -60º to over 400º F,
depending on the grade of the material. Some silicone seals can
operate at 330º F continuously and handle up to 400º
F intermittently, while others can take 375º and go as high
as 450º to 500º F intermittently without damage. Silicone
is a good high-temperature material, but costs four to five times
as much as nitrile.

In the mid-1980s, positive valve stem seals
made of fluoroelastomer materials (FKM and Viton) began to appear
in import and domestic overhead cam engines. Fluoroelastomer seals
cost roughly 12 times as much as nitrile, but have a temperature
range of -5º to 450º F, making them one of the best
high-temperature seals available.

Viton has good flexibility like
nitrile, which means it can handle some runout between the valve
stem and guide. It is also considered to be a more durable material
than silicone. Viton also has better wear resistance than most
other seal materials, making it a good choice for applications
where long term durability is a must.

The highest rated positive
seal material is Teflon, with a range of -5º to 600º
F. Like nylon, Teflon is a hard material so it cannot handle as
much runout between the stem and guide as more flexible seal materials
can. What’s more, Teflon is expensive – costing 20 to 25 times
as much as nitrile.

Material identification

It’s important
to know what type of material the valve stem seals are made of
when rebuilding an engine so you can replace same with same, or
better. Upgrading to a better grade of material should certainly
be considered if the original seals are badly deteriorated and
you have a choice as to the type of seal material that’s available
for the engine. Upgrading from nitrile to polyacrylate, silicone
or Viton, for example, would provide better durability and longevity
if the original nitrile seals were found to be hardened or falling

Identifying seal material

How can you tell one type
of seal material from another? Color is not necessarily an accurate
guide because the same material may come in several different
colors. Nitrile seals may be black, green or blue. Polyacrylate
is usually black, while Viton may be brown, orange or black. Nylon
has a translucent appearance while Teflon is white. Silicone is
usually black.

Replacement seals may not be the same color as the
OEM seals even if the materials are identical, while others may
be the same color but made of a different material. The color
identification information contained in some OEM service manuals
is also inaccurate. So going by color alone is not a very good
way to tell what type of material is in a valve stem seal.

engines may also have two different types of seal materials which
may be color coded to distinguish the intake and exhaust valve
guide seals (a higher temperature material being used for the
exhaust valves). AERA has published a technical bulletin (September
1997, TB 1488) identifying the seals used in 1984 to ’96 Chrysler/Jeep
2.5L and 4.0L engines. On this application, black seals (polyacrylate)
are used on the intake valves and brown seals (Viton) are used
on the exhaust valves.One way to identify an unknown seal material
is 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
  • 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 same
type of seal material as that used by the original equipment engine
manufacturer. That’s because many aftermarket suppliers source
their seals directly from the OEM supplier rather than make the
seals themselves. Others who do manufacture some of their own
seals may use the same or a higher grade of material in their

Some suppliers substitute silicone or Viton for nitrile
to provide better, higher temperature performance for extended
durability. But there are also aftermarket suppliers who cater
to those who are looking for the least expensive seals they can
buy. Such suppliers typically use the least expensive grade of
seal material (nitrile) to reduce cost.

The “right” seal
material for any given engine application will depend on the design
of the engine and OEM seal, the “normal” engine operating
temperature, how the engine will be used (normal service or heavy-duty
use), whether or not the OEM seals performed adequately in the
new engine application, and how important seal longevity is to
you and your customer.

A lower grade seal material such as nitrile
may be okay in a low-priced rebuilt engine for normal everyday
driving, but may not be adequate in a more demanding application.
Chuck Wible of Anderson Automotive, Louisville, TX, is an example
of a rebuilder who says he usually likes to go up a step when
replacing seals in his rebuilt engines. “If the original
seals are nitrile, I usually replace them with polyacrylate or
silicone. I prefer silicone for umbrella seals and Viton for positive
seals,” he said.

Seal design

Valve stem oil seals come
in two basic types – umbrella seals and positive seals. Used mostly
on older pushrod engines, umbrella or deflector style seals (which
also include O-rings) are installed on the valve stem and ride
the stem up and down as the valve opens and closes.

An umbrella
seal controls the amount of lubrication the valve guide receives
by deflecting oil splash away from the guide. An O-ring does the
same thing by preventing oil from flowing down the valve stem
into the guide. Umbrella seals are a simple and effective design,
and are easy to install. But they do not provide the same degree
of oil control as positive seals.

Positive seals are used on most
late model engines for two reasons: emissions control and oil
control. A positive valve stem seal provides a tighter seal which
reduces the amount of oil that enters the guides. This minimizes
oil consumption and hydrocarbon emissions, and also helps to keep
intake vacuum high for better idle quality (air being sucked past
worn valve guides and seals can cause lean misfire and a rough

A positive seal is also needed in most overhead cam engines
to prevent oil from flooding the guides. An umbrella seal cannot
handle the amount of oil that’s found in most OHC heads.

an umbrella seal, a positive seal does not move. It is pressed
in place on the end of the valve guide and wipes the oil off the
valve stem as the stem moves up and down. The seal does not actually
make direct contact with the stem but rides on a thin film of
oil creating a hydrodynamic seal. This allows a small amount of
oil to slip past the seal to lubricate the guide. For this reason,
a precise fit is extremely important with a positive seal to get
accurate oil metering.

If a positive seal fits too loose around
the valve stem, too much oil will get past the seal and flood
the guide. Oil consumption will go up along with all the problems
that go with too much oil in the combustion chamber. If a positive
seal fits the stem too tight, the hydrodynamic seal may be lost
as the oil film is scraped off the stem. This will starve the
guide 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 sealing
lip and the wire or spring around the neck area of the seal play
a big role in the seal’s ability to do its job. The wire or spring
in the neck area helps support the seal so it can conform to the
valve stem. Design differences here and in the design of the lip
determine how much deviation in valve stem diameter the seal can

Most positive seals can’t tolerate more than .005ý
difference in the valve stem diameter from the stock size. If
you’re installing new valves with oversized stems, therefore,
replacement seals with a larger inside diameter (I.D.) would be
required. Likewise, if you’re reusing valves and grinding the
stems, replacement seals with a smaller I.D. would be needed.

so, some aftermarket positive seals are designed to handle valve
stems from .005ý undersize to .015ý oversize. So
before choosing a seal, check with the seal supplier to find out
the range in valve stem sizes it can accommodate.

Lip abrasion
of a positive seal can occur if oversized valves are used with
standard sized seals. Lip damage can also occur if the valve stems
have been reground and the finish of the stems is too rough. But
one of the most common causes of lip damage is not lubricating
the seals and stems when the engine is assembled.

Some type of
lubrication must always be used with positive seals (motor oil
or assembly lube). The seal I.D. must also be protected during
installation by using a sleeve over the end of the valve. The
sharp edges around the keeper grooves may cut or tear a positive
seal, so that’s why some type of protection is required during

Another thing that needs to be considered with positive
seals is concentricity. The metal jacketed-type of positive valve
stem seals found on many Japanese engines and late model domestic
OHC engines provide good support and help hold the seal perpendicular
to the valve stem. However, they are more rigid than the nonjacketed-type
of positive seals (the same is true for Teflon positive seals).

the outside diameter (O.D.) of the guide chimney needs to be concentric
with the inside diameter of the guide for a good seal. On most
applications, there should be no more than about .010ý
of runout. Too much runout can deform the seal lip preventing
it from sealing properly resulting in increased oil consumption
and uneven seal wear.

On small block Chevy and Ford V8 engines
where positive seals are used on cast iron guides, lack of concentricity
is 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 7.3L
engines. Concentricity problems can usually be avoided by centering
off the valve guide I.D. when machining the guide chimney O.D.

newer engines such as GM’s 3.1L and 4.3L V6 and Ford’s 4.6L V8
use a positive seal design that has an integral spring seat. This
keeps the valve spring from galling the aluminum head and also
helps center the seal on the valve stem. On heavy-duty diesels
this design is often used to keep the seals from blowing off the
guides when the engine is under boost pressure.

Replace same
with same?

Most seal suppliers say rebuilders should stick
with 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 engines
usually have O-rings or umbrella style valve stem seals because
that was the type of seal design that was in general use at the
time the engine was originally designed and built. So, in most
instances, replacing same with same should provide the same degree
of oil control and lubrication.

Positive seals, on the other hand,
are used on most late model engines and OHC engines to minimize
oil consumption and emissions. Positive seals are also required
on most OHC engines because umbrella seals can’t handle the volume
of oil found in most OHC heads.

Some rebuilders, though, don’t
always replace same with same. The reasons vary depending on the
application. On some engines, a rebuilder may replace the original
umbrella style seals with positive seals to get better oil control.
Some rebuilders are also replacing positive seals in certain pushrod
engines with umbrella style seals to save money and make installation

Chuck Wible of Anderson Automotive says he’s had great
success converting newer small block Chevy and Fords as well as
173 Chevys from positive seals to umbrella seals. “It saves
half the cost, and makes it easier and quicker to install the
seals,” said Wible. “But it only works on some engines.
You have to look at the angle of the head. If there’s no risk
of flooding the guide area with oil, you can probably change to
an umbrella style seal. Otherwise, you should stick with a positive
type of seal. The Chevy 151, for example, has a flat head that
puddles oil so it would not be a good choice for an umbrella seal.”


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.

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).

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.

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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