The growth in the valve seat insert market can be traced back to the early 1970s when the switch to unleaded fuel took place. Most of the engines in use or that were in core or inventory storage had to have replacement seats inserted in the exhaust side to prevent valve seat recession that occurred when the engines were run on unleaded gas.
Many people think that lead was a lubricant and somehow prevented wear. In fact, the lead caused a chemical reaction with the cast iron of the cylinder head and the stainless steel valve, forming oxides and halides that locally hardened the wear surfaces. This local hardening is what actually helped to prevent seat recession.
During the changeover period, it was not uncommon for a vehicle
that had been initially run on leaded fuel to be switched to unleaded.
The initial use of leaded fuel had created the local hardening
required and the switch to unleaded created no problems. However,
if these same heads were then reconditioned, the machine shop
would machine away the protective layers and seat recession would
occur very rapidly, sometimes in as little as 3,000 miles.
The OEMs used an induction hardening technique to locally harden
the valve seat areas. This process was supposed to produce a hardness
depth of around .070″, but in many cases it was found to
not be deep enough to allow for re-machining during head rebuilding.
These early unleaded fuel heads also needed to have exhaust seat
inserts fitted to them when they were rebuilt.
The growth continues today in demand for rebuilt cylinder heads,
especially with the extensive use of aluminum. With the exception
of diesels and truck engines, almost all cylinder heads are now
produced in aluminum. These heads have inserts already fitted
at the factory; this has contributed to the growth in the seat
insert market at the OEM level.
When the time arrives for these aluminum heads to be rebuilt,
they are often cracked around the valve pocket areas; the factory
inserts must be removed to weld up the cracks. New inserts are
usually then required to complete the repair process. All of these
changes have contributed to a replacement valve seat market that
is estimated to be about 7 million units in North America alone.
Valve seat materials
The growth in the OEM seat market has led to the widespread use
of powder metallurgy to produce inserts in large volumes. Powder
met itself allows a greater variety of material “cocktails”
to be produced. Some of these are engine specific and can almost
exactly replicate the heat transfer characteristics of the parent
metal of the cylinder head.
In most cases a replacement seat insert is not offered through
the OEM service organizations as the replacement of seats is not
considered an approved service procedure. The use of powder met
demands very large production runs to justify the tooling costs,
but it does produce a part that is very close to finished size.
Very often there is little extra machining required after installation
to complete the job.
This lack of machining has led to the use of some very hard alloys
that are extremely difficult to re-machine during the head rebuilding
process. In fact some of these latest alloys work-harden after
one or two turns of the cutter blade, blunting the cutter almost
immediately. In most passenger car type heads running on gasoline,
these seats are overkill and can be replaced with a material with
much more machinability.
The volume requirement of powder met has so far precluded the
use of that technology for most of the replacement seat market.
All of the companies supplying the aftermarket offer individually
cast replacement seat inserts in varying grades of materials and
sizes to suit individual applications. The lower grade materials
are normally iron-based alloys that are perfectly capable of withstanding
the heat and corrosion in today’s passenger car engines.
These alloys are commonly at or around the composition of cast
XB. This material offers a good compromise of machinability with
good strength and corrosion resistance at a reasonable cost. Lesser
materials are available and can give good service provided they
are used in the correct application.
The cast XB type materials can be used in passenger car, light
truck and even some diesel engines, and will give excellent service.
Typically, these alloys contain about 20% chromium and 1.5% to
2% nickel and are referred to as “hard seats” in the
industry. Their hardness is about 40 HRC at room temperature;
most will work-harden in use to about 45 HRC or higher after a
few thousand miles.
High output diesels and gaseous fuel engines require upgraded
materials to provide good service life. These upgraded materials
are often nickel- or cobalt-based and come with a corresponding
increase in cost. The composition of these nickel-based alloys
is about SAE610b, numbers 11, 12 or 13 compositions. These seats
are capable of withstanding higher operating temperatures and
higher levels of corrosion found in LPG type engines.
Gasoline leaves behind an ash content that acts as a lubricant
between the valve face and seat insert. LPG type fuels burn very
cleanly and this ash content is missing. Severe wear will take
place if the correct grade of material is not used in LPG engines.
Very often the valve material must also be changed to provide
good service life in these applications.
The last series of materials are the cobalt- or stellite-based
alloys which are normally application specific. A good example
of this is the Cummins K Series engines. The intake valve in the
Premium #1 engine is made from Tribaloy and must be run with a
Tribaloy seat insert to give the best service.
These alloys have hardness values around 50 to 55 HRC and maintain
higher hardness at elevated operating temperatures. They are also
very abrasion resistant and cost more money to produce. They contain
about 30% chromium and typically would be around the composition
of SAE610b, number 14, which is also known as Stellite #3. These
seats are normally the hardest to machine of all the seat alloys
used in the replacement market.
It is no longer recommended to use plain cast iron for any valve
seat application in today’s engines. Most machine shops fit seats
by size. It is not uncommon to see old cast iron seats that come
back for warranty because they were still on the customer’s shelf
and were installed by mistake. Non-applicable, old cast iron seat
inventories should be thrown out to avoid such problems.
Press fits and surface finishes
The powder met OEM seats that we discussed earlier are often made
of a material that closely matches the expansion rate of the parent
material it is going to be installed in. For this reason they
often have press fits of about .003″, but can be as low as
.002″. The replacement cast seats, however, need varying
press fits to prevent them from falling out during heat soaks.
Most aftermarket seats need about .005″ press when installed
in iron heads and about .007″ press when installed in aluminum
heads. Seat suppliers usually build the required press fit into
the O.D. of the seat. A 1.500″ O.D. seat will measure 1.505″
for cast iron applications and 1.507″ for aluminum heads.
This is why it is best to consult your seat suppliers catalog
and use the correct part number listed for that specific application.
Always use the press fit recommended by your seat supplier not
the value listed in OEM manuals. Selecting a seat by size only
could create a problem in obtaining the correct interference fit.
The light press on some OEM seats can create problems when the
heads are cleaned in ovens. If the temperature is not closely
controlled the OEM seats can fall out during the heating process
or loosen to create problems later.
Many rebuilders find that aluminum heads can be oven cleaned with
the head upside down to prevent these type of problems from occurring.
Most seats are finished on the O.D. to about a 15 Ra surface finish.
The finish in the counterbore should be equally smooth and round
to within .001″ T.I.R. This will ensure good contact area
and excellent heat transfer properties for the valve to operate
Seat cutting techniques
More and more shops are changing to seat cutting equipment to
replace their older grinding systems. To ensure good tool life
with these systems it is necessary to keep close control over
feed and speed rates wherever possible. The spindle speed should
be adjusted from intake to exhaust valves especially where large
diameter differences are involved. The cutting speed increases
with the increase in the diameter from the exhaust to the intake
Generally speaking, uncoated carbide inserts work best for seat
inserts. A sharp cutting edge (no hone) on the uncoated carbide
will provide lower cutting forces overall. Although C2 grade carbide
can provide satisfactory results, our research suggests that C4
carbide will provide the best overall tool life and process flexibility.
Check with your tool supplier for availability of both these grades.
Carbides used for steel (grades C5 to C8) do not work well with
valve seat insert materials. If ceramics can be obtained they
will offer increased productivity, but they are more fragile and
need more careful handling. Cermet cutters also will provide excellent
results on iron-based materials. Chart 1 contains recommended
feed and speed rates for the different seat materials and different
Cutting speeds are in feet per minute. Feed speeds are in inches
To use the above information you first need to know your carbide
grade. Then the formula for calculating cutter rpm is: Surface
Speed x 12 / 3.142 / Seat Diameter (inches) = RPM.
To use the above formula assume a seat with a 1.500″ I.D.
and a C2 grade cutter. The material is iron-based. The calculation
would look like this: 200 (feet/min) x 12 / 3.142 / 1.5 = 509
This means that the C2 cutter should run at about 500 rpm on iron-based
materials with a 1.5″ cutting diameter. From this it can
be seen that larger diameter seats require a slower spindle speed
to maintain the correct cutting rate.
It should also be noted that the harder the material the slower
the surface speed required. If your machine requires you to provide
inches per minute of feed rate, the formula to calculate it looks
like this: Feed Rate (inches per rev) x Spindle RPM = Inches per
Using the same example as above, this works out to: .003 (inches
per rev) x 500 RPM = 1.5 Inches per Minute. The speeds and feeds
listed above should be used as a guide because of differences
in carbides. Your machine may require some fine tuning, but normally
you will end up pretty close to the above values.
There are three main requirements to be aware of, i.e., seat width,
seat angle and seat runout. The seat width is important because
about 70% of heat transferred from a valve goes out through the
seat contact area.
The old rule of thumb used to be to try to maintain a seat width
of about .070″. Today’s engines, however, have valves that
are so thin it is impossible to locate a seat that wide on the
valve face. The little Subaru Justy engine’s intake valve seat
width requirement is only .020″ to .040″. It is important
to remember that valve seat width problems show up on the valve
and rarely burn up the seat.
The photo at the top of this page shows a burnt valve with its
seat next to it. This seat was installed but never cut afterwards.
The end result of this error was a burnt valve that was returned
for a warranty claim. A list of valve seat contact width requirements
is available from S.B. International, Nashville, TN, free of charge.
The seat angle is also very important. By far, more mistakes are
made on seat angles on the 6.9/7.3L Navistar than any other engine.
The mistake made is to cut the exhaust seat at 30° instead
of 37.5°. The result is a fine line contact point that is
guaranteed to burn the valve out quickly.
Rebuilders should also keep in mind that tool holders wear more
than is recognized, and then allow the cutter to tip during operation.
The runout requirement is generally between .001″ and .002″.
The larger the valve head the more runout allowed.
Excessive runout will eventually break the valve head off at the
underhead radius due to the flexing that occurs every time the
valve opens and closes against the seat. The most common causes
of excessive runout are a loose fitting pilot and the condition
of the machine spindle bearings.
Alan Carver is director of marketing for S.B. International,
Inc., Nashville, TN. He has made numerous technical presentations
to both custom and production engine rebuilders.