One thing you can almost
always count on when rebuilding a cylinder head is worn valve
guides. The guides experience a lot of wear because of the constant
friction between the guide and stem. To make matters worse, positive
valve seals on late model engines prevent the guides from receiving
much lubrication. Side forces on the valve stem caused by changes
in valvetrain geometry, or by direct acting overhead cams, further
contribute to guide wear.
When the guides are worn
or there is too much clearance between the guide and valve stem,
the engine will use oil. This applies to both intake and exhaust
guides. Though oil consumption can be more of a problem on the
intake side because of constant exposure to engine vacuum, oil
can also be pulled down the exhaust guides by suction in the exhaust
port. The flow of exhaust past the exhaust guide creates a venturi
effect that can pull oil down the guide.
Oil in the exhaust system
of late model vehicles with catalytic converters may cause the
converter to overheat and suffer damage. On the intake side, oil
drawn into the engine past worn intake guides can foul spark plugs,
cause the engine to emit higher than normal unburned hydrocarbon
(HC) emissions, and contribute to a rapid buildup of carbon deposits
on the backs of the intake valves and in the combustion chamber.
Carbon deposits in the
combustion chamber can raise compression to the point where detonation
occurs under load. Deposits on the backs of the intake valves
in engines equipped with multipoint fuel injection can cause hesitation
and idle problems because the deposits interfere with proper fuel
delivery.
Inadequate valve cooling
and premature valve failure is another problem that can be caused
by worn guides or guides with excessive clearance. About 75% of
the heat from a typical valve is conducted to the seat, and the
remaining 25% goes up the stem and out through the guide. On late
mode engines with three-angle narrow seats, the amount of heat
transfer that takes place through the stem is even higher because
less heat can be dissipated through the seat. So if the guide
is worn, the valve may run hot and burn.
Worn guides can also
pass air. “Unmetered” air drawn into the intake ports
past the guides creates an effect similar to worn throttle shafts
on a carburetor. The extra air reduces intake vacuum and upsets
the air/fuel calibration of the engine at idle. The result may
be a lean, misfire problem and rough idle.
Worn guides can also
contribute to valve breakage. The guides support and center the
valves as they open and close. A worn guide will allow the valve
to wobble slightly as it opens, which cause it to drift off-center
with respect to the seat. This can cause the head of the valve
to flex slightly each time it closes (much like a valve with a
nonconcentric seat). After so many cycles, the metal fatigues
and the head of the valve breaks off from the stem.
Generally speaking, the
intake valve stem-to- guide clearance for most passenger cars
ranges from .001″ to .003″, and .002″ to .004″
for exhaust guides (which generally require .0005″ to .001″
more clearance than the intakes for thermal expansion). Diesel
engines, as a rule, have looser specs on both intake and exhaust
guides than gasoline engines, and heads with sodium-filled exhaust
valves usually require an extra .001″ of clearance to handle
the additional heat conducted up through the valve stems.
Checking guide wear
To check guide wear,
some machinists insert a valve stem into a guide and “feel”
for looseness by wobbling the valve. Others may use a valve seat
pilot tool to check the guides. Though either technique will reveal
badly worn guides, neither is a very accurate method of gauging
guide clearances or wear.
The best way to check
guide wear is with a gauge set designed for this purpose. A gauge
set will give you precise measurements and can be used to measure
any portion of the guide. To check guide wear (as well as taper)
using a telescoping or split ball gauge, measure the guide ID
at both ends and in the middle. Subtract the middle reading from
the ends to determine taper wear. Compare the smallest ID measurement
(usually in the middle of the guide) to the factory specs to determine
total wear.
Valve stems should also
be measured to check for wear and sizing. Nominal sizes vary quite
a bit depending on the application, and there’s no way of knowing
if the valve has been replaced previously with one of a different
size without measuring.
Many late model engines
have tapered valve stems. Taper stem valves are ground with the
stem diameter smaller at the head end of the valve. This is done
to create a larger clearance at the head where the temperatures
are highest. This reduces the chance of galling with unleaded
fuel and narrow three-angle valve seats. When measuring a tapered
stem, check the outside diameter about an inch in for each end.
Guide repair options
Whether you repair guides
on an “as needed” basis or automatically redo all the
guides anytime a head is rebuilt, you have a variety of guide
repair options from which to choose. Most rebuilders either go
with thin wall bronze liners and reclaimed valves, or install
new or rechromed valves with oversized stems. Replacing guides
is another option with aluminum heads as well as some cast iron
heads, as is knurling. Most rebuilders have tried all of these
techniques at one time or another, but usually stick with a single
technique that fits their operation best, or gives them the least
amount of problems.
Knurling
Though still used by
some small shops, most rebuilders see knurling as a short term
“quick fix” that doesn’t hold up as well as guide liners,
new replacement guides or valves with oversized stems. Knurling
should only be considered as a guide repair option if guide wear
is minimal (less than .006″). And even then, it may not provide
satisfactory results.
Knurling typically decreases
the inside diameter of the guide where it needs it the least,
namely in the center where there is the least wear, rather than
at the ends where the wear is usually greatest. When the knurling
tool is run through the guide, it leaves behind a spiral groove.
The groove acts like a furrow and raises the metal on either side.
This reduces the inside diameter of the guide so a reamer can
then be used to resize the guide back to (or close to) its original
dimensions.
The grooves also help
to retain and seal oil better than a smooth bore guide. This allows
somewhat tighter guide-to-stem clearances (as close as .0007″).
But the bearing surface area created by knurling is not that great,
so it won’t last as long as a guide that offers greater bearing
area.
Liners
Boring out the original
guides and installing thin wall bronze liners to restore proper
clearances is a fast and economical guide repair option. It also
provides the benefits of a phosphor/bronze guide surface which
offers better lubricity, scuff resistance and wear characteristics
than cast iron.
Though liners are most
often used to repair integral guides in cast iron heads, they
are also a very effective way to repair replaceable guides in
cast iron or aluminum heads — which saves time and eliminates
the risks associated with driving out the old guides and pressing
in new ones.
Liners also save the
cost of having to replace the valves. If the original valves are
not worn, standard sized liners can be used to restore the inside
diameter dimensions of the guides. If the valves are worn, the
stems can be turned down .0050″ to accommodate liners with
slightly undersized inside diameters.
Jerry Qualiana, vice
president of aftermarket sales at K-Line Industries, Holland,
MI, says K-Line’s Bronze Bullet Guide Liner system has been authorized
by Ford Motor Co. and meets Ford’s Q-1 quality standards.
According to Qualiana,
the Bronze Bullet Guide Liner design is an enhanced design over
previous bronze liners, incorporating an interrupted spiral which
assists in retaining oil in the guide, while eliminating oil flow
through the guide. In conjunction with the previously mentioned
lubricity characteristics of phosphor bronze, Bronze Bullet Guide
Liners offer improved guide life in today’s oil starved valve
guide environment.
Because of the lubricity
in the phosphor bronze, K-Line has always advocated valve-to-stem
clearance at the low side of the manufacturer’s recommended specifications.
Mike McElmurry, vice president of production at Sequel Corp.,
a production engine remanufacturer located in Willow Springs,
MO, agrees. “Because the final size is so easy to control,
we have been able to tighten all of our valve-to-guide tolerances
by at least .001”.
“This, along with
the liner’s ability to resist galling, has reduced our warranty
claims by as much as 75%. McElmurry added that for Sequel’s production
requirements, grinding valves to .003″ undersize in combination
with the liner has provided an $0.80 per guide savings over any
other valve guide repair option available.
As with any valve guide
liner, the key to success is proper installation. Qualiana says
if the original guides are not worn more than .030″ or cracked,
they can be lined. Otherwise, replacement would be recommended.
Qualiana offered the
following five-step installation procedure to ensure acceptable
performance of his company’s guide liners:
First, the old guides
have to be bored out to accept the liners. Qualiana recommends
using a KL1725CB Black Beauty carbide reamer in an air drill with
a no-load speed of 2,100 to 3,000 rpm. K-Line offers a KL9900
Boring Fixture with centering pilots that center the reamer off
the valve seat (which maintains seat concentricity), and an air
clamping fixture that holds the head securely in place while the
guides are being bored.
The guides should be
bored dry with no lubricant, using steady consistent pressure.
Once the guides have been bored out, they should be blown out
and checked with a go- no-go gauge to make sure they’re the proper
size.
The liners should then
be pressed in from the top side of the head using an air hammer
and K-Line’s Auto Installer tool. The liners go in with the tapered
side facing the guide hole. The liners are then driven in flush
with the top of the guide boss.
Next, the liners are
sized. Any of three different techniques may be used: roller burnishing
(use with lubrication), broaching (driving a calibrated ball through
the liner with an air hammer), or using K-Line’s ball broach tool
in an air hammer.
Sizing the liners is
a critical step because it accomplishes two things: it provides
the proper clearances between valve stem and liner for proper
lubrication and oil control, and it locks the liner in place so
it will transfer heat efficiently to the surrounding metal for
proper valve cooling.
Bronze actually conducts
heat more efficiently than cast iron, but requires a tight fit
and metal-to-metal contact with the surrounding guide for good
heat transfer. If the liner isn’t sized properly, it may cause
the valve to run hot — or worse yet, come loose.
After the liners have
been sized, turn the head over and trim the liner to length. The
liner should be cut flush with the guide boss in the port. This
step is not necessary if precut liners are being used that have
the correct length for the application.
The final step is to
Flex Hone the liner — after any seat work that’s necessary has
been completed. The Flex honing step removes any burrs left from
trimming the liner to length, and leaves a nice crosshatch finish
that improves oil retention. One pass in and out is all that’s
recommended to hone the liner. A flexible nylon brush should then
be passed through the liner to clean the hole.
Though this procedure
may sound complicated, a typical four cylinder or V8 can be relined
in six to seven minutes, says Qualiana. Also, the majority of
the detailed steps listed, regarding cleanliness and accuracy
in the guide area, are requirements no matter which method of
guide repair the rebuilder chooses.
Ron Bernstein, vice president
of Precision Engine Parts in Las Vegas, NV, says his company sells
a solid, one-piece smooth bore .030” oversize phosphor bronze
valve guide liner. “Ours is not a split design, so all you
do is ream out the guide and press it in,” explained Bernstein.
“You don’t have
to broach it afterwards because the liner is installed with an
interference press fit of about .001″_ to .0015″. This
saves a step and prevents the liner from falling out. But the
guide must be bored to exact dimensions, which means you have
to use the proper boring tool and replace it when it becomes worn.
Our liner restores the guide back to stock dimensions so a reclaimed
or new valve can be installed. It’s a very popular liner with
Mexican rebuilders,” said Bernstein.
Ertel Manufacturing Corp.
in Indianapolis, IN, makes cast iron liners as well as guides.
Engineer Bob Leszcynski says many people have a love/hate relationship
with bronze liners. “They love the fact that anybody with
a Black & Decker hand drill can install the liners, but they
hate the fact that if they’re not installed correctly the head
will come back with loose or worn liners.
“We say rebuilders
should always use some type of piloted installation equipment
that centers off the valve seat so the liner will be centered
properly in the guide. With a hand drill and no fixturing, you
have no control. Lean this way or that way on the drill a little
bit and your hole will be off.
“We also say you
must always broach the liners once they’ve been installed to seat
them, which is something we also require for our cast iron liners.
Most people don’t think cast iron will stretch, but it does when
you broach the liner to seat it.”
Leszcynski said cast
iron liners cost about the same as bronze liners. “Bronze
has good anti-seize properties and is popular for that reason.
But cast iron wears better and performs more like an integral
valve guide in a cast iron head. Cast iron is also a good replacement
choice for aluminum heads. In fact, you can use cast iron guides
or liners in virtually any application where bronze might be used.
We also have cast iron guides for the ’93 and newer engines that
have powder metal guides.”
Oversized valves
Another popular guide
repair option is reaming the guides to oversize and installing
new valves with oversized stems or used valves with oversize,
chromed-plated stems. Those who prefer this technique say it’s
a fast and easy way to restore guides because all you have to
do is ream the guide to oversize and drop in a new valve.
In many instances, the
exhaust valves have to be replaced anyway because of wear or burning,
so the added cost is not that great a factor. New valves also
eliminate the need to regrind the old valve stems, as well as
the sizing hassles that go with reusing reground valves or stock
valves that come in so many different nominal sizes.
According to Alan Carver,
director or marketing at SB International, Nashville, TN, a supplier
of replacement valves and seats, about 30% of production engine
rebuilders are installing new oversized valves. This compares
to about 30% that are using guide liners, and 30% that are grinding
or rechroming valves. Carver says that although they offer oversized
valves in .003″, .005″ and .015″ oversizes, the
+.015″ valve is the most popular because it can be used in
about 85% of all engine applications.
“It used to be that
rebuilders could reclaim about 80% of their valves. But because
of the accelerated valve stem wear in so many older engines that
didn’t have chrome plated valve stems, they now have to throw
out about 80% of those valves. The cost of replacing the valves
on popular engines is not that bad because the valves are relatively
inexpensive,” said Carver. “But on less popular engines,
they can be rather expensive.”
Bill McClusky at Manley
Valves, Dynagear Co. in Dallas, TX, said his company offers oversized
valves in +.003″, +.005″ and +.015″ sizes, with
most of the demand being for the +.015″. He said there hasn’t
been a lot of growth in the demand for oversized valves primarily
because many rebuilders are still reclaiming the old valves.
“It’s still about
a dollar per valve cheaper to reclaim valves than to replace them
even when you figure in the added labor cost to recondition the
old valves,” he said. “Even so, some rebuilders are
going with all new valves because they eliminate problems. Being
able to offer a customer a head with all new valves can also be
a good selling point,” said McClusky.
John Lynaugh, warranty
supervisor for Cloverland Manufacturing in Escanaba, MI, says
Cloverland Mfg. rebuilds about 150 cylinder heads a day. Of these,
99% go out the door with oversized, chrome-plated valves. Lynaugh
says they prefer to use +.008″ oversized valves for several
reasons:
“We tried knurling
and found that it only lasted about 5,000 miles,’ said Lynaugh.
“We also tried liners but saw failures as early as 10,000
to 12,000 miles. So we went to chrome-plated, oversized valves
and haven’t had any warranty problems since!”
Lynaugh says the chrome-plated
valves conduct heat better than unplated valves and resist scuffing
like a new OEM valve. The standard oversized stems also make installation
easy because all that’s needed is one pass with a reamer through
the guide to size the hole.
Fred Calouette of Cal
Grinding Inc., Escanaba, MI, supplies Cloverland with its chrome-plated
valves. Calouette says ease of installation, better durability
and service are just a few of the many reasons why his customers,
large and small, prefer the +.008″ valve program. Cal Valves
are plated with .008_ of hard chrome (not just a flash coating
as is used on many OEM valves), and finished to OEM specs plus
.008″.
“Our +.008″
oversized valve program really simplifies installation because
all you need is one reamer for each basic valve size,” said
Calouette. “The reason for this is we finish grind all valves
to one common size, be it a 7mm +.008″, 5/16 +.008″,
and so on. All valves are ready to install and do not require
any additional machining.”
Calouette says chrome
plating makes a valve more durable, and at the same time improves
its wear characteristics five to 10 times over an unplated valve.
Most OEM valves are chrome plated to prevent galling and scuffing
during dry starts. Ford, for example, has specified chrome stems
on both intake and exhaust valves since 1985. Chrome prevents
the stem from galling when cast iron guides are used, and it helps
prevent positive valve seal wear on intake valves.
When used valves are
salvaged and the stems are reground, grinding removes the chrome
flashing. This means a reground valve must be used with either
a bronze liner or guide, or replated to restore the original scuff
protection if used with a cast iron guide.
Gene Hailey, vice president
of tech services at Enginetech Inc., Carrollton, TX, says his
company offers a .015″ oversize “Rebuilders” valve
for restoring worn valve guides. “The larger oversize can
compensate for greater wear in the guides,” said Hailey.
“The stem diameters have also been consolidated to a common
size so only one reamer is needed, regardless of the application.
You can use the same reamer on a Ford, GM or Chrysler head.”
Hailey says his exhaust
valves are made of 21-4N stainless, and the intakes are HNV3 alloy.
All valves have a hardened tip, a .030″ oversized head (to
compensate for seat wear and refinishing), and a triple chrome
flashing on the stem .002″, thick. The chrome plating makes
the valves more scuff and wear resistant when used in cast iron
guides. All valves also follow the OEM design with respect to
taper. If an OEM exhaust valve is tapered (which many GM and stelite
diesel valves are), so is the Rebuilders valve. Intake valves
are .0005_ larger than exhaust valves.
John Brehm, shop manager
at Densmore Engines in Fresno, CA, says Densmore has been using
the Rebuilders valve for eight years with excellent results. “We
use all new valves in every head we remanufacture, including our
Ultra heads which we warranty for 30 months or 50,000 miles. So
we usually go with an oversized valve unless a valve isn’t available,
in which case we reline or replace the guide and use a new standard
sized valve.”
Brehm says he uses the
oversized valves mostly in cast iron heads, preferring to replace
the guides in aluminum heads.
Honing guides
When guides are reconditioned
by reaming to oversize (or knurling), the passage of a reamer
through the guide fractures the metal leaving microscopic pullouts,
tears and a relatively rough surface. This less than ideal bearing
surface will not wear as well as one that’s been honed. So honing
is usually recommended to smooth the guide bore by knocking the
peaks off the ridges left by the reamer. This produces a superior
bearing surface that will retain oil better and last longer than
an unhoned guide.
Even new guides can benefit
from honing. New guides are often rather rough. Honing provides
a more uniform surface finish which will reduce stem and guide
wear, and generally extend the life of the guides. This includes
cast iron guides as well as bronze guides.
Guide replacement
On aluminum or cast iron
heads with nonintegral guides, worn guides are often replaced.
Pressing out the old guides and installing new ones can be difficult
with some aluminum heads where the interference fit is considerable.
Cracking the head or galling the guide hole is always a risk.
One recommendation here
is to preheat the heads in an oven prior to guide removal and
to lubricate dry liners before driving them out. The head should
also be preheated before the new guides are installed. Chilling
the replacement guides can reduce the amount of interference during
installation. Lubricant also helps prevent galling. With tapered
guides, care must be taken to install them from the right side.
Most wet guides are tapered, and also require sealer to prevent
leaks.
Replacement guides come
in various alloys and varieties including bronze, cast iron and
powdered metal. Phosphor/bronze, silicon/aluminum/bronze and manganese/bronze
are generally more expensive than cast iron but are harder and
usually provide superior wear resistance.
With leaded gasoline,
bronze guides typically lasted three to five times longer than
cast iron. But with today’s unleaded gasoline the difference in
longevity between cast iron and bronze isn’t as great according
to one supplier of bronze guides. However, bronze still provides
superior heat transfer, resists seizing and can handle closer
tolerances (which improves valve life and reduces oil consumption).
That’s why thick wall bronze guides are preferred by many performance
shops.
