Connect with us

Features

Ensuring Bearing Life: Crankshaft Bearings Are Always Replaced When Rebuilding An Engine

Advertisement

Click Here to Read More
Advertisement

Crankshaft bearings are always replaced when rebuilding an engine
because they’re a wear component. Heat, pressure, chemical attack,
abrasion and loss of lubrication can all contribute to deterioration
of the bearings. Consequently, when an engine is rebuilt new bearings
are always installed.
"Reading" the old bearings can
reveal a great deal about conditions that may have contributed
to their demise. All bearings will show some degree of wear. A
close examination may reveal some scoring or wiping, dirt or other
debris embedded in the surface of the bearings, or pitting or
flaking. But when one or more crankshaft bearings are found to
be damaged or show unusual or uneven wear, it typically indicates
other problems that need correcting – problems that if left uncorrected
may cause the replacement bearings to suffer the same fate.

Advertisement

Causes of bearing failures

Dirt contamination often causes premature bearing failure. When
dirt or other abrasives find their way between the crankshaft
journal and bearing, it can become embedded in the soft bearing
material. The softer the bearing material, the greater the embedability
– which may or may not be a good thing depending on the size of
the abrasive particles and the thickness of the bearing material.

If a particle is small and becomes deeply embedded in a relatively
soft bearing material, it may cause no damage to the crankshaft
journal. But if it displaces bearing material around itself or
protrudes above the bearing surface, it can score the crankshaft.

Advertisement

Heat is another factor that accelerates bearing wear and may lead
to failure if the bearings get hot enough. Bearings are primarily
cooled by oil flow between the bearing and journal. Anything that
disrupts or reduces the flow of oil not only raises bearing temperatures,
but also increases the risk of scoring or wiping the bearing.

Conditions that can reduce oil flow and cause the bearings to
run hot include a worn oil pump, restricted oil pickup screen,
internal oil leaks, a low oil level in the crankcase, aerated
oil (oil level too high), fuel diluted oil from excessive blowby,
or coolant contaminated oil from internal coolant leaks.
Temperatures
in excess of 620° F can melt away the lead in copper/lead
bearings and those with babbitt overlays. Because copper doesn’t
melt until 1,980° F, burned copper/lead bearings will typically
have a copper appearance instead of the normal dull gray appearance.

Advertisement

Misalignment is another condition that can accelerate bearing
wear. If the center main bearings are worn more than the ones
towards either end of the crankshaft, the crankshaft may be bent
or the main bores may be out of alignment.
The straightness of
the crankshaft can be checked by placing the crank on V-blocks,
positioning a dial indicator on the center journal and watching
the indicator as the crank is turned one complete revolution.
The greater the shaft diameter, the greater the maximum amount
of allowable runout. If runout exceeds limits, the crank must
be straightened or replaced.
Main bore alignment can be checked
by inserting a bar about .001" smaller in diameter than the
main bores through the block with the main caps installed and
torqued. If the bar doesn’t turn easily, the block needs to be
align bored. Alignment can also be checked with a straight edge
and feeler gauge. A deviation of more than .0015" in any
bore calls for align boring. Line boring must also be done if
a main cap is replaced.
The concentricity of the main bores is
also important, and should be within .0015". If not, reboring
will be necessary to install bearings with oversized outside diameters.

Advertisement

Connecting rods with elongated big end bores can cause similar
problems. If the rod bearings show a diagonal or uneven wear pattern,
it usually means the rod is twisted. Rods with elongated crank
journal bores or twist must be reconditioned or replaced. On some
newer engines, such as Ford’s 4.6L V8 with powder metal rods and
"cracked" caps, rods with elongated bores cannot be
reconditioned by grinding the caps because the caps do not have
a machined mating surface. So the big end bores must be cut to
accept bearings with oversized outside diameters if the bores
are stretched or out-of-round.
Uneven bearing wear due to misalignment
can also result if the crankshaft journals are not true. To check
the roundness of the crank journals, measure each journal’s diameter
at either bottom or top dead center and again at 90° either
way. Rod journals typically experience the most wear at top dead
center.
Comparing diameters at the two different positions should
reveal any out-of-roundness that exists. Though the traditional
rule of thumb says up to .001" of journal variation is acceptable,
many of today’s engines can’t tolerate more than .0002" to
0005" of out-of-roundness.
To check for taper wear on the
journals (one end worn more than the other), barrel wear (ends
worn more than the center) or hourglass wear (center worn more
than the middle), measure the journal diameter at the center and
both ends. Again, the generally accepted limit for taper wear
has usually been up to .001", but nowadays it ranges from
.0003" to .005" for journals two inches or larger in
diameter.
The journal diameter itself should be within .001"
of its original dimensions, or within .001" of standard regrind
dimensions for proper oil clearances with a replacement bearing.
If a journal has been previously reground, there’s usually a machinist’s
mark stamped by the journal. A 10, 20 or 30 would indicate the
crank has already been ground to undersize, and that further regrinding
may be out of the question depending on how badly the crank is
worn.
Any crankshaft that does not meet all of the above criteria,
or has grooves, scratches, pitting or galling on the surface,
must be ground undersize to restore the journals. The journals
should also be polished to provide a smooth surface (10 microinches
or less is recommended), and the oil holes chamfered to promote
good oil flow to the bearings.
Ron Thompson, a bearing engineer
at Federal-Mogul, says improper crankshaft finish can be especially
hard on bearings. He recommends grinding the crank in the favorable
direction, then a two-step polishing procedure to achieve an optimum
finish. First, the journals should be polished in the "unfavorable"
direction (opposite the direction of rotation) with #280 grit,
then finished in the "favorable" direction (same direction
as rotation) with #320 grit.
Steve Williams of K-Line Industries,
Holland, MI, says that the type of polishing procedure will vary
depending on the type of metal in the crankshaft and how it is
ground. "With our equipment, we don’t recommend an unfavorable/favorable
polish," said Williams. "We recommend favorable only.
A 30-second polish using our 15 micron tape will produce journal
finishes in the 3-6 micron range."
Misassembly can be another
cause of premature bearing failure. Common mistakes include installing
the wrong sized bearings (using standard size bearings on an undersize
crank or vice versa), installing the wrong half of a split bearing
as an upper (which blocks the oil supply hole and starves the
bearing for oil), getting too much or not enough crush because
main and/or rod caps are too tight or loose, forgetting to tighten
a main cap or rod bolt to specs, failing to clean parts thoroughly,
and getting dirt behind the bearing shell when the bearing is
installed.
Corrosion can also play a role in bearing failure.
Corrosion results when acids accumulate in the crankcase and attack
the bearings, causing pitting in the bearing surface. This is
more of a problem with heavy-duty diesel engines that use high
sulfur fuel rather than gasoline engines, but it can also happen
in gasoline engines if the oil is not changed often enough and
acids are allowed to accumulate in the crankcase. Other factors
that can contribute to acid buildup include a restricted or plugged
PCV system, engine operation during extremely cold or hot weather,
excessive crankcase blowby (worn rings or cylinders) or using
poor quality oil or fuel.
Babbitt and lead are more vulnerable
than aluminum to this type of corrosion, so for engine applications
where corrosion is a concern aluminum bearings may offer better
corrosion resistance.

Advertisement

Clearances

Proper clearances are another factor that are extremely important
for bearing longevity and oil pressure. Crankshaft bearings generally
need at least a .0001" thick oil film between themselves
and their journals to prevent metal-to-metal contact. This requires
assembly clearances that are loose enough so oil can flow into
the gap between the bearing and journal to form an oil wedge that
can support the crankshaft. The clearance must also be sufficient
to allow enough oil flow to cool the bearings. But the clearance
must not be too great, otherwise the oil will escape before it
can form a supporting wedge.
Excessive bearing clearances (more
than about .001" per inch of diameter of the crankshaft journal)
can allow a drop in oil pressure that can adversely affect lubrication
elsewhere in the engine, such as the camshaft and upper valvetrain.
Excessive clearances also increase engine noise and pounding,
which over time can lead to bearing fatigue and failure. Fatigued
bearings will typically be full of microscopic cracks and have
flaking material on the surface.
The amount of clearance between
the bearings and crank journals will obviously vary depending
on the application and the preferences of rebuilders and their
customers. Some may want closer tolerances to maximize oil pressure,
while others may want looser tolerances to allow for machining
variances and contaminants that often end up in the crankcase.

Advertisement

One large production engine rebuilder says his company tries to
build all its passenger car and light truck engines with about
.001" to .002" clearance in the main and rod bearings.
This compares to as much as .004" of clearance that may have
been present in the OEM engine. But on some engines, such as the
General Motors 2.8L V6 (173), more than .0015" of clearance
can result in noise problems.

Eccentricity

Most crankshaft bearings are designed with a certain amount of
"eccentricity" so oil can more easily form a wedge to
support the crankshaft. The shell is typically about .00013"
to .0005" thicker at the crown than the parting line. This
allows the oil to get under the crank as the crank starts to turn,
lifting it off the bearing so it can glide on a film of oil.
Increasing
the amount of eccentricity can increase oil flow for greater bearing
cooling and longevity, which is why many racing bearings have
extra eccentricity. But at low rpm, too much eccentricity may
cause a slight drop in oil pressure. Since many production engine
rebuilders test newly assembled engines on a simulator or dyno,
bearings with a high amount of eccentricity may give the false
impression that something is amiss because the oil pressure readings
may be lower than "normal."
Jerry Hammann of SIMTEST,
Canyon County, CA, says the engine testers that his company manufacturers,
which he says are used by about 80% of all the production engine
rebuilders in the U.S., checks oil pressure as the engine is spun
at low rpm."
We treat the engine as a group of orifices and
look at total oil flow," said Hammann. "Our machine
takes 180 oil pressure readings per revolution, then averages
the readings to show the total amount of variation per revolution.
At low rpm, you can see the variations in oil pressure due to
the rod bearings, as well as eccentricity in the main bearings."

Advertisement

Hammann says that as oil clearances increase, so does oil flow,
which allows a rebuilder to catch misassembly problems before
an engine leaves the shop. He also said that bearings with more
eccentricity will show a greater variation in oil pressure."

It’s not our goal to tell rebuilders which bearings are best,
or to say when there’s too much variation in oil pressure or oil
flow to determine if a bearing is good or bad. What we provide
is a means of controlling quality so rebuilders can set their
own standards and rebuild engines with greater consistency. If
you build 100 engines the same way, they should all test the same.

Advertisement

Hammann says his company worked with one bearing manufacturer
to develop bearings with less eccentricity so the bearings would
give better readings on their test equipment.

Bearing materials

At the original equipment level, the use of aluminum main and
rod bearings is growing for a variety of reasons. One is that
aluminum bearings are less expensive to manufacture than bimetal
or trimetal copper/lead bearings. Switching to aluminum also gets
rid of lead, which is an environmental concern for manufacturers.
But there are many other reasons, too."
Federal-Mogul provides
both copper/lead and aluminum bearings. But perceptions are changing
with respect to aluminum versus copper/lead," said Federal-Mogul’s
Thompson. "Most of the original equipment manufacturers are
going to aluminum bearings, as are a growing number of rebuilders
in the aftermarket. Many people are switching to aluminum because
it provides improved durability and better control over tolerances."

Advertisement

Overplated bearings tend to trap and hold dirt that can score
the crankshaft. But aluminum bearings tend to flush out debris
rather than hold it. Aluminum bearing alloys also contain silicone
particles which help resist seizure and actually polish the crank.
"I can see the day when traditional copper/lead bearings
may only be used for racing," said Thompson.
Ed Pavelick
at King Engine Bearings, Cedar Grove, NJ, says that 95% of his
company’s aftermarket bearings are now aluminum. "We made
the decision to go to aluminum several years ago when we developed
our exclusive Alecular bearing material," said Pavelick.
"It’s an aluminum alloy that contains tin, copper and several
other elements. We think it provides the kind of longevity that
today’s market demands."
Pavelick said that traditional trimetal
rod and main bearings have a three-layer construction. The steel
backing plate is covered with a layer of copper/lead overlayed
with a thin (.0005" to .0008") coating of babbitt. King’s
aluminum alloy bearings, by comparison, use just two layers, a
.012" to .015" thick layer of its Alecular alloy over
the steel shell. Pavelick says this provides greater conformability
as well as better embedability for microparticles larger than
.0004" in diameter, which are most responsible for scoring
cranks and tearing or weakening thin babbitt overlays.
Another
plus with aluminum, says Pavelick, is that it has greater temperature
resistance than copper/lead. The melting point of its aluminum
alloy is more than 1,100° F, which is almost three times
as high as traditional babbitt. This provides added protection
against localized overheating due to detonation, overloading,
misalignment and similar conditions.
Bob Anderson, engine bearing
team leader at AE Clevite Engine Parts, Ann Arbor, MI, says that
although many OEMs are using aluminum, trimetal copper/lead bearings
are still the preferred bearing material for the aftermarket."

Advertisement

We’ve stayed with a traditional trimetal copper/lead bearing because
that’s what the aftermarket wants," said Anderson. "We
believe trimetal copper/lead offers the best combination of strength,
surface action and embedability. Copper/lead can carry 12,000
pounds per square inch (psi) versus about 7,000 to 8,000 psi for
aluminum, it can handle less than perfect conditions, and is a
more forgiving material than aluminum in a typical aftermarket
application."
Chris Worthington, a bearing engineer at ACL
Automotive America, Inc., Tucker, GA, said that although the Japanese
are using a lot of aluminum bearings, Ford and General Motors
are still using copper/lead bearings in many of their engines
because of the high strength of the material. As for the aftermarket,
most of it remains copper/lead for domestic engines and a mix
of copper/lead and aluminum bearings for import applications.
He said the high performance market is almost all copper/lead
bearings."
Although most rebuilders still prefer copper/lead
because it is a more forgiving material, others prefer to use
the same bearing material as the original bearings. So we have
both aluminum and copper/lead bearings," explained Worthington.

Advertisement

Gene Hailey, vice president of technical services at Enginetech,
Inc., Carrolton, TX, said his company is looking at aluminum bearings,
but for now is sticking with copper/lead because that’s what everybody
wants."
Our main concerns with aluminum are its load carrying
ability and embedability," said Hailey. "Oil filters
typically only screen out particles that are about seven microns
and larger in size, so the bearing material must be able to handle
the dirt that gets through."
As for the environmental issues
associated with lead, it is mostly a concern for bearing manufacturers
not end users. The government isn’t concerned about the amount
of lead in used engine oil because the amount is usually insignificant.

Advertisement

One change that Hailey said has been made in Enginetech’s bearings
is to reduce the amount of eccentricity and crush relief. Although
greater eccentricity increases oil flow to improve bearing cooling
and longevity, it also causes a slight drop in oil pressure readings
on engine test equipment used by many large rebuilders. So to
produce more traditional test results, eccentricity was reduced.

Bearing selection

Most rebuilders continue to prefer copper/lead replacement bearings.
Jerry Miller of Crankshaft Supply, Minneapolis, MN, says he recommends
trimetal copper/lead bearings because the material offers good
conformability, embedability and longevity. "About 90% of
the crankshaft kits we sell are sold with AE Clevite "P"
or Federal-Mogul "CP" bearings. We also sell kits with
ACL and Enginetech bearings, too."
The biggest problem we
see with any type of bearing are people who replace a crankshaft
but don’t clean the engine. Debris gets into the bearings and
wipes out the bearings and crank," said Miller.
Larry Erickson
of Crankshaft Rebuilders in Sandford, FL, says is company sells
about 100,000 crankshaft kits annually primarily to retailers.
"We use Federal-Mogul, AE Clevite, ACL, King and Enginetech
bearings. In most cases, we would rather go with a copper/lead
bearing because it is more forgiving in a dirty environment. But
we’re also using a lot of aluminum bearing these days, too."

Advertisement

Almost half of the warranty problems we see are worn flange bearings
that have failed at short mileages of 300 to 500 miles. We’ve
found that the underlying cause in almost every case is a ballooned
torque converter. Nine out of ten of the vehicles have a trailer
hitch. When pump pressure inside the automatic transmission exceeds
the preset pressure, it diverts the bypass pressure through the
oil cooler lines. If the lines are clogged, pressure can build
up inside the torque converter causing it to balloon and push
forward on the crankshaft," said Erickson.
John Kluemper,
quality control manager of gasoline engines at Jasper Engines,
Jasper, IN, says Jasper uses both types of bearing materials.
"We use mostly Federal-Mogul bearings, some of which are
trimetal copper/lead and others are aluminum. Both kinds work
fine, though we think trimetal copper/lead can handle more dirt
and debris in a dirty operating environment."
Kluemper says
Jasper live tests each engine after it has been rebuilt. He says
too much eccentricity in the bearings can cause an engine to lose
oil pressure. "Oil pressure can vary up to two pounds at
hot idle depending on the amount of eccentricity in the bearings,
so we prefer bearings that have less rather than more eccentricity.
We also try to maintain minimum oil clearances of about .001 to
.002 inches on most engines to minimize noise and maximize oil
pressure."
One mistake Kluemper said rebuilders should be
careful to avoid when installing bearings is failing to oil the
threads on the main cap bolts. "If you don’t oil the threads,
the cap may not tighten all the way down leaving too much clearance
in the bearings," Kluemper said. "We’ve seen caps installed
with dry threads that had .0045 inch of clearance. When the caps
were reinstalled with oiled threads, the clearance decreased to
.002 inches."

Advertisement

POPULAR POSTS

Engine of the Week

2,000+ HP Capable GT500 Short Block

Engine of the Week

1,004-HP 632 cid Big Block Chevy Engine

Engine of the Week

635 cid Big Block Chevy NA Engine

Diesel of the Week

Triple-Turbo Billet 5.9L 12-Valve Cummins Engine

Connect
Engine Builder Magazine