Some winter reading to digest in the cold months (65 here in Florida,
HAHAHAHAHA!)
-Aron-
----- Original Message -----
Ensuring Bearing Life, Larry Carley, Automotive
Rebuilder, July 1997
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.
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. 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 6200 F can melt away the lead in
copper/lead bearings and those with babbitt overlays. Because copper
doesn't melt until 1,9800 F, burned copper/lead bearings will
typically have a copper appearance instead of the normal dull gray
appearance. 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.
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 900 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.
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. 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." 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. 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." 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
1,1000 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." 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. 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. 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." 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."
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