Crankshaft bearings should always be replaced when you are rebuilding an engine because the bearings are 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 should always be 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.
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 embeddability, which may or may not be a good thing depending on the size of the abrasive particles and the thickness of the bearing material. Trimetal copper/lead bearings usually provide better embeddability than harder bimetal aluminum bearings.
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 620 degrees can melt away the lead in trimetal copper/lead bearings and those with babbitt overlays. Because copper does not melt until 1,980 degrees, 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. If runout exceeds limits (the greater the shaft diameter, the greater the maximum amount of allowable runout), the crank must be straightened or replaced.
Main bore alignment can be checked by inserting a bar about .001 in. smaller in diameter than the main bores through the block with the main caps installed and torqued. If the bar does not 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. 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 in. 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 90 degrees 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 if it exists. Though the traditional rule of thumb says up to .001 in. of journal variation is acceptable, many engines cannot tolerate more than .0002 to .0005 in. 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 in., but nowadays it ranges from .0003 to .0005 in. for journals two inches or larger in diameter.
The journal diameter itself should be within .001 in. of its original dimensions, or within .001 in. of standard regrind dimensions for proper oil clearances with a replacement bearing. If a journal has been previously reground, there is usually a machinists 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. If using traditional polishing equipment, he recommends 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 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 do not recommend an unfavorable/favorable polish. We recommend favorable only. A 30 second polish using our 15 micron tape will produce journal finishes in the 3 to 6 microinch 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.
Proper clearances are another factor that are extremely important bearing longevity and oil pressure. Crankshaft bearings generally need at least a .0001 inch 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 inch per inch of diameter of the crankshaft journal) can allow a drop in oil pressure that can adversely effect 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 the engine rebuilder. You may want closer tolerances to maximize oil pressure if you plan to use a lower viscosity motor oil such as 5W-20, or you may want to run a heavier racing oil such as 20W-50, in which case you will need looser bearing clearances. Thinner oils reduce friction and improve fuel economy but also require closer bearing clearances to maintain good oil pressure.
One large production engine rebuilder says they try to build all their passenger car and light truck engines with about .001 to .002 inch clearance in the main and rod bearings. This compares to as much as .004 inch of clearance that may have been present in the OEM engine. But on some engines, such as the General Motors 173, more than .0015 inch of clearance can result in noise problems.
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 0.00013 to 0.0005 inches 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 percent 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. 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 is not our goal to tell rebuilders which bearings are best or to say when there is too much variation in oil pressure or oil flow to call a bearing 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.
At the original equipment level, the use of aluminum main and rod bearings is now universal for a variety of reasons. One is that aluminum bearings are less expensive to manufacturer 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 have changed with respect to aluminum versus copper/lead," said Federal-Mogul's Ron Thompson. "Most of the original equipment manufacturers are using aluminum bearings, as are a growing number of rebuilders in the aftermarket. Many people have switched 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 which helps resist seizure and actually polishes 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 says that 95 percent 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. It is 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 in.) coating of babbitt. King's aluminum alloy bearings, by comparison, use just two layers, a .012 to .015 inch thick layer of their Alecular alloy over the steel shell. Pavelick says this provides greater conformability as well as better embedability for microparticles larger than .0004 inch 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 their aluminum alloy is over 1,100 degrees F, which is almost three times as high as 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 have stayed with a traditional trimetal copper/lead bearing because thatis what the aftermarket wants. 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 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" said Worthington.
Gene Hailey, vice president of technical services at Enginetech in Carrolton, TX, said his company is looking at aluminum bearings but for now is sticking with copper/lead because that is what everybody wants.
"Our main concerns with aluminum are its load carrying ability and embedability. 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 is not 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 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.
Many engine builders continue to prefer copper/lead replacement bearings, especially for older engines and performance engines, although many ae now using aluminum replacement bearings for late model engines. 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 percent 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 do not 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 are 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 have 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 in 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 he said you should be careful to avoid when installing engine bearings is failing to oil the threads on the main cap bolts. "If you do not oil the threads, the cap may not tighten all the way down leaving too much clearance in the bearings. We have 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," said Kluemper.