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. CAUSES OF ENGINE BEARING FAILURE 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. Inspecting the bearings will often tell you why they failed. Don't waste your time installing new bearings until the underlying cause of the failure is identified and fixed. 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. CRANKSHAFT REFINISHING 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." ENGINE BEARING MISASSEMBLY 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. ENGINE BEARING CLEARANCES 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 u…

Fonte: AA1Car.com