Of all the things that can take your engine out, bearing failure is one of the most insidious. Car-oriented types tend to try to ignore bearing issues, compensating for them by using ever-thicker oil and an eye on the gauges instead of just replacing them as necessary. But bearing wear can have broad-ranging consequences not only to your engine, but to anything bolted to it.
Silver Shavings in the Oil
A certain amount of silver-like, metal sheen -- actually aluminum dust -- in the oil and on the dipstick is a result of normal bearing wear. But this dust should be just that; a powder fine enough that you cannot make out any individual grain of aluminum. But when bearings as a result of overheating or running out of oil, the bearings' aluminum outer face will typically shave away in fine splinters or ribbons. If you find splinters or ribbons of aluminum stuck to your dipstick or in the oil during an oil change, then you've got a bearing either gone or on its way out. A good mechanic will always cut the top off of his oil filter after each oil change to check the filter media for early signs of excess aluminum dust and shavings.
Copper Sheen in the Oil
Many bearings use three layers; a soft aluminum outer layer, a layer of copper in the middle and a steel backing plate. Such tri-metal bearings use copper, which is a softer but slicker metal than aluminum, to give the owner a last line of defense before the bearing wears down to the steel or fails completely. The copper layer will typically last one or two thousand miles, giving you an opportunity to detect imminent bearing failure before it kills your motor. Once you bearing gets down to the copper layer, the bearing will send out a red flag in the form of copper dust in the oil. If you see a ceppory sheen on your dipstick, inside the valve cover or in the oil, then you've got a bearing on the verge of failure.
Loss of Oil Pressure
Your oil pump flows a certain amount of fluid: say 20 gallons per minute. If you've got 20 little pressure leaks in the engine -- leaks like the hydraulic lifters, the rocker arms and the gaps between your engine bearings -- that each flow one gallon per minute, then you've got no oil pressure because oil's going out of the oil channels as fast as it comes in. So excess clearances between the bearings as a result of excess bearing wear will bleed off oil pressure, particularly at low rpm when the pump's spinning slowest. While several things can cause a loss of oil pressure at low rpm, this is one sign of severely worn bearings in the lower end.
Noise in the Engine
Most neophyte car guys have heard of rod knock, if not heard it themselves. Rod knock -- a steady hammering that increases linearly with rpm -- happens when excess clearances causes by bearing wear cause the rod big-ends to hammer against the crankshaft at the top and bottom of the piston stroke. Rod knock is a sure indicator of worn bearings. But there's another kind of engine noise that can indicate worn lower-end bearings, and that can happen in the valvetrain and lifters. Many engines have a "priority main" oiling system, meaning that oil pressure goes to the crankshaft and rods before it does anywhere else. Excess oil leakage at the crank and rods can starve the valvetrain of needed pressure, resulting in a seemingly innocuous lifter tap that actually indicates worn crank or rod bearings.
Worn Belts and Transmission Noise
Here's an interesting symptom that tends to slip the average mechanic's mind. Your main bearings have two kinds of bearing face; the regular bearing face that the the crankshaft rides on, and a "thrust bearing" surface in the engine block that keeps the transmission from moving back and forth. The thrust-bearing surface, which is vertical in the engine block, can wear just like the main bearing face. If there's enough room on the rod journals, worn thrust bearings can allow the crank to move forward or backward enough to cause excessive edge-wear on the belts, or backward enough to jam the torque converter into the transmission. The latter may cause transmission noise at the least, and at worst can break the transmission. The metal dust resulting from the force of the torque converter pressing back on the oil pump can clog the filter, potentially causing fluid pressure loss -- and clutch slippage -- in the transmission as a result of the clogged filter and damage to the pump.
- How to Rebuild Small-Block Chevy LT1/LT4 Engines; Mike Mavrigan
- Racing Engine Builder's Handbook; Tom Monroe