Engine Bearings Analysis

The end of the racing season has come upon most of us. Now, we enter the process of tearing down and rebuilding, which will include changing the bearings. Regardless of whether this is done by you or your engine builder, the bearings are one of the keys to the condition of the lower end of the motor. As such, the ability to read the bearings can give valuable information on what is happening in the lower end, as well as the quality of the machining, assembly, and oiling system.

This reading and analysis is equally as applicable to engines operating flawlessly as it is to blown motors. Engine failures related to bearings and the oiling system are normally the easiest to spot, but finding and correcting other potential problems before they occur can be even more valuable.

John Havel at Clevite Engine Parts, after researching bearing failures, has compiled a list of causes of premature bearing failures. Although they were not the only causes of these failures, they were determined to be the main factors. The second, third, and a good percentage of the first problem on the list occur during the engine assembly phase. Most of the rest of the dirt and most of the insufficient lubrication are attributed to the oiling and filtration systems. These problems account for over 60 percent of the prime reasons for bearing failure.

Once the engine is apart, analysis of the appearance of the bearings can begin. Dirt and other foreign material in the lining are a common occurrence.

Indications of this include dirt embedded in the surface and scratches on the surfaces. If not corrected, this condition leads directly to bearing failure. Dirt and foreign matter displaces the soft bearing material and pushes this material into "high spots." These high spots can become big enough to contact the crank journal, causing a rubbing action. This action will eventually lead to breakdown and cause a rupture of the bearing lining. Additionally, the foreign material may stick out enough to scratch or grind on the crank journal.

Causes for this problem include improper cleaning of the parts before assembly, dirt entering through the air intake, and wear on internal parts of the motor resulting in small fragments in the oil supply. After replacing the bearings, corrections can include grinding the journal surfaces, cleaning the components with strong detergents and hot water, (especially after grinding and honing operations) and close monitoring of the filters. Surface failure or surface fatigue is a condition caused by excessive loading or even running the bearing beyond its life span. Excessive loading can be caused by detonation. Stress cracks occur on the bearing surface because of metal fatigue. As surface cracks widen and deepen, other cracks occur near the metal backing.

When this continues, metal flaking occurs. Intermediate layer bearing failure occurs in a similar way because of tremendous loading, especially from detonation. These areas look like craters or canyons on the bearing surfaces. Corrections include careful examination of the journals and possibly grinding the crank.

Another condition causing surface fatigue (but in localized areas) is foreign particles on the bearing&8217s back. A piece of dirt, abrasives, or other metallic particles lodged behind the bearing during assembly will cause this condition. A piece of the previous bearing surface welding itself to the housing bore will also cause this. This piece of debris will actually deform the bearing toward the crank and cause overheating and eventual flaking. Careful preparation of the bearing saddles can prevent this from developing. As for corrections, a close inspection and measurement of the housing bore is in order; based on that, a reconditioning of the housing may be necessary.

Two conditions that look very similar are an out-of-round bore and excessive crush. In the case of an out-of-round bore, the bearings are excessively worn near the parting faces. In rod bearings, this normally results from operation at high rpm with high inertial loads where the rod bore will become oblong and because of improperly aligned or bent rods. In the main bearing case, thermal distortion (caused by over-torqued cylinder heads and intake manifolds) while the engine is hot will cause an out-of-round bore. The bore will deflect and force the parting ends of the bearing to close in on the crank. In this area, the oil clearances become reduced because the ends of the bearings tend to "pinch" on the crank. Without any oil, metal-to-metal contact occurs and, over time, the bearing fails.

Excessive crush looks basically the same, but is caused by improper machining of the bearing cap or the bearing cap being torqued too tight. With an excessive crush condition, the ends of the bearing, at the parting surfaces, will show large amounts of wear. This wear will sometimes get to the point where copper will show where no bearing wear is expected to be seen at all. In this instance, the excessive force crushing the bearings causes actual distortion of the bearing. After determining which of these problems is occurring, machining the caps and seating surfaces is in order.

The opposite of excessive crush is insufficient crush. Without sufficient crush there is not enough radial pressure between the bearing and the housing. This allows the bearing to shift in the seat and polishes the backside of the bearing. Surface welding can occur during this polishing process. This is caused most often by an under-torqued bearing cap, improper machining of the bearing seats, and bearing caps not seating properly because of dirt or burrs on the contact surface. When the bearing is free to move or shift, it cannot adequately transfer heat. This leads to overheating of the bearing surface, flaking, and eventual failure. The seating surfaces must be machined, careful measurements of the tolerances should be taken, and extra care should be exercised upon reassembly.

There are a few clues to understanding and diagnosing excessive and insufficient crush conditions. In normal conditions, the wear pattern on a used bearing will be across 2/3 to 3/4 of the bearing surface. In an insufficient crush condition, the wear pattern on the bearing will be across less of the bearing surface.

In the excessive crush condition, the wear pattern will be across more than 3/4 of the bearing surface with the largest deterioration at the parting surface area.

A condition most often seen when a custom or high-performance crankshaft is installed without a correction in the bearings is fillet ride. To increase the strength of the crankshaft, extra material is added to provide a larger radius between the journal and the counter weight to reduce stress in the component. The bearings must allow space for this extra material. If it doesn’t, the fillet of the crank will rub directly on the bearing. This will appear as excessive wear on the extreme edges of the bearings. The new bearings or the crank must be machined to provide the additional space. Many special high performance bearings are manufactured in this way for these applications.

A condition that results in a delamination of the bearing material is called a "hot short," where an excessive amount of heat builds up in the bearing. The lead in the lining of the bearing melts, resulting in large patches of bearing lining lifting from the surface. At first, this tends to look like a manufacturing problem. Any form of stress resulting in wiping and friction causing high bearing temperature is known to cause it.

Wiping is an early sign of future problems. This occurs when the crank begins to break through the oil film and contacts the bearing. This will result in a highly polished and slightly worn bearing surface. Uneven patterns in the bearings show alignment problems or a bent crankshaft or rods. Corrections for this include increasing oil pressure, reducing oil temperature, or switching to higher viscosity oil.

The last condition to mention is oil starvation. This can be seen as a more advanced stage of wiping, usually occurring first near the center of the bearing. The bearing will look very shiny or, in advanced cases, discolored, and will most probably have excessive wear all over. This condition is easily the most recognizable. Causes include insufficient oil clearance, plugged oil passages, blocked oil filter, incorrect oil for the application, incorrect oil pan design, incorrect placement of the oil pump pickup, or a malfunctioning oil pump.

To correct this problem, the incorrect, failed, or blocked part must first be diagnosed. Then, it becomes necessary to run a check of the affected parts to determine if they can be used again. Finally, a careful recheck of tolerances and clearances before reassembly should finish the preparation.

The bearings at the end of the season should look like they have little to almost no wear. If everything works together correctly, the bearings should have little or no contact with the crankshaft.

One of the best ways to extend bearing life is to run up the oil pressure before starting the motor every time. Too often, the ignition is on as soon as the starter begins cranking. The motor starts before the oil has reached the bearings and this is where a high percentage of total bearing wear occurs. This wear can greatly be reduced if the ignition switch was turned on after oil pressure is achieved.

These descriptions of failures and problems should provide insight into the workings of the lower-end of the race motor. Analysis of problems with oil starvation, metal fatigue, or abnormal crush should allow simple corrections. This will end up saving your racing budget for more important things.