The process of tearing down and rebuilding an engine, which should include changing all the bearings, needs to have an option for reviewing bearing damage. 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, however finding and correcting other potential problems before they occur can be even more valuable.

John Havel at Clevite Engine Parts, after researching many 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’s 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.

We'll take a look at fillet rides, hot shorts, wiping and oil starvation in the next issue LubeTalk.

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