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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 engine rebuilder. Some want closer tolerances to maximize oil pressure while others want looser tolerances to allow for machining variances and contaminants that often end up in the crankcase.

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 skf bearings  .0015 inch of clearance can result in noise problems. 

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."

"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 [engine bearings] as well as eccentricity in the main bearings."

"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.

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 koyo 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.

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