To measure the wall thickness of a main or rod bearing correctly thelocation on the bearing that you measure and the tool you use willdetermine the level of accuracy of the measurement. A micrometer with aball anvil must be used to fit the curvature of the bearing I.D., asshown in Figure 1. Standard flat anvil micrometer, shown in Figure 2 will bridge across the curved I.D. and give false readings.
Micrometers with pointed or blade anvils and dial calipers should notbe used as they can give false readings and damage the bearing surface.Obviously using an accurate, recently calibrated micrometer is a mustfor reliable measurement.
Just as important as using the correct tool is the location on thebearing that is measured. Crankshaft bearings should be measured 90° tothe parting line. By design most crankshaft bearings have an eccentricdesign, which means the wall thickness at the parting line is not equalto that at the 90° centerline.
The purpose of the eccentric design isto compensate for casting distortion during operation, create a “wedge”affect that promotes the formation of oil film, and for slighttolerance differences of the jointing components. To determine theamount of eccentricity, measure the bearing at the centerline and thenat approximately 3/8” above the parting line since some bearings mayalso have an additional “relief” near the parting ends.
The amount ofeccentric dimension and the presence of a relief cut vary by enginemake and/or model. The centerline measurement determines the amount ofclearance between the bearing and crankshaft and is the commondimension given when wall specifications are stated.
–Tech Tip courtesy of IPD (www.ipdparts.com)
Engine Builder Staff
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