One method of measuring oil clearance is to measure the diameter of the journal with a micrometer caliper. The diameter of the shaft is measured at several points around the circumference to determine the size and to check for roundness. Measuring each end of the bearing surface will determine the amount of taper, if any.
The inside of the bearing is measured with the cap bolted in place, using a telescoping gauge or an inside micrometer. The difference of these two measurements represents the clearance between the journal and the bearing.
An alternate method is the use of a plastic material which deforms or flattens between the journal and the bearing when the cap is drawn down to the proper tightness. The amount of increase in the width of the plastic material, as it flattens out, is then measured with a furnished gauge to determine the clearance between journal and bearing.
The oil pressure test will also disclose if there is excessive clearance between the inserts and the crankpins. The amount of diametral clearance on automobile crankshaft bearings is specified by the manufacturer. These dimensions should be followed. In the absence of specific instructions, it is customary to use a minimum of .0005 to .001 in. for small shafts and up to .0015 to .002 in. for a large shaft. Any clearance in excess of .005 in. on either main or rod bearings usually calls for the installation of new bearings.
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