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Every journal needs to be measured in four places.
Imagine the journal for a particular cylinder at bottom dead center. Imagine the orientation of the connecting rod at that point. Two of the diameters need to be parallel to that orientation of the rod, one at each end of the journal.
The other two need to be 90 degrees off of those first two; again at each end of the journal.
These four measurements allow you to evaluate for out of round and taper.
The maximum for taper or out of round is 0.0005 in.
Also, your journal needs to be visually inspected for scoring, pitting, or serious discoloration that manages to be indicative of some other issue. Certain of those issues can be polished out by a machine shop.
Plastigage is only a check, and I would never use that to determine the condition of an installed bearing set or to evaluate a crankshaft. It is no substitute for measuring. It is not very accurate and is pretty subjective in its interpretation.
The very best way to determine bearing clearance is to measure the OD [outside diameter] of the crank journals as described above, and then to measure the ID [inside diameter] of the installed bearings with the caps torqued. If you measure, you know.
The average person will not be very successful in measuring the ID of a bearing set that is torqued in place. Both the inside micrometer and the combination of a telescoping gauge and outside micrometer are very affected by feel and the experience level of the user.
A dial bore gauge is the most accurate way to measure the ID of a bearing set.
Practice measuring a particular journal, or even one location on a journal, until you get the very same measurement five times in a row, without cheating. Then do your measurements. Recheck yourself often.
Imagine the journal for a particular cylinder at bottom dead center. Imagine the orientation of the connecting rod at that point. Two of the diameters need to be parallel to that orientation of the rod, one at each end of the journal.
The other two need to be 90 degrees off of those first two; again at each end of the journal.
These four measurements allow you to evaluate for out of round and taper.
The maximum for taper or out of round is 0.0005 in.
Also, your journal needs to be visually inspected for scoring, pitting, or serious discoloration that manages to be indicative of some other issue. Certain of those issues can be polished out by a machine shop.
Plastigage is only a check, and I would never use that to determine the condition of an installed bearing set or to evaluate a crankshaft. It is no substitute for measuring. It is not very accurate and is pretty subjective in its interpretation.
The very best way to determine bearing clearance is to measure the OD [outside diameter] of the crank journals as described above, and then to measure the ID [inside diameter] of the installed bearings with the caps torqued. If you measure, you know.
The average person will not be very successful in measuring the ID of a bearing set that is torqued in place. Both the inside micrometer and the combination of a telescoping gauge and outside micrometer are very affected by feel and the experience level of the user.
A dial bore gauge is the most accurate way to measure the ID of a bearing set.
Practice measuring a particular journal, or even one location on a journal, until you get the very same measurement five times in a row, without cheating. Then do your measurements. Recheck yourself often.
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