1.3.4 Main Bearings
DETROIT DIESEL 53
Fig. 7 - Main Bearing Measurements
Fig. 8 - Measuring Thickness of Bearing Shell
The bearing shell thickness will be the total thickness of the steel ball and the bearing shell, less the diameter of the ball.
This is the only practical method for measuring the bearing thickness, unless a special micrometer is available for this
purpose. The minimum thickness of a worn standard main bearing shell is .1230"and, if any of the bearing shells are
thinner than this dimension, replace all of the bearing shells. A new standard bearing shell has a thickness of .1245 " to
.1250" (in-line engine), Refer to Table 1.
In addition to the thickness measurement, check the clearance between the main bearings and the crankshaft journals.
This clearance may be determined with the crankshaft in place by means of a soft plastic measuring strip which is
squeezed between the journal and the bearing (refer to Shop Notes in Section 1.0). With the crankshaft removed,
measure the outside diameter of the crankshaft main bearing journals and the inside diameter of the main bearing shells
when installed in place with the proper torque on the bearing cap bolts. When installed, the bearing shells are .001 "larger
in diameter at the parting line than 90° from the parting line.
The bearing shells do not form a true circle when not installed. When installed, the bearing shells have a squeeze fit in
the main bearing bore and must be tight when the bearing cap is drawn down. This crush assures a tight, uniform contact
between the bearing shell and bearing seat. Bearing shells that do not have sufficient crush will not have uniform seat
contact, as shown by shiny spots on the back, and must be replaced. If the clearance between any crankshaft journal and
its bearing shells exceeds .0060", all of the bearing shells must be discarded and replaced. This clearance is .0010" to
.0040" with new parts.
Before installing new replacement bearings, it is very important to thoroughly inspect the crankshaft journals. Very often,
after prolonged engine operation, a ridge is formed on the crankshaft journals in line with the journal oil holes. If this ridge
is not removed before the new bearings are installed, then, during engine operation, localized high unit pressures in the
center -area of the bearing shell will cause pitting of the bearing surface. Also, damaged bearings may cause bending
fatigue and resultant cracks in the