DETROIT DIESEL 53
1.3.4 Main Bearings
TABLE 1
clearance between the main bearings and the
crankshaft journals. This clearance may be determined
Fig. 7 Main Bearing Measurements
with the crankshaft in place by means of a soft plastic
measuring strip which is squeezed between the journal
The bearing shell thickness will be the total thickness
and the bearing (refer to Shop Notes in Section 1.0).
of the steel ball and the bearing shell. less the
With the crankshaft removed, measure the outside
diameter of the ball. This is the only practical method
diameter of the crankshaft main bearing journals and
for measuring the bearing thickness. unless a special
the inside diameter of the main bearing shells when
installed in place with the proper torque on the
miminum thickness of a worn standard main bearing
bearing cap bolts. When installed, the bearing shells
shell is .l230" and, if any of the bearing shells are
are .001" larger in diameter at the parting line than
thinner than this dimension, replace all of the bearing
shells. A new standard bearing shell has a thickness of
.1245" to .I250"(in-line engine),
The bearing shells do not form a true circle when not
Refer to Table 1.
installed. When installed, the bearing shells have a
squeeze fit in the main bearing bore and must be tight
In addition to the thickness measurement, check the
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 0100" 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
Fig. 8 - Measuring Thickness of Bearing Shell
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