DETROIT DIESEL 53
Main Bearings 1.3.4
Tool J 4757, placed between the shell and a micrometer
2. Remove the oil pump intake pipe and screen
as illustrated in Fig.7.
will give an accurate
assembly.
measurement. The bearing shell thickness will be the
3. Remove one main bearing cap at a time and
total thickness of the steel ball and the bearing shell,
inspect the bearing shells as outlined under Inspection.
less the diameter of the ball. This is the only practical
Complete replacement of the bearing shells and
method for measuring the shell thickness, unless a
reinstallation of the bearing cap before removing
special micrometer is available for this purpose.
another bearing cap.
a. To remove all except the rear main
Minimum thickness of a worn standard main bearing
bearing shell, insert a 1/4" x 3/4" bolt, with
shell is .123" and, if any of the shells are thinner than
a 1/2" diameter and 1/16" thick head
this dimension, all of the shells must be replaced.
(made from a standard bolt), into the
crankshaft journal oil hole, then revolve
Check the clearance between the main bearings and the
the shaft to the right (clockwise) and roll
crankshaft journals. This clearance (Section 1.0) may
the bearing shell out of position. The
be determined with the crankshaft in place by means of
head of the bolt should not extend beyond
a soft plastic measuring strip which is squeezed
the outside diameter of the shell (Fig.3 ).
between the journal and the bearing, or, with the
b. The lower halves of the crankshaft thrust
crankshaft removed, by measuring the outside diameter
washers will be removed along with the
of the crankshaft main bearing journals and the inside
rear main bearing cap. Remove the
diameter of the main bearing shells when installed in
upper halves of the washers by pushing
place with the proper torque on the main bearing cap
on the ends of the washers with a small
bolts. If the clearance between any crankshaft main
rod, thus forcing them around and out of
bearing journal and its bearing shells exceed .006", all
the main bearing support.
of the bearing shells must be discarded and replaced
c. Remove the rear main bearing upper
with new shells.
shell by driving on the edge of t e bearing
shell with a small curved rod (Fig. 4 ) and
The two bearing shells do not form a true circle when
revolving the crankshaft at the same time,
not installed. Measure the inside diameter with the
thus rolling the shell from its position.
bearing shells installed in the cylinder block and the
caps bolted in place (crankshaft removed). The two
Inspection
halves of the shells have a squeeze fit in the main
Bearing failures may result from deterioration (acid
bearing bore, and must be tight when the cap is drawn
formation) or contamination of oil or loss of oil which
down. This crush assures a tight, uniform contact
results in scratching, etching, scoring or excessive wear.
between the bearing shell and seat. Bearing shells that
An analysis of the lubricating oil may be required to
do not have sufficient crush will not have uniform seat
determine if corrosive acid and 'sulphur are present
contact, as shown by shiny spots on the bearing shell
which cause acid etching, flaking and pitting. Bearing
backs, and must be replaced.
seizure may be due to low oil or no oil.
Before installing new replacement bearings, it is very
Check the oil filter elements and replace them if
important to thoroughly inspect the crankshaft journals.
necessary. Also, check the oil by-pass valve to make
Very often, after prolonged engine operation, a ridge is
sure it is operating freely.
formed on the crankshaft journals in line with the journal
oil holes. If this ridge is not removed before new
After removal, clean the bearings and inspect them for
bearings are installed, then, during engine operation,
scoring, pitting, flaking, etching and dirt grooving. If any
localized high unit pressures in the center area of the
of these defects are present, the bearings must be
bearing shell will cause pitting of the bearing surface.
discarded. The lower bearing shells, which carry the
Also, damaged bearings may cause bending fatigue and
load, will normally show signs of distress before the
resultant cracks in the crankshaft. See Section 1.3 under
upper shells do.
Crankshaft Inspection for removal of the ridge and
inspection of the crankshaft.
Inspect the backs of the bearing shells for bright spots
The crankshaft journals may be inspected for scoring,
which indicate they have been moving in the caps or the
over-heating or wear without removing the crankshaft.
cylinder block. If such spots are present, discard the
However, to measure the diameter of the journals, the
bearing shells.
crankshaft must be removed. Refer to Crankshaft
Inspection in Section 1.3.
Measure the thickness of the bearing shells at point "C",
90 from the parting line, as shown in Fig; 1972
Do not replace one main bearing shell alone. If one
General Motors Corp.
bearing shell requires replacement, install both new
1972 General Motors Corp.
February, 1997
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