TM 5-3810-300-24&P-3
Main Bearings 1.3.4
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 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 .0014" to .0044"
with new parts.
Before installing new replacement bearings, it is very important
Fig. 8. Crankshaft Thrust Washers in Place
to thoroughly inspect the crankshaft journals. Very often, after
prolonged engine operation, a ridge is formed on the
after dressing or regrinding the thrust surfaces, new standard
crankshaft journals in line with the journal oil holes. If this
size thrust washers do not hold the crankshaft end play within
ridge is not removed before the new bearings are installed,
the specified limits, it may be necessary to install oversize
then, during engine operation, localized high unit pressures in
thrust washers on one or both sides of the rear main bearing.
the center area of the bearing shell will cause pitting of the
A new standard size thrust washer is .1190" to .1220" thick.
bearing surface. Also, damaged bearings may cause bending
Thrust washers are available in .005" and .010" oversize.
fatigue and resultant cracks in the crankshaft. Refer to
Section 1.3 under Crankshaft Inspection for removal of ridges
Install Main Bearing Shells (Crankshaft in Place)
and inspection of the crankshaft.
Make sure all of the parts are clean. Then apply clean engine
oil to each crankshaft journal and install the upper main
Do not replace one main bearing shell alone. If one shell
bearing shells by reversing the sequence of operations given
requires replacement, install both new upper and lower
for removal.
bearing shells. Also, if a new or reground crankshaft is to be
used, install all new bearing shells.
The upper and lower main bearing shells are not alike; the
upper bearing shell is grooved and drilled for lubrication - the
Bearing shells are available in .010", .020" and .030"
lower bearing shell is not. Be sure to install the grooved and
undersize for service with reground crankshafts.
drilled bearing shells in the cylinder block and the plain
bearing shells in the bearing caps, otherwise the oil flow to the
To determine the size bearings required, refer to Crankshaft
bearings and to the upper end of the connecting rods will be
Grinding in Section 1.3. Bearings which are .002" undersize
blocked off. Used bearing shells must be reinstalled on the
are available to compensate for slight journal wear where it is
same journal from which they were removed.
unnecessary to regrind the crankshaft.
1. When installing an upper main bearing shell with the
crankshaft in place, start the plain end of the bearing
NOTE: Bearing shells are NOT reworkable from
shell around the crankshaft journal so that, when the
one undersize
to
another
under
any
bearing is in place, the tang will fit into the groove in
circumstances.
the bearing support.
Inspect the crankshaft thrust washers. If the washers are
scored or worn excessively or the crankshaft end play is
excessive, they must be replaced. Improper clutch adjustment
can contribute to excessive wear on the thrust washers.
Refer to Install
Inspect the crankshaft thrust surfaces.
Crankshaft
in
Section
1.3.
If,
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