TM 5-3810-300-24&P-3
1.3.4
CRANKSHAFT MAIN BEARINGS
The lower main bearing shells have no oil grooves;
The crankshaft main bearings shells (Fig. 1) are
therefore, the upper and lower bearing shells must not
precision made and are replaceable without machining.
be interchanged.
They consist of an upper bearing shell seated in each
cylinder block main bearing support and a lower bearing
Thrust washers (Fig. 1), on each side of the rear main
shell seated in each main bearing cap. The bearing
bearing, absorb the crankshaft thrust. The lower halves
shells are prevented from endwise or radial movement
of the two-piece washers are doweled to the bearing cap:
by a tang at the parting line at one end of each bearing
the upper halves are not doweled.
shell. The tangs on the lower bearing shells are off-
center and the tangs on the upper bearing shells are
Main bearing trouble is ordinarily indicated by low or no
centered to aid correct installation.
oil pressure. All of the main bearing load is carried on
the lower bearings; therefore, wear will occur on lower
Various types of bearings have been used. Currently,
bearing shells first. The condition of lower bearing shells
multiple layer copper-lead coplated or aluminum triplated
may be observed by removing the main bearing caps.
bearings are in use. These bearings have an inner
surface, called the matrix, of copper-lead or aluminum.
If main bearing trouble is suspected, remove the oil pan,
A thin deposit of babbitt is then plated onto the matrix.
then remove the main bearing caps, one at a time, as
outlined below and examine the bearing shells.
corrosion and scoring tendencies which, combined with
the material of the matrix, provides improved load
carrying characteristics. These bearings are identified by
Remove Main Bearing Shells (Crankshaft in Place)
the satin silver sheen of the babbitt when new and a dull
gray after being in service.
The bearing caps are numbered 1, 2, 3, etc., indicating
their respective positions and, when removed, must
The former copper-lead bearings had a copper color
always be reinstalled in their original position.
when new and turned very dark during engine operation.
All crankshaft main bearing journals, except the rear
An oil hole in the groove of each upper bearing shell,
journal, are drilled for an oil passage. Therefore, the
midway between the parting lines, registers with a
procedure for removing the upper bearing shells with the
vertical oil passage in the cylinder block. Lubricating oil,
crankshaft in place is somewhat different on the drilled
under pressure, passes from the cylinder block oil gallery
journals than on the rear journal.
by way of the bearing shells to the drilled passages in the
crankshaft, then to the connecting rods and connecting
Remove the main bearing shells as follows:
rod bearings.
1. Drain and remove the oil pan to expose the main
bearing caps.
Fig. 1. Main Bearing Shells, Bearing Caps
Fig. 2. Removing Main Bearing Cap
and Crankshaft Thrust Washers
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