TM 5-3895-346-14
MAIN BEARINGS
ENGINE OVERHAUL
CRANKSHAFT MAIN BEARINGS
The crankshaft main bearings shells (Fig. 1) are precision made and are replaceable without machining. They
consist of an upper bearing shell seated in each cylinder block main bearing support and a lower bearing shell
seated in each main bearing cap. The bearing shells are prevented from endwise or radial movement by a tang
at the parting line at one end of each bearing shell. The tangs on the lower bearing shells are off-center and the
tangs on the upper bearing shells are centered to aid correct installation.
On in-line engines, a 7/16-inch oil hole in the groove of each upper bearing shell, midway between the parting
lines, registers with a vertical oil passage in the cylinder block. Lubricating oil under pressure passes from the
cylinder block oil gallery by way of the bearing shells to the drilled passages in the crankshaft, then to the
connecting rods and connecting rod
bearings.
Figure 1. Main Bearing Shells, Bearing
Caps, and Crankshaft Thrust Washers -
In-Line Engines
The lower main bearing shells have no oil grooves; therefore, the upper and lower bearing shells must not be
interchanged.
Thrust washers (Fig. 1) on each side of the rear main bearing, absorb the crankshaft thrust. The lower halves of
the two-piece washers are doweled to the bearing cap; the upper halves are not doweled.
Main bearing trouble is ordinarily indicated by low or no oil pressure. All of the main bearing load is carried on
the lower bearings; therefore, wear will occur on the lower bearing shells first. The condition of the lower
bearing shells may be observed by removing the main bearing caps.
If main bearing trouble is suspected, remove the oil pan, then remove the main bearing caps, one at a time (as
outlined below), and examine the bearing shells.
Remove Main Bearing Shells (Crankshaft in Place)
The bearing caps are numbered 1, 2, 3, etc., indicating their respective positions and, when removed, must
always be reinstalled in their original position.
All crankshaft main bearing journals, except the rear journal, are drilled for an oil passage. Therefore, the
procedure for removing the upper bear- ing shells with the crankshaft in place is somewhat different on the
drilled journals than on the rear journal.
Remove the main bearing shells as follows:
1.
Drain and remove the oil pan to expose the main bearing caps.
2.
Remove the oil pump and the oil inlet pipe and screen assembly.
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