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 TM 55-1905-220-14-4
3-41. CRANKSHAFT BEARINGS.
a. The crankshaft main bearings shells 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.
b. The bearing caps are numbered 1,2,3, etc. indicating their respective positions and, when removed,
must always be reinstalled in their original position.
c. An 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.
d. The lower main bearing shells have no oil grooves; therefore, the upper and lower bearing shells must
not be interchanged.
e. Thrust washers 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.
f. 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.
g. Bearing failures may result from deterioration (acid formation) or contamination of the oil or loss of oil.
An analysis of the lubricating oil may be required to determine if corrosive acid and sulphur are present which
cause acid etching, flaking and pitting. Bearing seizure may be due to low oil or no oil.
h. Check the oil filter elements and replace them if necessary. Also check the oil by-pass valve to make
sure it is operating freely.
3-873
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