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 TM 55-1905-217-34
(7) Minimum thickness of a worn standard main bearing shell is .153 inch and, if any shells are thinner
than this dimension, all shells must be discarded and replaced with new shells. A new bearing shell has a thickness of
.1548 inch to .1553 inch. In addition to this thickness measurement, the clearance between main bearings and crankshaft
journals should be checked. This clearance may be determined with the crankshaft in place by means of a soft plastic
measuring strip, which is squeezed between journal and bearing, or, with the crankshaft removed, by measuring 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. If the clearance between any crankshaft main bearing journal and its bearing shells
exceeds .006 inch, all bearing shells must be discarded and replaced with new shells. This clearance is .0014 inch to
.0044 inch when new parts are used.
(8) Bearing shells when in place have .001 inch larger diameter at the parting line than 90 o from the
parting line.
NOTE
The two shells do not form a true circle when not installed and when measured
for inside diameter should be installed in the cylinder block with caps bolted in
place (crankshaft removed).
(9) The two halves of the shells have a squeeze fit in the main bearing bore, and must be tight when the
cap is drawn down. This "crush" assures a tight, uniform contact between the bearing shell and seat. Bearing shells that
do not have sufficient crush will not have uniform seat contact, as shown by shiny spots on the bearing shell backs, and
should be replaced.
(10) When main bearing replacement is necessary, it is very important that the crankshaft journals be
thoroughly inspected before new replacement bearings are installed. Very often, after prolonged engine operation, a ridge
is formed on the circumference of the crankshaft journals in line with the journal oil holes. This ridge must not exceed
.0002 inch and, if it is not removed before new bearings are in- stalled, then, during engine operation, localized high unit
pressures in the center area of the bearing shell will cause pitting of the bearing surface. Bearings that have failed may
result in bending fatigue and resultant cracks in the crankshaft.
(11) The crankshaft journals may be inspected for scoring, over-heating or wear without removing the
crankshaft. To measure journal diameters, however, removal of the crankshaft is necessary.
(12) The crankshaft thrust washers, located at the rear main bearing consist of two pieces on each side of
the bearing cap. Excessive end play from use of an improper flywheel or improper .clutch adjustment can be contributory
factors to excessive wear on the thrust washers. If the washers have become scored or otherwise damaged, or if the
crankshaft end play is excessive, they must be replaced. The specified crankshaft end play, with new thrust washers, is
.004 inch to .011 inch. The maximum allowable end play with used washers is .018 inch.
d. Installation (Crankshaft in Place).
(1) Make sure all of the parts are clean. Then apply clean engine oil to the crankshaft journals and install
the main bearing shells by reversing the sequence of operations given for removal.
(2) Upper and lower main bearing shells are not alike; the upper shell is grooved and drilled for lubrication-
-the lower shell is not. Be sure to install the grooved and drilled shells in the cylinder block and the plain shells in the
bearing caps. Used bearing shells must be reinstalled on the same journal from which they were moved.
(a) When installing the upper main bearing shells with the crankshaft in place, start the plain
end of the shell around the crankshaft journal so that, when the shell is in place, the tank will fit into the
groove in the bearing sup- port.
(b) Assemble the crankshaft thrust washers before installing the rear main bearing cap. Clean
both halves of each thrust washer carefully, removing any burrs from the washers and the bearing cap--
the slightest burr or particle of dirt may decrease the clearance between the washers and the crankshaft
beyond specified limits.
(c) Slide the upper halves of the thrust washers in place in the grooves as shown in figure 3-85;
then assemble the lower halves over the dowel pins in the bearing cap.
NOTE
Main bearing caps are bored in position and marked 1, 2, 3, etc. They must be
replaced in their original positions with the marked side of each cap toward the blower
side of the cylinder block.
(d) With the lower bearing shells installed in the bearing caps, lubricate the bolt threads and the
bolt head contact areas with SAE 30 engine oil. Install the caps and draw the bolts up snug. Then rap
the caps sharply to seat them properly and uniformly tighten the bolts to torque specified in table 4,
chapter one. Tighten nuts and bolts to the high side of the torque specification, but do not exceed the
limit.
3-119
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