TM 5-3895-346-14
MAIN BEARINGS
Figure 5. Main Bearing Measurements
Figure 6. Measuring Thickness of
Bearing Shell
installed in place with the proper torque on the bearing cap bolts. When installed, the bearing shells are 0.001
inch larger in diameter at the parting line than 90from 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 0.0060 inch, all of the bearing shells
must be discarded and replaced. This clearance is 0.0010 to 0.0040 inch with new parts.
Before installing new replacement bear- ings, it is very important to thoroughly inspect the crankshaft journals.
Very often, after prolonged engine operation, a ridge is formed on the crankshaft journals in line with the journal
oilholes. If this ridge is not removed before the new bearings are installed, then, during engine operation,
localized high unit pressures in the center area of the bearing shell will cause pitting of the bearing surface.
Also, damaged bearings may cause bending fatigue and resultant cracks in the crankshaft. Refer to Crankshaft
Inspection for removal of ridges and inspection of the crankshaft.
Do not replace one main bearing shell alone. If one bearing shell requires replacement, install all new upper
and lower bearing shells. Also, if a new or reground crankshaft is to be used, install all new bearing shells.
Bearing shells are available in 0.010, 0.020 and 0.030 inch undersize for service with reground crankshafts. To
determine the size bearings required, refer to Crankshaft Grinding. Bearing which are 0.002 inch undersize are
available to compensate for slight journal wear where it is unnecessary to regrind the crankshaft.
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