TM 5-3805-255-14
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BEARING FITTING PROCEDURE
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General
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1. Undersize precision-type bearing shells should be installed when, because of wear, bearing-to-crankshaft run-
ning clearances are to be reduced.
NOTE
Bearing caps must not be filed, lapped, or modified in any other manner.
2. Premature bearing failure will result from attempts to reduce journal-to-bearing running clearances by
modifying bearing caps, bearings, or both, because such modification will alter engineered fit of bearing shells
in their bores and destroy specifically desired crush.
3. When installing precision-type connecting rod or main bearings, it is important that bearing shells fit tightly in
rod or case bore. To accomplish this, the bearing manufacturer makes the diameter at right angles to the part-
ing line slightly larger than the actual diameter of the bore into which they are assembled. When the assembly
is drawn up tight, the bearing is compressed, assuring a positive contact between bearing back and bore. This
increased diameter is referred to as bearing crush as shown in Figure 1.
Figure 1. Illustrating Bearing Crush.
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4. To obtain proper bearing assembly with correct crush, be careful when tightening clamping bolts or nuts to be
sure they are drawn down alternately and evenly, using a torque wrench and tightening as specified.
5. Rods, caps, or blocks must not be filed, lapped, or modified in any other manner in order to reduce clearance.
While such practice will make a tighter fit at top and bottom, it will result in an out-of-round bore and bearing
shell distortion.
6. Main and connecting rod bearings are designed with a spread (width across open ends) slightly larger than the
diameter of the crankcase bore or connecting rod bore into which they are assembled. For example, the width
across a rod bearing not in place is approximately 0.025 in. (0.64 mm) more than when the bearing is in
position. This condition is originally designed into the bearing causing it to spread outward at the parting line
when crush load is applied by tightening nuts or bolts. Some of this snap may be lost in normal use, but the
bearing need not be replaced because of a nominal loss of this condition.
7. This condition causes the bearing to fit snugly in the rod bore and the bearing must be snapped or lightly
forced into its seat as shown in Figure 2.
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