TM 5-3895-346-14
ENGINE OVERHAUL
CONNECTING ROD BEARINGS
CONNECTING ROD BEARINGS
5. Install the bearing shells and bearing cap before another connecting rod bearing cap is removed.
Inspection
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.
After removal, clean the bearings and inspect them for scoring, pitting, flaking, etching, or signs of over-
heating. If any of these defects are present, the bearings must be discarded. The upper bearing shells, which
carry the load, will normally show signs of distress before the lower bearing shells do.
Inspect the backs of the bearing shells for bright spots, which indicate they have been shifting in their supports.
If such spots are present, discard the bearing shells. Also inspect the connecting rod bearing bore for burrs,
foreign particles, etc.
Measure the thickness of the bearing shells, using a micrometer and ball attachment J 4757, as described
under Inspection. The minimum thickness of a worn, standard connecting rod bearing shell should not be less
than 0.1230 inch and, if either bearing shell is thinner than this dimension, replace both bearing shells. A new
standard bearing shell has a thickness of 0.1245 to 0.1250 inch (in-line engine). Refer to Table 1.
In addition to the thickness measurement, check the clearance between the connecting rod bearing shells and
the crankshaft journal. This clearance may be checked by means of a soft plastic
Table 1
Minimum
Bearing
*New Bearing
Worn
Size
Thickness
Thickness
In-Line Engines
Standard
0.1245 -
0.1230 inch
0.1250 inch
0.002 inch
0.1255 -
0.1240 inch
Undersize
0.1260 inch
0.010 inch
0.1295 -
0.1280 inch
Undersize
0.1300 inch
0.020 inch
0.1345 -
0.1330 inch
Undersize
0.1350 inch
0.030 inch
0.1395 -
0.1380 inch
Undersize
0.1400 inch
*Thickness 90from parting line of bearing.
measuring strip which is squeezed between the journal and the bearing (refer to Shop Notes). The maximum
connecting rod bearing-to-journal clearance with used parts is 0.006 inch.
Before installing the bearings, inspect the crankshaft journals (refer to Inspection).
Do not replace one connecting rod bear-ing shell alone. If one bearing shell requires replacement, install both
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. Bearings which are 0.002
inch undersize are available to compensate for slight journal wear where it is unnecessary to regrind the
crankshaft.
154
