a. Connecting rod bearings. Periodic inspection of connecting rod bearings can be made
without removing the piston and connecting rod assembly; this is done by blocking the bearing and
backing the shaft away from the shell.
When the connecting rod assemblies are removed, inspect the face and fillets of the
crankshaft journal for scored condition. If the crankshaft journal is found scored in the bearing area,
smooth off all irregularities by stoning with a fine grit stone. Do not stone lengthwise or use a file.
Check to see that the crankshaft is not out of round; then clean all parts.
Be sure to line up the oil holes when the connecting rod bearings are replaced. In place,
they must have proper clearance on the crankshaft journal. Their clearance should be 0.001 inch per
inch of crankshaft diameter, unless otherwise specified; their thrust must be parallel with respect to the
crankcheek or adjoining bearing and they must be free to move laterally with the crankshaft in any
angular position. Connecting rod bolts should be tightened to the torque specified by the manufacturer
or to proper bolt elongation or stretch.
b. Crankshaft bearings. Under normal conditions, crankshaft bearings receive lubrication by
the forced lubrication system built into the engine, and proper bearing alinement is assured by the rigid
engine frame and method of mounting. However, misalinement may occur if one or more bearings are
permitted to wear unduly. If a bearing becomes low, the span will be increased between the bearings
supporting the shaft and this will introduce heavy stresses in the crankpins; if the condition is permitted
to continue, it will result in a crankshaft bearing failure.
Crankshaft bearings can be checked in several ways; one method follows. Locate
crankpin at upper dead center and force the shaft down to a firm seat in the bearings. After removing
the valve levers, connect an air hose to a T at the indicator cock connection. Provide a pressure gage on
the cylinder head side of the T. Lock flywheel from turning and maintain air pressure in the cylinder at
125 pounds or more by regulating the valves located on the inlet side of the T. Measure distance
between gage points on crankcheeks with micrometer or strain gage; then after locating crankpin at
lower dead center, again force the shaft into the bearings. Measure distance between gage points for the
two opposite horizontal positions; do not use air pressure in the cylinder. When the bearings are in line,
all measurements will be the same.