Chevrolet 1980 Light Duty Truck Service Manual page 439

Fig. 6 A 1 -2 8 ~ M e a s u rin g Gaging Plastic
5. If within specifications measure new or used bearing
clearances with Plastigage or its equivalent.
If a bearing is being fitted to an out-of-round crankpin,
be sure to fit to the maximum diameter of the crankpin. If
the bearing is fitted to the minimum diameter and the
crankpin is out-of-round .001" interference between the
bearing and crankpin will result in rapid bearing failure.
a. Place a piece of gaging plastic, the length of the
bearing (parallel to the crankshaft), on the crankpin or
bearing surface (fig. 6A1-27). Plastic gage should be
positioned in the middle of upper or lower bearing shell.
(Bearings are eccentric and false readings could occur if
placed elsewhere).
b. Install the bearing in the connecting rod and cap.
c. Install the bearing cap and evenly torque nuts to 35
lb. ft. (47N*m).
Do not turn the crankshaft with the gaging plastic
installed.
Fig. 6 A 1 -2 9~ M easu rin g Connecting Rod Side Clearance
d. Remove the bearing cap and using the scale on the
gaging plastic envelope, measure the gaging plastic width
at the widest point (fig. 6A1-28).
6. If the clearance exceeds specifications, select a new,
correct size, bearing and remeasure the clearance.
Be sure to check what size bearing is being removed
in order to determine proper replacement size bearing.
Undersize is marked on the back of the bearing. If clearance
cannot be brought within specification, the crankpin will
have to be ground undersize. If the crankpin is already at
maximum undersize, replace crankshaft.
7. Coat the bearing surface with oil, install the rod cap
and torque nuts to 35 lb. ft. (47N -m ).
8. When all connecting rod bearings have been
installed, tap each rod lightly (parallel to the crankpin) to
make sure they have clearance.
9. Measure all connecting rod side clearances between
the connecting rod cap and side of crankpin (fig. 6A1-29).
CRANKSHAFT M A IN BEARINGS
Main bearings are of the precision insert type and do
not utilize shim for adjustment. If clearances are found to
be excessive, a new bearing, both upper and lower halves,
will be required. Service bearings are available in standard
size and .001", .002", .009", .010" and .020" undersize.
Selective fitting of both rod and main bearing inserts
is necessary in production in order to obtain close
tolerances. For this reason you may find one half of a
standard insert with one half of a .001" undersize insert
which will decrease the clearance .0005" from using a full
standard bearing.
When a production crankshaft cannot be prescision
fitted by this method, it is then ground .009" undersize on
only those main journals th a t cannot be properly
fitted. All journals w ill not necessarily be ground.
A .009" undersize bearing or .010" undersize bearing will
then be used for precision fitting in the same manner as
previously described.
A reground crankshaft will be identified by the
following:
• The crankshaft throw will be stamped on one side of
the undersize journal with "9", along with a large spot of
light green paint.
• The main bearing cap will be painted light green on
each side of the affected.
If, for any reason, main bearings caps are replaced,
shimming may be necessary. Laminated shims for each cap
are available for service. Shim requirement will be
determined by bearing clearance.
Inspection
In general, the lower half of the bearing (except # 1
bearing) shows a greater wear and the most distress from
fatigue. If upon inspection the lower half is suitable for use,
it can be assumed that the upper half is also satisfactory.
If the lower half shows evidence of wear or damage, both
upper and lower halves should be replaced. Never replace
one half w ith o u t replacing the other half.
Checking Clearance
To obtain the most accurate results with "Plastigage",
or its equivalent, a wax-like plastic material which will
compress evenly between the bearing and journal surfaces
without damaging either surface, certain precautions should
IN-LINE 6 6A1-21
LIG H T T R U C K S ER V IC E M AN U AL