Chevrolet 1980 Light Duty Truck Service Manual page 476

6A 4-24 SMALL BLOCK
Fig. 6A4-42--Measuring Connecting Rod Side Clearance
Identification of a reground crankshaft is by the
following:
• The crankshaft throw will be stamped on one side of
the undersize journal with
green paint.
• The main bearing cap will be painted light green on
each side of the affected journal.
If, for any reason, main bearing 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 WITHOUT 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
be observed.
If the engine is out of the vehicle and upside down, the
crankshaft will rest on the upper bearings and the total
clearance can be measured between the lower bearing and
journal. If the engine is to remain in the vehicle, the
crankshaft must be supported upward to remove any
clearance from the upper bearing. The total clearance can
then be measured between the lower bearing and journal.
To assure the proper seating of the crankshaft, all
bearing cap bolts should be at their specified torque. In
addition, preparatory to checking fit of bearings, the surface
along with a large spot of
"9",
Fig. 6A4-43-Gaging Plastic on Journal
of the crankshaft journal and bearing should be wiped clean
of oil.
1. With the oil pan and oil pump removed, and starting
with the rear main bearing, remove bearing cap and wipe
oil from journal and bearing cap.
2. Place a piece of gaging plastic the full width of the
bearing (parallel to the crankshaft) on the journal (fig. 6A4-
43).
Do not rotate the crankshaft while the gaging plastic
is between the bearing and journal.
3. Install the bearing cap and evenly torque the
retaining bolts to specifications. Bearing cap M U S T be
torqued to specifications in order to assure proper reading.
Variations in torque affect the compression of the plastic
gage.
4. Remove bearing cap. The flattened gaging plastic
will be found adhering to either the bearing shell or journal.
5. On the edge of gaging plastic envelope there is a
graduated scale which is correlated in thousandths of an
inch. Without removing the gaging plastic, measure its
compressed width (at the widest point) with the graduations
on the gaging plastic envelope (fig. 6A4-44).
Normally main bearing journals wear evenly and are
not out-of-round. However, if a bearing is being fitted to an
out-of-round (.001" max.), be sure to fit to the maximum
diameter of the journal: If the bearing is fitted to the
minimum diameter and the journal is out-of-round .001",
interference between the bearing and journal will result in
rapid bearing failure. If the flattened gaging plastic tapers
toward the middle or ends, there is a difference in clearance
indicating taper, low spot or other irregularity of the bearing
or journal. Be sure to measure the journal with a micrometer
if the flattened gaging plastic indicates more than .001"
difference.
6. If the bearing clearance is within specifications, the
bearing insert is satisfactory. If the clearance is not within
LIG H T T R U C K S ER V IC E M AN UAL