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305-350 ENGINE
6A6-19
Inspection
In general, the lower half of the bearing (except No. 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 should be supported both front and rear (damper
and flywheel) to remove the clearance from the upper
bearing. The total clearance can then be measured between
the lower bearing and journal.
NOTE: To assure the proper seating of the crankshaft,
all bearing cap bolts should be at their specified torque.
In addition, before checking fit of bearings, the surface
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. 6A6-
42).
Fig.
6 A 6 - 4 2
Gaging Plastic On Journal
CAUTION: Do not rotate the crankshafl while
the gaging plastic is between the bearing and
journal.
3. Install the bearing cap and evenly torque the retaining
bolts to specifications.
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 displayed 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. 6A6-43).
Fig.
6A6-43
Measuring Gauging Plastic
NOTE: Normally, main bearing journals wear evenly
and are not out-of-round. However, if a bearing is being
fitted to an out-of-round journal (.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 .00lV difference.
6. If the bearing clearance is within specifications, the
bearing insert is satisfactory. If the clearance is not within
specifications, replace the insert. Always replace both upper
and lower inserts as a unit.
NOTE: If a new bearing cap is being installed and
clearance is less than .00lW, inspect for burrs or nicks;
if none are found then install shims as required.
7. A standard, ,001" or .002" undersize bearing should
produce the proper clearance.
8. Proceed to the next bearing. After all bearings have
been checked, rotate the crankshaft to see that there is no
excessive drag.
9. Measure crankshaft end play (see specifications) by
forcing the crankshaft to the extreme front position. Measure
at the front end of the rear main bearing with a feeler gage
(Fig. 6A6-44).
20. Install a new rear main bearing oil seal in the cylinder
block and main bearing cap.
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