G643 (E) Service Manual Chapter 3. Engine Mechanical System 92
Plastigage Method
Install or Connect
1. Main bearing inserts and crankshaft.
2. Gauging plastic.
A. Begin with the rear main bearing.
B. Wipe the oil from the crankshaft journal and
the lower main bearing insert.
C. Place a piece of gauging plastic the full width
of the lower bearing insert (parallel to the
crankshaft) on the journal (figure 12-62). Do
not rotate the crankshaft while the gauging
plastic is between the bearing and journal.
3. Main bearing cap and bolts.
4. Tighten bolts to specification.
Remove or Disconnect
• Main bearing cap. DO NOT REMOVE THE
GAUGING PLASTIC FROM THE JOURNAL OR
LOWER MAIN BEARING INSERT.
Figure 12-62 Placing Gauging Plastic on Journal
Measure
• Gauging plastic as follows:
a. The flattened gauging plastic will be found
adhering to either the lower bearing insert or
journal.
b. On the edge of the gauging plastic envelope
there is a graduated scale. Without removing
the gauging plastic, measure its compressed
width (at the widest point) with the graduations
on the gauging plastic envelope (figure 12-63).
Figure 12-63 Measuring Gauging Plastic
c. If the flattened 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.
d. Normally, crankshaft bearing journals wear
evenly and are not out-of-round. However, if a
bearing is being fitted to an out-of-round
0.0254 mm (0.001 in.) (maximum) journal, be
sure to fit to the maximum diameter of the
journal. If the bearing is fitted to the minimum
diameter and the journal is excessively out-of-
round, interference between the bearing and
the journal will result in rapid bearing failure.
e. If the bearing clearance is within specifications,
the bearing is satisfactory. If the clearance is
not within specifications, replace the bearing.
Always replace both upper and lower bearings
as a unit.
f. A standard or undersize bearing combination
may result in the proper clearance. If the
proper bearing clearance cannot be achieved
using standard or undersize bearings, it will be
necessary to replace the crankshaft.
NOTICE: Crankshaft bearings must not be shimmed,
scraped, or filed. Do not touch the bearing surface
with bare fingers. Skin oil will etch the bearing
surface.
g. Remove the flattened gauging plastic.
h. Measure remaining journals.
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