GMC 1989 Light Duty Truck, Repair Manual, Page: 354 / 998

6A-20 GENERAL ENGINE MECHANICAL
Figure 38— Measuring the Gaging Plastic
the crankshaft while the gaging plastic is be­
tween the bearing and journal.
3. Main bearing cap and bolts.
T ig h ten
• Bolts to “ Specifications.”
+-► R em ove or D isconnect
Main bearing cap.
DO NOT REMOVE THE GAGING PLASTIC FROM
THE JOURNAL OR LOWER MAIN BEARING IN­
SERT
M easure (Figure 38)
• Gaging plastic as follows:
1. The flattened gaging plastic will be found adhering
to either the lower bearing insert or journal.
2. On the edge of the gaging plastic envelope there is
a graduated scale. W ithout removing the gaging
plastic, measure its compressed width (at the widest
point) with the graduations on the gaging plastic en­
velope (figure 38).
3. If the flattened gaging plastic tapers toward the m id­
dle or ends, there is a difference in clearance indica­
ting 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 indi­
cates more than 0.001-inch (0.0005-inch on 2.5 L en­
gines) difference.
4. 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, be sure to fit to the m axi­
mum diameter of the journal. If the bearing is fitted
to the minimum diameter and the journal is exces­
sively out-of-round, interference between the bear­
ing and the journal will result in rapid bearing
failure.
5. 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 inserts as a unit.
6. A standard or undersize bearing may produce the
proper clearance. Refer to the proper section for
bearing availability. If not, it will be necessary to re­
grind the crankshaft journal for use with the next
undersize bearing. Do not grind rolled fillet crank­
shafts, such as used on 2.5 L, 2.8 L, and 6.2 L en­
gines. After selecting the new bearing, recheck the
clearance.
7. Remove the flattened gaging plastc.
8. Perform the preceding steps on the remaining main
bearings.
MEASURING CONNECTING ROD BEARING CLEARANCE
Connecting rod bearings are of the precision insert type
and do not use shims for adjustment. DO NOT FILE
RODS OR ROD CAPS. If clearances are found to be ex­
cessive, a new bearing (both upper and lower halves) will
be required.
The simplest, most accurate way to measure connect­
ing rod bearing clerarance is with the use of gaging plas­
tic. This wax-like material compresses evenly between the
bearing and journal surfaces with dam aging them. Pro­
ceed as follows.
jn y
Clean
All oil from the crankshaft journal and the connect­
ing rod bearing inserts.
Install or C onnect (Figure 39)
• Refer to the proper section for additional information
on connecting rod bearing sizing.
1. Connecting rod with the upper connecting rod bear­
ing insert to the crankshaft journal.
2. A piece of gaging plastic the length of the bearing
(parallel to the crankshaft) on the journal (figure 39).
The plastic gage should be positioned in the middle
of the upper and lower bearing insert. (Bearings are
eccentric and false readings could occur if placed
elsewhere.)
Im portant
If a bearing is being fitted to an out-of-round
crankpin, be sure to fit the maximum diameter
of the crankpin. If the bearing is fitted to the
minimum diameter and the crankpin is exces-