Chevrolet 1991 Light Duty Truck Repair Manual page 354

6A-18 GENERAL ENGINE MECHANICAL
— 6.2L:
— Intake and exhaust: 0.08 mm (0.003-inch), 0.38
mm (0.015-inch).
Select a reamer which will provide a straight, clean
bore through the guide's entire length (figure 36).
Reamer availability is as follows. Sizes given are
nominal.
• All engines except 6.2L and 7.4 L:
— J 5830-1: 0.08 mm (0.003-inch) oversize.
— J 6621: 0.13 mm (0.005-inch) oversize.
— J 5830-2: 0.38 mm (0.015-inch) oversize.
— J 5830-3: 0.76 mm (0.030-inch) oversize.
— These reamers (except J 6621) also available
in Reamer Set J 5830-02.
• 6.2L and 7.4 L engines:
— J 7049-1: 0.08 mm (0.003-inch) oversize.
— J 7049-2:. 0.38 mm (0.015-inch) oversize.
— J 7049-3: 0.76 mm (0.030-inch) oversize.
— These reamers also available in Reamer Set
J 7049.
ASSEMBLY
Assemble
Valves and components. Refer to the proper sec­
tion.
MEASURING MAIN BEARING CLEARANCE
Main bearings are of the precision insert type and do
not use shims for adjustment. If clearances are found
to be excessive, new upper and lower inserts will be
required.
The simplest, most accurate way to measure main
bearing clearance is with the use of gaging plastic.
This wax-like material compresses evenly between the
bearing and journal surfaces without damaging them.
Proceed as follows:
Clean
All oil from the crankshaft journal and main bear­
ing inserts.
Install or Connect (Figure 37)
Refer to the proper section for additional informa­
tion.
Main bearing inserts and crankshaft, as outlined in
the proper section.
Gaging plastic.
• Begin with the rear main bearing.
• Wipe the oil from the crankshaft journal and
the lower main bearing insert.
• Place a piece of gaging plastic the full width of
the lower bearing insert (parallel to the crank­
shaft) on the journal (figure 37). Do not rotate
the crankshaft while the gaging plastic is
between the bearing and journal.
3. Main bearing cap and bolts.
^ Tighten
• Bolts to "Specifications."
Remove or Disconnect
• Main bearing cap.
DO NOT REMOVE THE GAGING PLASTIC FROM
THE JOURNAL OR LOWER MAIN BEARING INSERT.
Measure (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. Without removing the gaging
plastic, measure its compressed width (at the wid­
est point) with the graduations on the gaging
plastic envelope (figure 38).
3. 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 0.001-inch (0.0005-inch on 2.