GMC S Series 1994 Repair Manual Download

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V Important

• In order to provide an effective compression

seal, the ring gaps must be staggered upon
installation, refer to Figure 49.

MAIN AND ROD BEARINGS

MAIN AND ROD BEARING INSPECTION

Figure 29

Main bearings are of the precision insert type and

do  not  utilize  shims  for  adjustment.  If  clearances  are
found  to  be  excessive,  a  new  bearing,  both  upper  and
lower  halves,  will  be  required.  Service  bearings  are

available in standard size and undersize.

Inspect

1. Bearing surfaces for:

-   Wear.
-   Gouges.
-   Grooves.
-   Cracks.
-   Chips.
-   Imbedded  foreign  material.

• If foreign material is found, determine its

nature and source. Inspect oil pan sludge
and residue.

2. Outer bearing surfaces for:

- Overheating (discoloration).
- Looseness or rotation (flattened tangs and

wear grooves).

- Wear.

• Surface wear indicates either movement of

the insert or high spots in the surrounding
material (spot wear).

3. Thrust surfaces (main thrust bearing) for:

- Wear.
- Grooving.

• Grooves are caused by irregularities of the

crankshaft

thrust

surface.

Refer

“ Crankshaft Servicing” in this section.

Important

• Bearing failure, other than normal wear, must

investigated

carefully.

Inspect

the

crankshaft, connecting rod, bearing bores and
bearing oil feed circuit.

4. Bearing cap bolts. If bolts are stretched, replace

them.

- To determine if a bolt is stretched, compare it

to a new one.

MEASURING BEARING CLEARANCE

Measure

• Bearing clearance. To determine the correct

replacement insert size, the bearing clearance

must  be  measured  accurately.  Either  of  the
following  two  methods  may  be  used,  however,
method  “ A ”  gives  more  reliable  results  and  is

preferred.

Figure 29 - Measuring Bearing Clearance

|~f~| Important

• Method “ A” yields measurements from which

the bearing clearance can be

computed.

Method “ B” yields the bearing clearance

directly.

Method “ B” does

not

give any

indication of bearing runout.

• Do not mix inserts of different nominal size in

the same bearing bore.

Method “A”

1. Measure the crankshaft journal diameter with a

micrometer in several places, approximately 90°
apart, and average the measurements.

2. Compute taper and runout, refer to “ Engine

Specifications” in this section for allowable

limits.

3. Bearing insert I.D. with an inside micrometer.

Measure using new inserts, if the inserts are
being replaced.

a. Install bearing inserts.

• C o n n e c t i n g r o d n u t s to 52 N*m

(38 lbs. ft.).

• Main bearing cap bolts to 95 N*m

(70 lbs. ft.).

b. Measure inside diameter with an inside

micrometer. Measure at 90° to the split line of
the bearing.

c. Subtract journal diameter from bearing inside

diameter to obtain bearing clearance. Refer to

“ Specifications” in this section for proper

clearance.

If  measurements  and  clearances  obtained  meet
specifications,  bearing  size  used  for  measurement
is  correct  and  can  be  used.
If measurements  do  not  meet  specifications  and/or
bearing  clearance  cannot  be  corrected  by  selecting
service  bearings  available,  the  crankshaft  must  be
replaced.

GMC S Series 1994, Repair Manual

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