Chevrolet Silverado / GMC Sierra. Service manual — part 145

Fig. 486: Measuring Crankpins For Out-Of-Round & Taper
Courtesy of GENERAL MOTORS CORP.

The crankshaft bearings are of the precision insert type and do not use shims for adjustment.
If the clearances are excessive, then new upper and lower crankshaft bearings will be
required. The service crankshaft bearings are available in the standard size and an undersize.

The selective fitting of the crankshaft bearings are necessary in production in order to obtain
close tolerances. For this reason, in one journal bore you may use one-half of a standard
crankshaft bearing with one-half of an undersize crankshaft bearing.

In order to determine the correct replacement bearing size, the bearing clearance must be
measured accurately. When checking main bearing clearances, either the micrometer or
plastic gage method may be used; however, the micrometer method gives more reliable

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Mechanical - 4.3L - Sierra & Silverado

results and is preferred. When checking connecting rod bearing clearances, the plastic gage
method will result in unreliable measurements. The use of J 43690 is preferred. See Special
Tools.

Normally the crankshaft bearing journals wear evenly and are not out-of-round. However, if
a crankshaft bearing is being fitted to an out-of-round crankshaft bearing journal, be sure to
fit to the maximum diameter of the crankshaft bearing journal. If the crankshaft bearing is
fitted to the minimum diameter and the crankshaft bearing journal is excessively out-of-
round, the interference between the crankshaft bearing and the crankshaft bearing journal
will result in rapid crankshaft bearing failure.

If the crankshaft bearing clearance is within specifications, the crankshaft bearing is
satisfactory. If the clearance is not within specifications, replace the crankshaft bearing.
Always replace both the upper and lower crankshaft bearings as a set.

A standard or undersize crankshaft bearing combination may result in the proper clearance.
If the proper crankshaft bearing clearance cannot be achieved using the standard or the
undersize crankshaft bearings, it may be necessary to repair or replace the crankshaft.

Measuring Crankshaft Bearing Clearances - Micrometer Method

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Mechanical - 4.3L - Sierra & Silverado

Fig. 487: View Of Crankshaft Bearings At Engine Block
Courtesy of GENERAL MOTORS CORP.

1. Measure the crankshaft journal diameter with a micrometer in several places, approximately

90 degrees apart. Average the measurements.

2. Determine the taper and out-of-round of the journal. Refer to Engine Mechanical

Specifications.

3. Install the bearings into the engine block or connecting rod assembly.

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Mechanical - 4.3L - Sierra & Silverado

Fig. 488: Bearing Cap Bolts
Courtesy of GENERAL MOTORS CORP.

4. Install the bearing cap bolts and tighten to specifications. Refer to Fastener Tightening

Specifications.

5. Measure the bearing inside diameter (ID) at two points 90 degrees apart. Average the

measurements.

6. In order to determine the bearing clearance, subtract the average journal diameter from the

2008 Chevrolet Silverado 1500

2008 ENGINE Engine Mechanical - 4.3L - Sierra & Silverado