Cylinder Block - Improvements and Main Bearing Usage

Bulletin No.
T-93-1

File in Group
6

Number
27

Corp. Ref. No.
216104R

Date
Nov. 1992



SUBJECT:
CYLINDER BLOCK IMPROVEMENTS AND MAIN BEARING USAGE

MODELS AFFECTED:
ALL 1991-1993 4.5L AND 4.9L V8 ENGINES



During the 1991 model year the cylinder block used for the 4.5L and 4.9L transverse V8 engines received a series of structural enhancements that have made dimensional adjustments in main bearing clearance necessary to prevent main bearing knock (mallet noise) in some vehicles. These crankcase changes are not incorporated in all 1991 vehicles, but are included for all 1992 and later engines. The improved crankcase is also used to assemble service engine assemblies produced after 09/01/91 (i.e., a service engine built in 1992 calendar year for use in any model year including past models will include the structurally updated transverse crankcase). Refer to Figures 1 and 2 to identify the updated crankcase.





The procedure necessary to establish which bearing (standard or oversize) should be installed in the # 1 or # 5 position, is contained in 1987 and later Service Information Manuals. This procedure applies to all 4.5L and 4.9L V8 engines. Bearing position # 1 has the most influence on mallet noise.









The bearings should be used only in engines with the structurally improved crankcase shown in Figure 2. For bearings to be used in the earlier crankcase shown in Figure 1, refer to Service bulletin T-92-132.

Crankcase Identification:

To determine which style crankcase is being worked on and hence which bearing chart applies, refer to the figures. Note the 1991 model year vehicles may contain either crankcase (as well as earlier model year vehicles that have experienced an engine replacement).



Structural changes have been incorporated in the transverse crankcase to keep up with steadily increasing power outputs. Figure 1 shows a "typical" crankcase that shows the only significant differences for purposes of bearing selection. Earlier blocks may not exhibit all of the features shown. 1990 style blocks have:

^ Less extensive ribbing
^ A "thinner" pan rail
^ A letter identifier other than "Z"

The 1992 style block shown in Figure 2 has:

^ More ribbing
^ A thicker pan rail
^ The letter "Z" cast into the side just above the oil pan rail near cylinder # 8

Parts are currently available from GMSPO.

For warranty purposes use Labor Operation J1150 from the appropriate Labor Time Guide.