GM Full Size Vans 1967-1986 Repair Guide

Connecting Rod Bearings

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Connecting rod bearings for the engine covered in this guide consist of two halves or shells which are interchangable in the rod and cap. When the shells are placed in position, the ends extend slightly beyond the rod and cap surfaces so that when the rod bolts are torqued the shells will be capped tightly in place to insure positive seating and to prevent turning. A tang holds the shells in place.

The ends of the bearing shell must never be filed flush with the mating surface of the rod and cap.

If a rod bearing becomes noisy or is worn so that its clearance on the crank journal is sloppy, a new bearing of the correct undersize must be selected and installed since there is a provision for adjustment.


WARNING
Under no circumstances should the rod end or cap be filed to adjust the bearing clearance, nor should shims of any kind be used.

Inspect the rod bearings while the rod assemblies are out of the engine. If the shells are scored or show flaking, they should be replaced. If they are in good shape check for proper clearance on the crank journal (see below). Any scoring or ridges on the crank journal means the crankshaft must be replaced, or reground and fitted with undersized bearings.

CHECKING BEARING CLEARANCE AND REPLACING BEARINGS



See Figures 1 and 2



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Fig. Fig. 1: Apply a strip of gauging material to the bearing journal, then install and torque the cap



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Fig. Fig. 2: After the cap is removed again, use the scale supplied with the gauge material to check clearances

Make sure connecting rods and their caps are kept together, and that the caps are installed in the proper direction.

Replacement bearings are available in standard size, and in undersizes for reground crankshafts. Connecting rod-to-crankshaft bearing clearance is checked using Plastigage® at either the top or bottom of each crank journal. The Plastigage® has a range of 0.001-0.003 in. (0.0254-0.0762mm).

  1. Remove the rod cap with the bearing shell. Completely clean the bearing shell and the crank journal, and blow any oil from the oil hole in the crankshaft; Plastigage® is soluble in oil.
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  3. Place a piece of Plastigage® lengthwise along the bottom center of the lower bearing shell, then install the cap with shell and torque the bolt or nuts to specification. DO NOT turn the crankshaft with Plastigage® in the bearing.
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  5. Remove the bearing cap with the shell. The flattened Plastigage® will be found sticking to either the bearing shell or crank journal. Do not remove it yet.
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  7. Use the scale printed on the Plastigage® envelope to measure the flattened material at its widest point. The number within the scale which most closely corresponds to the width of the Plastigage® indicates bearing clearance in thousandths of an inch.
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  9. Check the specifications chart for the desired clearance. It is advisable to install a new bearing if clearance exceeds 0.003 in. (0.0762mm). However, if the bearing is in good condition and is not being checked because of bearing noise, bearing replacement is not necessary.
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  11. If you are installing new bearings, try a standard size, then each undersize in order until one is found that is within the specified limits when checked for clearance with Plastigage®. Each undersize shell has its size stamped on it.
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  13. When the proper size shell is found, clean off the Plastigage®, oil the bearing thoroughly, reinstall the cap with its shell and torque the rod bolt nuts to specification.
  14.  

With the proper bearing selected and the nuts torqued, it should be possible to move the connecting rod back and forth freely on the crank journal as allowed by the specified connecting rod and clearance. If the rod cannot be moved, either the rod bearing is too far undersize or the rod is misaligned.

 
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