Chevrolet Full-size Cars 1968-1978 Repair Guide

Connecting Rod Bearings

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Connecting rod bearings for the engines covered in this guide consist of two halves or shells which are interchangeable 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 clamped tightly in place to insure positive seating and to prevent turning. A tang holds the shells in place.

The ends of the bearing shells 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 no 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 & REPLACING BEARINGS





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Fig. Fig. 1 Check connecting rod side clearance with a feeler gauge and a small prybar-carefully spread the connecting rods while inserting the feeler gauge



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Fig. Fig. 2 The notch on the the side of the bearing cap matches the groove on the bearing insert



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Fig. Fig. 3 Apply a strip of gauging material to the bearing journal, then install and torque the cap



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Fig. Fig. 4 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 crankshaft. 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.

  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.
  2.  
  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 to 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 in this information for the desired clearance. It is advisable to install a new bearing if clearance exceeds 0.003 in.; 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 units to specification.
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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 end clearance. If the rod cannot be moved, either the rod bearing is too far undersize or the rod is misaligned.

PISTON & CONNECTING ROD ASSEMBLY & INSTALLATION





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Fig. Fig. 5 Most pistons are marked to indicate positioning in the engine (usually a mark means the side facing front)



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Fig. Fig. 6 Installing the piston into the block using a ring compressor and the handle of a hammer

Install the connecting rod to the piston, making sure the piston installation notches and any marks on the rod are in proper relation to one another. Lubricate the wrist pin with clean engine oil, and install the pin into the rod and piston assembly, either by hand or by using a wrist pin press as required. Install snaprings if equipped, and rotate them in their grooves to make sure they are seated. To install the piston and connecting rod assembly:

  1. Make sure connecting rod big-end bearings (including end cap) are of the correct size and properly installed.
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  3. Fit rubber hoses over the connecting rod bolts to protect the crankshaft journals, as in the Piston Removal procedure. Coat the rod bearings with clean oil.
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  5. Using the proper ring compressor, insert the piston assembly into the cylinder so that the notch in the top of the piston faces the front of the engine (this assumes that the dimple(s) or other markings on the connecting rods are in correct relation to the piston notch(s).
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  7. From beneath the engine, coat each crank journal with clean oil. Pull the connecting rod, with the bearing shell in place, into position against the crank journal.
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  9. Remove the rubber hoses. Install the bearing cap and cap nuts and torque to specification.
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When more than one rod and piston assembly is being installed, the connecting rod cap attaching nuts should only be tightened enough to keep each rod in position until all have been installed. This will ease the installation of the remaining piston assemblies.

  1. Check the clearance between the sides of the connecting rods and the crankshaft using a feeler gauge. Spread the rods slightly with a screwdriver to insert the gauge. If clearance is below the minimum tolerance, the rod may be machined to provide adequate clearance. If clearance is excessive, substitute an unworn rod, and recheck. If clearance is still outside specifications, the crankshaft must be welded and reground, or replaced.
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  3. Replace the oil pump is removed and the oil pan.
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  5. Install the cylinder head(s) and intake manifold.
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