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 ensure 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 excessive, a new bearing of the correct undersize must be selected and installed since there is no provision for adjustment.
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 re-ground and fitted with undersized bearings.
If journals are deeply scored or ridged the crankshaft must be replaced, as regrinding will reduce the durability of the crankshaft.
ROD BEARING INSPECTION AND REPLACEMENT
See Figure 1
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 0.001-0.003inch.
- 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® 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.
- 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.
- 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.
- Check the specifications chart in this section for the desired clearance. It is advisable to install a new bearing if clearance exceeds 0.003inch; however, if the bearing is in good condition and is not being checked because of bearing noise, bearing replacement is not necessary.
- 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.
- 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.
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.
MEASURING THE OLD PISTONS
See Figure 2
Check used piston-to-cylinder bore clearance as follows:
- Measure the cylinder bore diameter with a telescope gauge.
- Measure the piston diameter. When measuring the pistons for size or taper, measurements must be made with the piston pin removed.
- Subtract the piston diameter from the cylinder bore diameter to determine piston-to-bore clearance.
- Compare the piston-to-bore clearances obtained with those clearances recommended. Determine if the piston-to-bore clearance is in the acceptable range.
- When measuring taper, the largest reading must be at the bottom of the skirt.
SELECTING NEW PISTONS
- If the used piston is not acceptable, check the service piston size and determine if a new piston can be selected. (Service pistons are available in standard, high limit and standard oversize.
- If the cylinder bore must be reconditioned, measure the new piston diameter, then hone the cylinder bore to obtain the preferred clearance.
- Select a new piston and mark the piston to identify the cylinder for which it was fitted. (On some vehicles, oversize pistons may be found. These pistons will be 0.254mm (0.010 inch) oversize).
- When cylinders are being honed, follow the manufacturer's recommendations for the use of the hone.
- Occasionally, during the honing operation, the cylinder bore should be thoroughly cleaned and the selected piston checked for correct fit.
- When finish-honing a cylinder bore, the hone should be moved up and down at a sufficient speed to obtain a very fine uniform surface finish in a cross-hatch pattern of approximately 45-65° included angle. The finish marks should be clean but not sharp, free from embedded particles and torn or folded metal.
- Permanently mark the piston for the cylinder to which it has been fitted and proceed to hone the remaining cylinders.
- Thoroughly clean the bores with hot water and detergent. Scrub well with a stiff bristle brush and rinse thoroughly with hot water. It is extremely essential that a good cleaning operation be performed. If any of the abrasive material is allowed to remain in the cylinder bores, it will rapidly wear the new rings and cylinder bores. The bores should be swabbed several times with light engine oil and a clean cloth and then wiped with a clean dry cloth. CYLINDERS SHOULD NOT BE CLEANED WITH KEROSENE OR GASOLINE! Clean the remainder of the cylinder block to remove the excess material spread during the honing operation.
PISTON AND CONNECTING ROD ASSEMBLY AND INSTALLATION
See Figures 3 and 4
Install the connecting rod to the piston, making sure piston installation notches and 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, by using a 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 into the engine, proceed as follows:
- Make sure connecting rod bearings are of the correct size and properly installed.
- 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.
- 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 and the connecting rod bearing tang slots on the side opposite the camshaft (this assumes that the dimple(s) or other markings on the connecting rods are in correct relation to the piston notch).
- 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.
- Remove the rubber hoses. Install the bearing cap and cap nuts and torque to specification.
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 assembles.
- Check the clearance between the sides of the connecting rods and the crankshaft using a feeler gauge. Spread the rods slightly with a small prybar 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.
- Replace the oil pump, if removed, and the oil pan.
- Install the cylinder head(s) and intake manifold, as previously described.