GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Pistons And Connecting Rods

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REMOVAL



See Figures 1, 2, 3, 4 and 5

Before removing the pistons, the top of the cylinder bore must be examined for a ridge. A ridge at the top of the bore is the result of normal cylinder wear, caused by the piston rings only traveling so far up the bore in the course of the piston stroke. The ridge can be felt by hand; it must be removed before the pistons are removed.

A ridge reamer is necessary for this operation. Place the piston at the bottom of its stroke, and cover it with a rag. Cut the ridge away with the ridge reamer, using extreme care to avoid cutting too deeply. Remove the rag, and remove the cuttings that remain on the piston with a magnet and a rag soaked in clean oil. Make sure the piston top and cylinder bore are absolutely clean before removing the piston.

  1. Remove intake manifold and cylinder head or heads.
  2.  
  3. Remove oil pan.
  4.  
  5. Remove oil pump assembly if necessary.
  6.  
  7. Matchmark the connecting rod cap to the connecting rod with a scribe; each cap must be reinstalled on its proper rod in the proper direction. Remove the connecting rod bearing cap and the rod bearing. Number the top of each piston with silver paint or a felt tip pen for later assembly.
  8.  
  9. Cut lengths of 3 / 8 in. (9.5mm) diameter hose to use as rod bolt guides. Install the hose over the threads of the rod bolts, to prevent the bolt threads from damaging the crankshaft journals and cylinder walls when the piston is removed.
  10.  
  11. Squirt some clean engine oil onto the cylinder wall from above, until the wall is coated. Carefully push the piston and rod assembly up and out of the cylinder by tapping on the bottom of the connecting rod with a wooden hammer handle.
  12.  
  13. Place the rod bearing and cap back on the connecting rod, and install the nuts temporarily. Using a number stamp or punch, stamp the cylinder number on the side of the connecting rod and cap; this will help keep the proper piston and rod assembly on the proper cylinder.
  14.  

On V6 engines, starting at the front the cylinders are numbered 2-4-6 on the right bank and 1-3-5 on the left. On all V8s, starting at the front the right bank cylinders are 2-4-6-8 and the left bank 1-3-5-7.

  1. Remove remaining pistons in a similar manner.
  2.  

On all engines, the notch on the piston will face the front of the engine for assembly. The chamfered corners of the bearing caps should face toward the front of the engine for the right bank and to the rear of the engine on the left bank. On some Pontiac built engines, the rods have three dimples on one side of the rod and a single dimple on the rod cap. The dimples must face to the rear on the right bank and forward on the left. Where there are numbers on the pistons and rods, the numbers must be on the same side when the two are assembled.

On various engines, the piston compression rings are marked with a dimple, a letter T , a letter O , GM or the word TOP to identify the side of the ring which must face toward the top of the piston.



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Fig. Fig. 1: Match the connecting rods to their caps with a scribe mark



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Fig. Fig. 2: Cover the connecting rod bolts with small pieces of hose to prevent damage to the crank journals and cylinder walls during removal



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Fig. Fig. 3: Push the piston and rod out with a hammer handle



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Fig. Fig. 4: Match the connecting rods to their cylinders with a number stamp



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Fig. Fig. 5: Removing the rod bearing cap from the connecting rod

CLEANING AND INSPECTION



See Figures 6, 7, 8, 9 and 10

A piston ring expander is necessary for removing piston rings without damaging them; any other method (screwdriver blades, pliers, etc.) usually results in the rings being bent, scratched or distorted, or the piston itself being damaged. When the rings are removed, clean the ring grooves using an appropriate ring groove cleaning tool, using care not to cut too deeply. Thoroughly clean all carbon and varnish from the piston with a non-caustic solvent.


CAUTION
Do not use a wire brush or caustic solvent (acids, etc.) on pistons.

Inspect the pistons for scuffing, scoring, cracks, pitting, or excessive ring groove wear. If these are evident, the piston must be replaced.

The piston should also be checked in relation to the cylinder diameter. Using a telescoping gauge and micrometer, or a dial gauge, measure the cylinder bore diameter perpendicular to the piston pin, 2 1 / 2 in. (63.5mm) below the cylinder block deck (surface where the block mates with the heads). Then, with the micrometer, measure the piston perpendicular to its wrist pin on the skirt. The difference between the two measurements is the piston clearance. If the clearance is within specifications or slightly below (after the cylinders have been bored or honed), finish honing is all that is necessary. If the clearance is excessive, try to obtain a slightly larger piston to bring clearance to within specifications. If this is not possible, obtain the first oversize piston and bore the cylinder to size. Generally, if the cylinder bore is tapered 0.005 in. (0.127mm) or more or is out-of-round 0.003 in. (0.0762mm) or more, it is advisable to rebore for the smallest possible oversize piston and rings.

After measuring, mark pistons with a felt-tip for reference and for assembly.



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Fig. Fig. 6: Measuring the piston with a micrometer



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Fig. Fig. 7: Measuring the cylinder bore with a dial gauge



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Fig. Fig. 8: A ring expander tool should be used to remove the piston rings



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Fig. Fig. 9: Clean the piston grooves using a ring groove cleaner



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Fig. Fig. 10: A telescoping gage and micrometer may be used to measure the cylinder bore diameter

RIDGE REMOVAL AND HONING



See Figures 11, 12, 13 and 14

Cylinder honing and/or boring should be performed by a reputable, professional machine shop with the proper equipment. In some cases, clean-up honing can be done with the cylinder block in the vehicle, but most excessive honing and all cylinder boring must be done with the block stripped and removed from the vehicle.

As the cylinder bore wears, a ridge is formed at the top of the cylinder where the rings never scrape. Removing this ridge makes piston removal easier and prevents damage to the rings. A ridge reamer is installed into the cylinder, tightened, then the cutters of the reamer are moved upward as the threaded portion of the reamer is rotated.

When new piston rings are installed in an engine, the cylinder bore must have a cross-hatch pattern finish. This allows the rings and bore wear together and provide better oil control and compression. This cross-hatch pattern is achieved by honing. No more than 0.0008 in (0.02 mm) should be removed during this process, since the finished roundness of the cylinder will be destroyed.



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Fig. Fig. 11: Ridge formed by piston ring travel



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Fig. Fig. 12: Proper cross hatch pattern angles



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Fig. Fig. 13: Remove the ridge from the cylinder bore using a ridge cutter



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Fig. Fig. 14: Honing the cylinder bore with a flexible hone

PISTON PIN REPLACEMENT



See Figure 15

Most of the engines covered in this guide utilize pistons with pressed-in wrist pins; these must be removed by a special press designed for this purpose and should only be performed by a machine shop. Other pistons have their wrist pins secured by snaprings, which are easily removed with snapring pliers. Separate the piston from the connecting rod once the snaprings have been removed.


CAUTION
DO NOT try to press out the piston wrist pin without using the specified tools (unless retained by snaprings). Severe damage may occur to the piston and connecting rod, causing internal engine damage and failure.



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Fig. Fig. 15: Exploded view of a piston assembly; note that the piston pin is secured with snap rings

PISTON RING REPLACEMENT



See Figures 16, 17, 18, 19, 20, 21 and 22

Piston ring end gap should be checked while the rings are removed from the pistons. Incorrect end gap indicates that the wrong size rings are being used; ring breakage could occur.

Compress the piston rings to be used in a cylinder, one at a time, into that cylinder. Squirt clean oil into the cylinder, so that the rings and the top 2 in. (51mm) of cylinder wall are coated. Using an inverted piston, press the rings approximately 1 in. (25mm) below the deck of the block (on diesels, measure ring gap clearance with the ring positioned at the bottom of ring travel in the bore). Measure the ring end gap with a feeler gauge, and compare to the Ring Gap chart in this section. Carefully pull the ring out of the cylinder and file the ends squarely with a fine file to obtain the proper clearance.

Check the pistons to see that the ring grooves and oil return holes have been properly cleaned. Slide a piston ring into its groove, and check the side clearance with a feeler gauge. On gasoline engines, make sure you insert the gauge between the ring and its lower land (lower edge of the groove), because any wear that occurs forms a step at the inner portion of the lower land. On diesels, insert the gauge between the ring and the upper land. If the piston grooves have worn to the extent that relatively high steps exist on the lower land, the piston should be replaced, because these will interfere with the operation of the new rings and ring clearances will be excessive. Piston rings are not furnished in oversize widths to compensate for ring groove wear.

Install the rings on the piston, lowest ring first, using a piston ring expander. There is a high risk of breaking or distorting the rings, or scratching the piston, if the rings are installed by hand or other means.

Position the rings on the piston as illustrated; spacing of the various piston ring gaps is crucial to proper oil retention and even cylinder wear. When installing new rings, refer to the installation diagram furnished with the new parts.



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Fig. Fig. 16: Removing the piston rings with a ring expander



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Fig. Fig. 17: Clean the piston ring grooves with a ring groove cleaner



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Fig. Fig. 18: Checking the piston ring end gap with a feeler gauge



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Fig. Fig. 19: Checking the ring side clearance



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Fig. Fig. 20: Ring gap spacing on Chevrolet, Oldsmobile and Pontiac built engines



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Fig. Fig. 21: Ring gap spacing on Buick built engines



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Fig. Fig. 22: Most rings are marked to show which side should face upward

ROD BEARING REPLACEMENT



See Figures 23, 24 and 25

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.


CAUTION
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. Any scoring or ridges on the crank journal means the crankshaft must be replaced, or reground and fitted with undersized bearings.

If turbo V6 crank journals are scored or ridged the crankshaft must be replaced, as regrinding will reduce the durability and strength of the crankshaft.

Make sure connecting rods and their caps are kept together, and that the caps are installed in the proper direction. On some engines like the Buick built 350 V8, the caps can only be installed one way.

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.
  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.
  4.  
  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.
  6.  
  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.
  8.  
  9. Check the specifications chart in this section 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.
  10.  
  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.
  12.  
  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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Fig. Fig. 23: Checking rod bearing clearance with Plastigage®



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Fig. Fig. 24: Apply a strip of gauging material to the bearing, then install the cap and torque the cap



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Fig. Fig. 25: After the bearing cap has been removed, use the gauge supplied with the gauge material to check bearing clearances

INSTALLATION



See Figures 26, 27, 28, 29 and 30

Install the connecting rod to the piston, making sure 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.
  2.  
  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 high quality assembly lube (not engine oil).
  4.  
  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(es).
  6.  
  7. From beneath the engine, coat each crank journal with assembly lube. Pull the connecting rod, with the bearing shell in place, into position against the crank journal.
  8.  
  9. Remove the rubber hoses. Install the bearing cap and cap nuts and torque to specification. On the 1988-1990 5.0 L V8, torque the nuts to 18 ft. lbs (24 Nm), then turn the nuts an additional 70 degrees.
  10.  

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 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.
  2.  
  3. Install the oil pump and oil pan.
  4.  
  5. Install the cylinder head(s) and intake manifold.
  6.  



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Fig. Fig. 26: Piston positioning in block, diesels up to 1981. On 1982 and later models face the notches on the pistons towards the front



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Fig. Fig. 27: Using a wooden hammer handle, gently tap the piston down through the ring compressor and into the cylinder



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Fig. Fig. 28: Checking the connecting rod side clearance with a feeler gauge



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Fig. Fig. 29: Match the tang on the bearing insert with the tang on the bearing cap



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Fig. Fig. 30: Be sure to torque the bearing caps to specification

 
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