See Figures 1, 2, 3, 4, 5 and 6
Although in some cases the pistons and connecting rods may be removed with the engine still in the vehicle, it is rarely worth the aggravation, especially when you are not working with a lift. On vehicles where this is possible (cylinder head and oil pan removal are both possible with the engine installed and there is sufficient working clearance) take EXTREME care to assure no dirt or contamination is allowed into the cylinders during assembly and installation.
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. If 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 moving the piston. For more details, refer to the ridge removal and honing procedures later in this section.
- Remove cylinder head or heads.
- Remove the oil pan.
- If necessary, remove the oil pump assembly.
- 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 reference during assembly.
The cylinders on a 4-cylinder engine are numbered 1-2-3-4 (front-to-rear); on the V6 2.8L engine, are numbered 1-3-5 (front-to-rear) on the right-side and 2-4-6 (front-to-rear) on the left-side and on the V6 4.3L, are numbered 2-4-6 (front-to-rear) on the right side and 1-3-5 (front-to-rear) on the left side.
- Cut lengths of 3 / 8 in. (10mm) 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.
- 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.
- Place the rod bearing and cap back on the connecting rod, and install the nut 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.
- Remove remaining pistons in similar manner.
- Clean and inspect the engine block, the crankshaft, the pistons and the connecting rods.
CLEANING AND INSPECTION
See Figures 7, 8, 9, 10, 11 and 12
A piston ring expander is necessary for removing piston rings without damaging them; any other method (screwdriver blades, pliers, etc.) usually results in the ring being bent, scratched or distorted, or the piston itself being damaged. When the rings are removed, clean the piston 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 solvent.
Clean the varnish from the piston skirts and pins with a cleaning solvent. DO NOT WIRE BRUSH ANY PART OF THE PISTON. Clean the ring grooves with a groove cleaner and make sure that the oil ring holes and slots are clean.
Inspect the piston for cracked ring lands, scuffed or damaged skirts, eroded areas at the top of the piston. Replace the pistons that are damaged or show signs of excessive wear.
Inspect the grooves for nicks of burrs that might cause the rings to hang up.
Measure the piston in relation to cylinder diameter. Refer to the cylinder bore cleaning and inspection procedures later in this section.
Wash the connecting rods in cleaning solvent and dry with compressed air. Check for twisted or bent rods and inspect for nicks or cracks. Replace connecting rods that are damaged.Cylinder Bores
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 (90 degrees) to the piston pin, about 1-2 1 / 2 in. (25-64mm) 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 the clearance within specifications. If this is not possible obtain, the first oversize piston and hone the cylinder or (if necessary) bore the cylinder to size. Generally, if the cylinder bore is tapered more than 0.005 in. (0.127mm) or is out-of-round more than 0.003 in. (0.0762mm), it is advisable to rebore for the smallest possible oversize piston and rings. After measuring, mark the pistons with a felt-tip pen for reference and for assembly.
Boring of the cylinder block should be performed by a reputable 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.
RIDGE REMOVAL & HONING
See Figures 13, 14 and 15
- Before the piston is removed from the cylinder, check for a ridge at the top of the cylinder bore. This ridge occurs because the piston ring does not travel all the way to the top of the bore, thereby leaving an unworn portion of the bore surface.
- Clean away any carbon buildup at the top of the cylinder with sand paper, in order to see the extent of the ridge more clearly. If the ridge is slight, it will be safe to remove the pistons without damaging the rings or piston ring lands. If the ridge is severe, and easily catches your fingernail, it will have to be removed using a ridge reamer.
A severe ridge is an indication of excessive bore wear. Before removing the piston, check the cylinder bore diameter with a bore gauge, as explained in the cleaning and inspection procedure. Compare your measurement with engine specification. If the bore is excessively worn, the cylinder will have to bored oversize and the piston and rings replaced.
- Install the ridge removal tool in the top of the cylinder bore. Carefully follow the manufacturers instructions for operation. Only remove the amount of material necessary to remove the ridge. 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 moving the piston.
- After the piston and connecting rod assembly have been removed, check the clearances as explained earlier in this section under the cleaning and inspection procedure, to determine whether boring and honing or just light honing are required. If boring is necessary, consult an automotive machine shop. If light honing is all that is necessary, proceed with the next step.
- Honing is best done with the crankshaft removed, to prevent damage to the crankshaft and to make the post-honing cleaning easier, as the honing process will scatter metal particles. However, if you do not want to remove the crankshaft, position the connecting rod journal for the cylinder being honed as far away from the bottom of the cylinder bore as possible, and wrap a shop cloth around the journal.
- Honing can be done either with a flexible glaze breaker type hone or with a rigid hone that has honing stones and guide shoes. The flexible hone removes the least amount of metal, and is especially recommended if your piston-to-cylinder bore clearance is on the loose side. The flexible hone is useful to provide a finish on which the new piston rings will seat. A rigid hone will remove more material than the flexible hone and requires more operator skill.
- Regardless of which type of hone you use, carefully follow the manufacturers instructions for operation.
- The hone should be moved up and down the bore at sufficient speed to obtain a uniform finish. A rigid hone will provide a definite cross-hatch finish; operate the rigid hone at a speed to obtain a 45-65 degree included angle in the cross-hatch. The finish marks should be clean but not sharp, free from embedded particles and torn or folded metal.
- Periodically during the honing procedure, thoroughly clean the cylinder bore and check the piston-to-bore clearance with the piston for that cylinder.
- After honing is completed, thoroughly wash the cylinder bores and the rest of the engine with hot water and detergent. Scrub the bores well with a stiff bristle brush and rinse thoroughly with hot water. Thorough cleaning is essential, for if any abrasive material is left in the cylinder bore, it will rapidly wear the new rings and the cylinder bore. If any abrasive material is left in the rest of the engine, it will be picked up by the oil and carried throughout the engine, damaging bearings and other parts.
- After the bores are cleaned, wipe them down with a clean cloth coated with light engine oil, to keep them from rusting.
PISTON PIN REPLACEMENT
See Figures 16 and 17
Most engines covered by this information utilize a piston and pin assembly which is pressed together. Disassembly will require an arbor press, a suitable support and the proper drivers.
The following procedure requires the use of the GM fixture/support assembly tool No. J-24086-20, GM piston pin removal tool No. J-24086-8, and the GM piston pin installation tool No. J-24086-9 or the equivalents.
Use care at all times when handling and servicing the connecting rods and pistons. To prevent possible damage to these units, DO NOT clamp the rod or piston in a vise since they may become distorted. DO NOT allow the pistons to strike one another, against hard objects or bench surfaces, since distortion of the piston contour or nicks in the soft aluminum material may result.
- Using an arbor press, the GM fixture/support assembly tool No. J-24086-20 or equivalent, and the GM piston pin removal tool No. J-24086-8 or equivalent, place the piston assembly in the fixture/support tool and press the pin from the piston assembly.
The piston and the piston pin are a matched set which are not serviced separately.
- Using a suitable solvent, wash the varnish and oil from the parts, then inspect the parts for scuffing or wear.
- Using a micrometer, measure the diameter of the piston pin. Using an inside micrometer or a dial bore gauge, measure the diameter of the piston bore.
If the piston pin-to-piston clearance is in excess of 0.001 in. (0.0254mm), replace the piston and piston pin assembly.To install:
- Lubricate the piston pin and the piston bore with engine oil.
- Install the piston pin into the piston assembly, use an arbor press, the GM fixture/support assembly tool No. J-24086-20 or equivalent, and the GM piston pin installation tool No. J-24086-9 or equivalent, then press the piston pin into the piston/connecting rod assembly.
When installing the piston pin into the piston/connecting rod assembly and the installation tool bottoms onto the support assembly, DO NOT exceed 5000 lbs. of pressure or structural damage may occur to the tool.
- After installing the piston pin, make sure that the piston has freedom of movement with the piston pin. The piston/connecting rod assembly is ready for installation into the engine block.
See Figure 18
- Using a pair of snapring pliers, remove the pin snapring from the piston.
- Slide the piston pin from the connecting rod and piston assembly.
When separating the piston from the connecting rod, be sure to mark them for reassembly purposes.
- Clean, then inspect both the piston and the connecting rod bearing surfaces for damage and/or wear; if necessary, replace the damaged part.
- Lubricate the piston pin and bearing surfaces with clean engine oil.
- Installation is the reverse of removal.
PISTON RING REPLACEMENT
Piston Ring End-Gap
Piston ring end-gap should be checked while the rings are removed from the pistons. Incorrect end gap indicates the wrong size rings are being used; ring breakage could occur and engine damage could result.
Squirt clean engine oil into the cylinder, then carefully compress and insert the piston rings to be used in a cylinder, one at a time, into that cylinder. Position the rings approximately 1 in. (25.4mm) below the deck of the block at a point where bore diameter is smallest. The ring can be carefully positioned using the top of the piston, this will assure that the ring is properly squared to the cylinder walls. Measure the ring end-gap with a feeler gauge, and compare to the piston and ring chart earlier in this chapter. Carefully pull the ring out of the cylinder and, if a larger clearance is necessary, file the ends squarely with a fine file to obtain the proper clearance.Piston Ring Side Clearance Check
See Figure 19
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 used pistons, 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. 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 clearance will be excessive. Piston rings are not furnished in oversize widths to compensate for ring groove wear.Ring Installation
See Figures 20, 21, 22, 23, 24, 25 and 26
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.
CONNECTING ROD BEARING REPLACEMENT
See Figures 26, 27, 28 and 29
Connecting rod bearings for the engines covered in this guide consist of two halves or shells which are usually 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.
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.
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® or an equivalent gauging material 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).
- Remove the rod cap with the bearing shell. Completely clean the bearing shell and the crank journal, then blow any oil from the oil hole in the crankshaft; Plastigage® is soluble in oil.
- Place a piece of Plastigage® lengthwise along the bottom center of the lower bearing shell, then install the cap with shell and torque the bolts 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 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.
- Check the specifications chart earlier this section for the desired clearance. It is advisable to install a new bearing if clearance exceeds 0.003 in. (.0762mm); 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® and oil the bearing thoroughly, then reinstall the cap with its shell and torque the rod fasteners 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.
See Figures 30, 31 and 32
The following procedure requires the use of a ring compressor tool such as J-8037 or equivalent, and the ring expander (removal/installation) tool mentioned earlier.
Assemble the piston, connecting rod and rings as detailed earlier in this section.
- Make sure that the connecting rod big-end bearings (including the end cap) are of the correct size and are properly installed.
- Fit rubber hoses over the connecting rod bolts to protect the crankshaft journals, as done during removal. Coat the rod bearings with clean oil.
It is a good idea to lightly coat the walls of the cylinder bore with clean engine oil during assembly.
- Using the proper ring compressor tool, compress the rings around the piston head, then insert the piston assembly into the cylinder so that the notch (or other mark) 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 marking(s). The marks made during disassembly should serve as a guide here.
- 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 from the studs. Install the bearing cap (with bearing shell) onto the connecting rod and the cap nuts. Torque the connecting rod cap nuts to the following:
- 1.9L engine: 43 ft. lbs. (59 Nm).
- 2.0L engine: 36 ft. lbs. (49 Nm)
- 2.2L diesel: 62 ft. lbs. (84 Nm)
- 2.5L engine: 32 ft. lbs. (43 Nm)
- 2.8L engine: 39 ft. lbs. (53 Nm)
- 4.3L engine: 45 ft. lbs. (61 Nm) for vehicles through 1990 or to 20 ft. lbs. (27 Nm), plus a 70 degree additional turn for 1991-93 vehicles.
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.
- 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 and 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.
- If removed, install the oil pump
- Install the oil pan.
- Install the cylinder head(s), as applicable.
- If removed, install the engine to the vehicle.