See Figures 1, 2 and 3
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. 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.
If you plan on consulting a machine shop for hot tanking, honing, boring or other block service, you should do so BEFORE disassembling your engine. They may have specific preferences on whether or not you remove a cylinder ridge or how to you are to label parts.
- Remove the engine assembly from the vehicle.
- Remove the intake manifold and the cylinder head(s).
- Remove the oil pan and the oil pump assembly.
- Check the connecting rods and caps for identification marks. If none are present, stamp the cylinder number on the machined surfaces of the bolt bosses on the connecting rod and cap for identification when reinstalling. If the pistons are to be removed from the connecting rod, mark the cylinder number on the piston with a silver pencil or quick drying paint for proper cylinder identification and cap to rod location.
The 2.5L (4-cyl) engine is numbered 1-2-3-4 (front-to-rear); on the 4.3L (V6) engine, is numbered 1-3-5 (front-to-rear) on the left side and 2-4-6 (front-to-rear) on the right side.
- Examine the cylinder bore above the ring travel. If a ridge exists, remove it with a ridge reamer before attempting to remove the piston and rod assembly.
- Remove the rod bearing cap and bearing.
- Install lengths of short rubber hose over the rod bolt threads; this will help prevent damage to the bearing journal and rod bolt threads.
- Remove the rod and piston assembly through the top of the cylinder bore by gently tapping outward using a wooden dowel or wooden tool handle; remove the other rod and piston assemblies in the same manner.
BE SURE to note the direction in which the piston was facing. These engines usually use pistons which are notched to show proper orientation. Make sure you note where any notch (if present) is located, or mark the piston using paint for installation purposes. Also, if you had to mark the piston, be careful during cleaning either not to accidentally remove the mark (when you are not paying attention), or to place a new mark on the piston once it has been cleaned.
CLEANING AND INSPECTION
Using a piston ring expanding tool, remove the piston rings from the pistons; any other method (screwdriver blades, pliers, etc.) usually results in the rings being bent, scratched or distorted and/or the piston itself being damaged.Pistons
See Figures 4, 5, 6 and 7
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 skirt (across the center line of the piston pin) and check the piston clearance (the difference between the measurement you get at the piston skirt and the measurement of the cylinder bore). Compare the clearance to the specifications found in the charts at the beginning of this section. If clearance is excessive, consult a machine shop about boring the cylinders for oversize pistons.Connecting Rods
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 any connecting rods that are damaged.Cylinder Bores
See Figures 8, 9 and 10
Using a telescoping gauge or an inside micrometer, measure the diameter of the cylinder bore, perpendicular (90°) to the piston pin, at 2 1 / 2 in. (63.5mm) below the surface of the cylinder block. The difference between the cylinder bore measurement and the piston skirt measurement 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 11, 12, 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.
- 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 manufacturer's instructions for operation. Remove only 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 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 and BEFORE the piston is checked for fit, 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
The following procedure requires the use of the GM Fixture/Support Assembly tool No. J-24086-20 or equivalent, the GM Piston Pin Removal tool No. J-24086-8 or equivalent, and the GM Piston Pin Installation tool No. J-24086-9 or equivalent.
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 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 a 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.
- Before installation, lubricate the piston pin and the piston bore with engine oil.
- To 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 psi (35,000 kPa) 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.
PISTON RING REPLACEMENT AND SIDE CLEARANCE MEASUREMENT
See Figures 18, 19, 20 and 21
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. Make sure the feeler gauge is inserted 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 been 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.
Once the ring end-gap has been checked and shown to be within specification, install the rings on the piston, bottom ring first, using a piston ring expander. There is a high risk of breaking or distorting the rings and/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 the proper oil retention and cylinder wear. When installing the new rings, refer to the installation diagram furnished with the new parts.
CHECKING RING END-GAP
See Figure 22
The 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 result .
- Compress the new piston ring into a cylinder (one at a time).
- Squirt some clean oil into the cylinder so that the ring and the top 2 in. (51mm) of the cylinder wall are coated.
- Using an inverted piston, push the ring approximately 1 in. (25.4mm) below the top of the cylinder.
- Using a feeler gauge, measure the ring gap and compare it to the Ring Gap chart in this section. Carefully remove the ring from the cylinder.
ROD BEARING REPLACEMENT
See Figures 23, 24, 25 and 26
Replacement bearings are available in standard size and undersize (for reground crankshafts). Connecting rod-to-crankshaft bearing clearance is checked using Plastigage® or an equivalent gauging material, at either the top or the 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, blow any oil from the oil hole in the crankshaft; place the Plastigage® lengthwise along the bottom center of the lower bearing shell, then install the cap with the shell and torque the bolt or nuts to specification. DO NOT turn the crankshaft with the Plastigage® on the bearing.
- Remove the bearing cap with the shell. The flattened Plastigage® will be found sticking to either the bearing shell or the crank journal. DO NOT remove it yet.
- Use the scale printed on the Plastigage® package 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 the 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 the 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.
- 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 tighten 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.
See Figures 21, 27, 28, 29 and 30
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.
Install the connecting rod to the piston, making sure that the piston installation notches and marks (if any) on the connecting rod are in proper relation to one another.
- Make sure that the connecting rod big-end bearings (including the end cap) 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. Lubricate the connecting rod bearings with clean engine oil.
- Using a suitable ring compressor positioned over the piston, compress the rings around the piston head. Insert the piston assembly into the cylinder, so that the notch (on top of the piston) faces the front of the engine.
- From beneath the engine, coat each crank journal with clean oil. Using a hammer handle, gently drive the connecting rod/piston assembly into the cylinder bore. Align the connecting rod (with bearing shell) onto the crankshaft journal.
Remove the rubber hoses from the studs. Install the bearing cap (with bearing shell) onto the connecting rod and the cap nuts. Tighten the connecting rod cap nuts to specification.
2.5L engine (1985-88): 32 ft. lbs. (43 Nm)
When more than one connecting rod/piston assembly are being installed, the connecting rod cap 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 to insert the feeler gauge. If the clearance is below the minimum tolerance, the rod may be machined to provide adequate clearance. If the clearance is excessive, substitute an unworn rod and recheck. If clearance is still outside specifications, the crankshaft must be welded and reground or replaced.
- Install the oil pump assembly and the oil pan.
- Install the cylinder head(s) and the intake manifold.
- Install the engine assembly to the vehicle.