GM Storm/Spectrum 1985-1993 Repair Guide

Pistons and Connecting Rods

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REMOVAL



See Figures 1 through 6

  1. Remove the engine from the vehicle and mount it on a suitable workstand.
  2.  
  3. Remove the cylinder head and the oil pan.
  4.  
  5. Check the connecting rod side clearance vertically along the side of the rod using a feeler gauge while moving the rod back and forth. The maximum allowable clearance is 0.158 in. (0.40mm). Replace the connecting rod assembly or the crankshaft to correct extreme clearance(s).
  6.  
  7. The position of each piston, connecting rod and connecting rod cap should be noted before any are removed, to assure that they can be reinstalled in the same location. Connecting rods and caps may be color coded or tagged during to prevent interchanging of parts. If necessary, prior to disconnecting the rods from the crankshaft, use a punch or numbering stamp to place match marks on the rod and cap to ensure correct reassembly.
  8.  
  9. Loosen and remove the connecting rod cap nuts, then tap the connecting rod bolts with a plastic-faced hammer to loosen the bearing cap. Remove the bearing cap from the connecting rod.
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  11. Clean the rod journal and bearing. Inspect the crank pin and bearing for pitting and scratches to determine if rod journals must be ground, or if the bearings and/or crankshaft must be replaced.
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  13. Lay a strip of Plastigage® across the rod journal, then install the rod cap and tighten the retainers to specification; refer to the torque chart. Remove the retainers and the rod cap, then determine oil clearance by comparing the widest point of the Plastigage® to the chart as per the manufacturers instructions. The maximum allowable clearance is 0.0047 in. (0.12mm). Completely remove the Plastigage® after the measurement has been taken.
  14.  
  15. Cover the connecting rod bolts with a short piece of 1 / 4 in. (6.34mm) inner diameter rubber hose to protect the crankshaft from scoring or damage. The connecting rod should not be removed until measurements are taken and the cylinder ridge, if present, has been removed.
  16.  
  17. Check the top of the cylinder liner for excessive carbon. If necessary, carefully remove the carbon deposits before attempting to remove the piston.
  18.  
  19. Using a hammer handle or piece of wood or plastic, tap the rod and piston upward in the bore until the piston rings clear the cylinder block. Remove the piston and connecting rod assembly from the top of the cylinder bore.
  20.  



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Fig. Fig. 1: Check the connecting rod side (thrust) clearance using a feeler gauge



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Fig. Fig. 2: Apply a strip of plastic-type gauging material to the rod journal



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Fig. Fig. 3: Install the bearing cap and torque to specification



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Fig. Fig. 4: Remove the bearing cap and compare the plastic strip to the gauge



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Fig. Fig. 5: Protect the crankshaft and cylinder walls from scoring or damage by installing short lengths of rubber hose over the connecting rod bolts before removing the piston



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Fig. Fig. 6: Using a piece of wood such as a hammer handle, gently tap the piston through the cylinder bore

CLEANING AND INSPECTION



See Figures 7 through 11

  1. Remove the piston rings using a piston ring expander. Refer to the Piston Ring Replacement procedure.
  2.  
  3. Clean the ring grooves with a ring groove cleaner or a broken piston ring, being careful not to cut into the piston metal. Heavy carbon deposits can be cleaned from the top of the piston with a wire brush, however, do not use a wire wheel on the ring grooves or lands. Clean the oil drain holes in the ring grooves. Clean all remaining dirt, carbon and varnish from the piston with a suitable solvent and a brush; do not use a caustic solution.
  4.  
  5. After cleaning, inspect the piston for scuffing, scoring, cracks, pitting or excessive ring groove wear. Replace any piston that is obviously worn.
  6.  
  7. If the piston appears okay, measure the piston diameter using a micrometer and note the measurement for calculations later. Measure the piston diameter at the grading position, located 1.12 in. (28.5mm) on 1.5L engines or 1.18 in. (30mm) on 1.6L and 1.8L engines, from the top of the piston, perpendicular to the piston pin bore.
  8.  
  9. Measure the cylinder bore diameter using a bore gauge, or using a telescope gauge and micrometer. Make 2 measurements longitudinal and transverse, then average the 2 results and record for calculations.
  10.  
  11. Subtract the piston diameter measurement made in Step 4 from the cylinder bore measurement made in Step 5 to determine the piston-to-bore clearance. Compare the clearance measurements with the specifications. If the clearance is within specification, light finish honing is all that is necessary. If the clearance is excessive, the cylinder must be bored and the piston replaced. Consult an automotive machine shop. If the pistons are replaced, the piston rings must also be replaced.
  12.  
  13. If the piston-to-bore clearance is okay, check the ring groove clearance. Insert the ring that will be used in the ring groove and check the clearance with a feeler gauge, as shown. Compare your measurement with specification. Replace the piston if the ring groove clearance is greater than 0.0059 in. (0.15mm).
  14.  
  15. Next, check the piston ring end gaps. This is done by inserting the ring into the cylinder bore and using the inverted piston to push the ring slightly beyond standard ring travel. The ring must be squared within the bore. Then use a feeler gauge to check the size of the ring gap. If the end gap is greater than 0.059 in. (1.5mm), replace the ring and remeasure. If the gap is still excessive, rebore the cylinder and install oversize pistons and rings.
  16.  
  17. Check the connecting rod for damage or obvious wear. Check for signs of fractures and check the bearing bore for out-of-round and taper.
  18.  
  19. A shiny surface on the pin boss side of the piston usually indicates that the connecting rod is bent or the wrist pin hole is not in proper relation to the piston skirt and ring grooves.
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  21. Abnormal connecting rod bearing wear can be caused by either a bent connecting rod, an improperly machined journal, or a tapered connecting rod bore.
  22.  
  23. Twisted connecting rods will not create an easily identifiable wear pattern, but badly twisted rods will disturb the action of the entire piston, rings, and connecting rod assembly and may be the cause of excessive oil consumption.
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  25. Remove and inspect the piston pin from the assembly; refer to the procedure later in this section. If measurements are good, but a connecting rod problem is still suspected, consult an automotive machine shop to have the rod checked further.
  26.  



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Fig. Fig. 7: The top of the piston may be cleaned using a wire brush



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Fig. Fig. 8: An old, broken ring may be used to clean the piston ring grooves



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Fig. Fig. 9: Measuring the piston outer diameter - Storm pistons shown



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Fig. Fig. 10: Measuring ring groove clearance



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Fig. Fig. 11: Checking piston ring end gap

PISTON PIN REPLACEMENT



See Figures 12, 13 and 14

  1. With the pistons removed from the block, check the fit between the piston and pin. Grasp the piston and try to move it back and forth on the pin. If movement is felt, remove the piston pin and connecting rod in order to check pin to connecting rod clearances.
  2.  
  3. If applicable, remove the snaprings from the piston and pin end.
  4.  
  5. Inspect the piston bore for burrs and remove, if found, using a sharp blade. This will prevent piston pin scoring during removal.
  6.  
  7. Using a wooden block to support the piston, tap or press out the pin from the piston and connecting rod. For the 1.6L engines, a pin service tool kit is available and a press must be used to free the pins.
  8.  
  9. Using an outside micrometer, measure the piston pin diameter. Replace a piston pin whose diameter is less than 0.7075 in. (17.97mm) for all engines except the 1.8L whose piston pins may not be less than 0.7862 in. (19.97mm) in diameter.
  10.  
  11. Using a small hole gauge and outside micrometer, measure the connecting rod small end. Compare the measurements to the piston pin diameter and replace the rod and piston pin if the clearance exceeds 0.00161 in. (0.041mm).
  12.  

For the 1.5L engine, weigh the piston pin and connecting rods and compare the results. They should not vary more than 0.3527 oz. (10g) or the offending pin and/or rod must be replaced. Before attempting to install the pin, heat the connecting rod in an oven in order to expand the bore and ease installation. Be very careful to use special gloves and precautions when handling the hot connecting rods. Also, be very cautious of overprotective homemakers who have just cleaned their ovens.

  1. When ready for assembly, position the piston and connecting rod sideways on a block of wood.
  2.  
  3. Coat the pin with a thin layer of clean engine oil, and if present, align the pin forward marks with the piston forward marks.
  4.  
  5. Tap or press the pin through the piston and connecting rod. Proceed slowly to make sure the pin is properly routed through the bores. For the 1.6L engines, the service tool kit or equivalent must be used along with a base press assembly.
  6.  
  7. If applicable, install the pin retaining snaprings.
  8.  
  9. Make sure the piston and connecting rod move smoothly back and forth on the pin.
  10.  



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Fig. Fig. 12: Check the piston pin outer diameter using an outside micrometer



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Fig. Fig. 13: Measure the connecting rod small end and compare the measurement to the pin diameter in order to check clearance



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Fig. Fig. 14: Example of a piston pin removal/installation service tool set - 1.6L engines

PISTON RING REPLACEMENT



  1. Remove the rings from the piston using a piston ring expander tool.
  2.  
  3. Clean the piston ring grooves, check the piston-to-cylinder bore clearance and check the ring groove clearance as explained in the piston and connecting rod cleaning and inspection procedure.
  4.  
  5. After the cylinder bores have been finish honed and cleaned, check the piston ring end gap. Compress the piston rings to be used in the cylinder, one at a time, into that cylinder. Using an inverted piston, push the ring down into the cylinder bore area where normal ring wear is not encountered.
  6.  
  7. Measure the ring end gap with a feeler gauge and compare to specification. A gap that is too tight is more harmful than one that is too loose (If ring end gap is excessively loose, the cylinder bore is probably worn beyond specification).
  8.  
  9. If the ring end gap is too tight, carefully remove the ring and file the ends squarely with a fine file or replace the ring to obtain the proper clearance.
  10.  
  11. Install the rings on the piston, lowest rings first. Compression rings should always be installed using a piston ring expander. There is a high risk of breaking or distorting the compression rings if they are installed by hand.
  12.  
  13. Position each ring with it's gap about 90 degrees from the next ring.
  14.  

If the instructions on the ring packaging differ from this information regarding ring gap positioning, follow the ring manufacturer's instructions.

ROD BEARING REPLACEMENT



  1. Inspect the rod bearings for scoring, chipping or other wear.
  2.  
  3. Inspect the crankshaft rod bearing journal for wear. Measure the journal diameter in several locations around the journal and compare to specification. If the crankshaft journal is scored or has deep ridges, or its diameter is below specification, the crankshaft must be removed from the engine and reground. Consult an automotive machine shop.
  4.  
  5. If the crankshaft journal appears usable, clean it and the rod bearing shells until they are completely free of oil. Blow any oil from the oil hole in the crankshaft.
  6.  

The journal surfaces and bearing shells must be completely free of oil to get an accurate reading with Plastigage®.

  1. Place a strip of Plastigage® lengthwise along the crankshaft rod journal as done during disassembly, then install the cap with the shell and torque the connecting rod nuts to specification. Do not turn the crankshaft with the Plastigage® installed in the bearing.
  2.  
  3. Remove the bearing cap with the shell. The flattened Plastigage® will either be sticking to the bearing shell or the crankshaft journal.
  4.  
  5. Using the printed scale on the Plastigage® package, measure the flattened Plastigage® at its widest point. The number on the scale that most closely corresponds to the width of the Plastigage® indicates the bearing clearance in thousandths of an inch or hundredths of a millimeter.
  6.  
  7. Compare your findings with the bearing clearance specification. If the bearing clearance is excessive, the bearing must be replaced or the crankshaft must be ground and the bearing replaced.
  8.  
  9. After clearance measuring is completed, be sure to remove the Plastigage® from the crankshaft and/or bearing shell.
  10.  
  11. For final bearing shell installation, make sure the connecting rod and rod cap bearing saddles are clean and free of nicks or burrs. Install the bearing shells in the connecting rod, making sure the bearing shell tangs are seated in the notches.
  12.  

Be careful when handling any plain bearings. Your hands and the working area should be clean. Dirt is easily embedded in the bearing surface and the bearings are easily scratched or damaged.

INSTALLATION



See Figure 15

  1. Make sure the cylinder bore and crankshaft journal are clean.
  2.  
  3. Position the crankshaft journal at its furthest position away from the bottom of the cylinder bore.
  4.  
  5. Coat the cylinder bore and bearing faces with light coat of clean engine oil.
  6.  
  7. Make sure the rod bearing shells are correctly installed. Install the rubber hoses over the connecting rod bolts to protect the crankshaft during installation.
  8.  
  9. Make sure the piston rings are properly installed and the ring end gaps are correctly positioned. Install a piston ring compressor over the piston and rings and compress the piston rings into their grooves. Follow the ring compressor manufacturers instructions.
  10.  
  11. Place the piston and connecting rod assembly into the cylinder bore. Make sure the assembly is the correct one for that bore and that the piston and connecting rod are facing in the proper direction. Most pistons have an arrow or notch on the top of the piston to indicate which side should face the front of the engine.
  12.  
  13. Make sure the ring compressor is seated squarely on the block deck surface. If the compressor is not seated squarely, a ring could pop out from beneath the compressor and hang up on the deck surface, as the piston is tapped into the bore, possibly breaking the ring and/or scoring the cylinder bore as the broken piece is forced into the cylinder.
  14.  
  15. Make sure that the connecting rod is not hung up on the crankshaft counterweights and is in position to come straight on to the crankshaft.
  16.  
  17. Tap the piston slowly into the bore, making sure the compressor remains squarely against the block deck. When the piston is completely in the bore, remove the ring compressor.
  18.  
  19. Coat the crankshaft journal and the bearing shells with engine assembly lube or clean engine oil. Pull the connecting rod onto the crankshaft journal. After the rod is seated, remove the rubber hoses from the rod bolts.
  20.  
  21. Install the rod bearing cap. Match the marked cap to the marked connecting rod. Also, be sure to align the cap and rod tangs on the same side of the connecting rod bore. Lightly oil the connecting rod bolt threads, then install the cap nuts and tighten to specification.
  22.  
  23. After each piston and connecting rod assembly is installed, turn the crankshaft over several times and check for binding. If there is a problem and the crankshaft will not turn, or turns with great difficulty, it will be easier to find the problem (rod cap on backwards, broken ring, etc..) than if all the assemblies are installed.
  24.  
  25. 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 the clearance is below the minimum specification, the connecting rod will have to be removed and machined to provide adequate clearance. If the clearance is excessive, substitute an unworn rod and recheck. If the clearance is still excessive, the crankshaft must be welded and reground, or replaced.
  26.  
  27. Install the oil pan and cylinder head.
  28.  
  29. Install the engine in the vehicle.
  30.  



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Fig. Fig. 15: Tapping the piston into the cylinder bore

 
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