GM Metro/Sprint 1985-1993 Repair Guide

Pistons and Connecting Rods

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This procedure assumes that the engine has already been partially disassembled, including removing the cylinder head and oil pan to allow access to the internal components. It is recommended that the engine be removed to facilitate servicing.

REMOVAL



  1. Remove the engine from the vehicle.
  2.  
  3. Remove the intake manifold and the cylinder head(s).
  4.  
  5. Remove the oil pan and the oil pump assembly.
  6.  
  7. Stamp the cylinder number on the machined surfaces of the bolt bosses on the connecting rod and cap for identification when reinstalling.
  8.  
  9. 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.
  10.  
  11. 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.
  12.  
  13. Remove the connecting rod bearing cap and bearing.
  14.  
  15. Install a 3 / 8 inch rubber guide hose over the rod bolt threads; this will prevent damage to the bearing journal and rod bolt threads.
  16.  
  17. Remove the rod and piston assembly through the top of the cylinder bore; remove the other rod and piston assemblies in the same manner.
  18.  
  19. Clean and inspect all components.
  20.  

CLEANING AND INSPECTION



Use 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

Clean the varnish from the piston skirts and pins with a cleaning solvent. Clean the ring grooves with a ring cleaner and make sure that the oil ring holes and slots are clean.

Do not wire brush any part of the piston.

Inspect the piston for cracked ring lands, scuffed or damaged skirts, eroded areas at the top of the piston. Inspect the grooves for nicks of burrs that might cause the rings to hang up. Replace the pistons that are damaged or show signs of excessive wear.

Measure the piston skirt perpendicular to the piston pin at a point approximately 1 / 2 inch up from the bottom of the position. Use this figure when checking the piston clearance. If installing replacement pistons, follow the manufacturers recommendations on where to measure the piston. After measuring, mark the pistons with a felt-tip pen for assembly reference.

Cylinder Bores

Using a telescoping gauge or an inside micrometer, measure the diameter of the cylinder bore, perpendicular (90°) to the piston pin, at 1-2 1 / 2 " below the surface of the cylinder block.

The difference between the piston outside diameter and the cylinder bore inside diameter is the piston clearance. If clearance exceeds specifications, the cylinder block must be bored to the next standard oversize and new pistons installed. If the clearance is within specifications or slightly below, finish honing is all that is necessary

Boring or honing 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.

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 the connecting rods that are damaged.

RIDGE REMOVAL AND HONING



Prior to removing the piston and rod assemblies, an inspection must be made to determine if a ridge exists at the top of the cylinder bore, above the travel of the piston.

  1. Use a dial bore gauge or inside micrometer to measure the difference between the ridge at the top of the bore and the portion of the bore where the piston travels.
  2.  
  3. If a ridge exists, it must be removed using a ridge reamer.
  4.  
  5. Place the piston at the bottom of its bore and cover it with a rag.
  6.  
  7. Cut the ridge away using a ridge reamer, exercising extreme care to avoid cutting too deeply.
  8.  
  9. Remove the rag and remove the cuttings that remain on the piston.
  10.  
  11. Using a hammer handle or a wooden bar, force the pistons out through the top of the cylinder block.
  12.  

Boring or honing of the cylinder block should be performed by a reputable machine shop with the proper equipment. In some cases, clean-up honing can be accomplished by the do-it-yourselfer, however, most boring or honing operations should be left to an experienced machine shop.

PISTON PIN REPLACEMENT



Non-Turbocharged Engine

See Figures 1 and 2



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Fig. Fig. 1: Piston pin replacement using a piston pin tool - Non-turbocharged engine



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Fig. Fig. 2: Correct position of the piston on the connecting rod

The piston pins on non-turbocharged engines are held in place by an interference fit with the connecting rod. A special tool, Piston Pin Remover/Installer J-34838, must be used to prevent damage to the piston when removing the pin.

  1. Fit the piston and connecting rod assembly into the piston pin remover so that the arrow mark on the piston head faces upward.
  2.  
  3. Place the piston pin remover and piston assembly into a press and press the piston pin out of the connecting rod.
  4.  

To install:
  1. After applying engine oil to piston pin holes in piston and connecting rod, fit connecting rod to piston with the dot on the connecting rod facing you and the arrow on the piston facing to the right.
  2.  
  3. Fit the piston and connecting rod assembly into the piston pin remover so that the arrow mark on the piston head faces upward.
  4.  
  5. Place the piston pin remover and piston assembly into a press and press the piston pin through the piston and connecting rod.
  6.  
  7. Press the piston pin until the line marked on the driver handle is flush with the flat surface of the piston.
  8.  

Turbocharged Engine

See Figures 2, 3 and 4



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Fig. Fig. 3: Piston and connecting rod assembly - Turbocharged engine



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Fig. Fig. 4: Removing the piston pin circlip with needle-nose pliers

The piston pins on turbocharged engines are of the full floating type and are held in place by circlips at the ends of the pin. Special tools are not needed to remove the piston pin.

  1. Use a ring expander to remove the piston rings.
  2.  
  3. Use needle-nose pliers to remove the circlip from one end of the piston.
  4.  
  5. Remove the piston pin and separate the piston from the connecting rod.
  6.  
  7. Remove the circlip from the opposite side of the piston.
  8.  

To install:
  1. Measure the inside diameter of the piston pin bore in the connecting rod. Inside diameter should be 0.7481-0.7485 in. (19.003-19.011mm).
  2.  
  3. Measure the outside diameter of the piston pin. Outside diameter should measure 0.7478-0.7481 in. (18.995-19.000mm).
  4.  
  5. The difference between the measurements is the piston pin-to-bore clearance. Clearance should be 0.0001-0.0006 in. (0.003-0.016mm).
  6.  
  7. Install the circlip at one end of the piston.
  8.  

Always use new circlips when reinstalling piston pins.

  1. Join the piston and connecting rod with the dot on the piston facing upward and the arrow on the piston facing the right.
  2.  
  3. Install the piston pin after lubricating with oil.
  4.  
  5. Install the circlip on the other end of the piston. Ensure that the circlip is fitted in piston groove securely.
  6.  
  7. Move the piston around to ensure it does not bind.
  8.  

PISTON RING REPLACEMENT



See Figures 5, 6 and 7



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Fig. Fig. 5: Measuring ring groove clearance



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Fig. Fig. 6: Measuring piston ring end gap



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Fig. Fig. 7: Install piston rings with the identification mark facing up

  1. Use a ring expander to remove the piston rings. Take care not to scratch the piston during ring removal.
  2.  
  3. Compress the piston ring into a cylinder (one at a time).
  4.  
  5. Squirt some clean oil into the cylinder so that the ring and the top 2" (51mm) of the cylinder wall are coated.
  6.  
  7. Using an inverted piston, push the ring approximately 1 inch(25.4mm) below the top of the cylinder.
  8.  
  9. Using a feeler gauge, measure the ring gap and compare it to the specifications. If ring gap is smaller than specifications, it may be filed to enlarge the gap. Although, this is rarely necessary if the correct rings are used.
  10.  
  11. Carefully remove the ring from the cylinder.
  12.  
  13. Check the pistons to see that the ring grooves and oil return holes have been properly cleaned.
  14.  
  15. 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.
  16.  
  17. 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.
  18.  

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.

  1. Install the rings on the piston, bottom ring first, using a piston ring expander. Place the identification mark on the ring upward.
  2.  

CONNECTING ROD BEARING REPLACEMENT



See Figures 2, 8 and 9

This procedure assumes that the engine has been disassembled and the connecting rod and piston assembly has been removed from the engine. However, the connecting rod bearing replacement can be performed with the engine partially disassembled and installed in the vehicle.

Replacement bearings are available in standard size and undersize (for reground crankshafts). Check the back of the existing bearing inserts to determine the undersize, if any, of the existing bearings.

Connecting rod-to-crankshaft bearing clearance is checked using Plastigage® at either the top or the bottom of each crank journal. The Plastigage® has a range of 0.001-0.003" (0.0254-0.0762mm).



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Fig. Fig. 8: Measuring rod bearing clearance with Plastigage®



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Fig. Fig. 9: Bearing undersize is stamped into the back of the insert

  1. Remove the rod cap from the connecting rod. Completely clean the bearing shell and the crank journal. Blow any oil from the oil hole in the crankshaft.
  2.  

Plastigage® will dissolve in oil or solvent. The components being checked must be dry and completely free of dirt, oil and solvent prior to checking the bearing clearance.

  1. Lightly lubricate the cylinder wall and install the piston and connecting rod assembly in the cylinder with the upper bearing insert in place.
  2.  

The piston rings do not have to be installed for this step as the assembly will be removed prior to finally assembly.

  1. Place the Plastigage® lengthwise along the bottom center of the lower bearing shell, then install the cap with the shell and torque nuts to 24-26 ft. lbs.. (33-35 Nm).
  2.  

Do not turn the crankshaft with the Plastigage® on the bearing.

  1. 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 the Plastigage® until measuring it with the scale on the package.
  2.  
  3. 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 the bearing clearance in thousandths of an inch.
  4.  
  5. Compare the measurement to specifications and replace the bearing if clearance is not within range. However, if the bearing is within specification and in good condition, bearing replacement is not necessary.
  6.  
  7. If the crankshaft is in good condition, install new standard size bearings and check the clearance again. If clearance is not within specification, undersize bearings are available.
  8.  
  9. After all measurements have been made, remove the piston and connecting rod assembly.
  10.  

INSTALLATION



See Figures 2, 10, 11, 12, 13, 14 and 15

This procedure assumes the crankshaft has already been installed in the engine. The crankshaft must be installed in the engine prior to installing the piston and connecting rod assemblies.

During installation, lubricate all moving components with oil. When the engine is first started, oil pressure will take time to build. The oil used to lubricate the engine during the first few seconds of operation must be placed on moving surfaces during installation.



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Fig. Fig. 10: 1985-91 piston ring end gap positions



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Fig. Fig. 11: 1992-93 piston ring end gap positions - Base and LSi models



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Fig. Fig. 12: 1992-93 piston ring end gap positions - XFi models



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Fig. Fig. 13: Install guide hoses on the connecting rod bolts to avoid damaging the crankshaft journals



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Fig. Fig. 14: Measuring connecting rod side clearance



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Fig. Fig. 15: Correct installation of connecting rod bearing caps

  1. Install the piston rings on the piston using a ring expander.
  2.  
  3. Install the upper rod bearing insert on the connecting rod.
  4.  
  5. Position the piston rings as shown in the illustration and install the ring compressor. Install rubber boots over the rod bolts, as required.
  6.  
  7. Install the piston into its cylinder with the arrow facing the crankshaft pulley side of the engine. The dot on the connecting rod should be facing upward.
  8.  
  9. Tap the piston into place using a wooden hammer handle. Remove the rubber boot, install the rod cap so that the numbers stamped into the connecting rod during removal align.
  10.  

Tap the piston and connecting rod assembly into the engine carefully. If the upper bearing insert should hit the crankshaft hard, it may damage the insert, causing premature bearing failure.

  1. Tighten the connecting rod nuts to 24-26 ft. lbs.. (33-35 Nm).
  2.  
  3. 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.
  4.  
  5. Measure the connecting rod side clearance using a feeler gauge and compare to specifications. If not within specifications inspect the crankshaft and connecting rod for excessive wear and replace as necessary.
  6.  

 
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