GM Caprice 1990-1993 Repair Guide

Pistons and Connecting Rods


See Figures 1 and 2

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Fig. Fig. 1: Cylinder bore ridge

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Fig. Fig. 2: Pushing the piston assembly from the engine using a hammer handle


Before removal of piston(s), connecting rod(s) and cap(s), mark them with their respective cylinder numbers. Place the mark on the side of the connecting rod as in the figure and also on the top of the piston, nearest to the front of the engine. This will ensure a proper match during reinstallation.

This procedure is easily completed if the engine has been removed from the car.

  1. Remove the cylinder head(s), intake manifold, exhaust manifold, oil pan, and oil pump as outlined in this Section.
  3. Mount the engine on a stand. In order to facilitate removal of the piston and connecting rod, the ridge at the top of the cylinder (unworn area; see illustration) must be removed. Place the piston at the bottom of the bore, and cover it with a rag. Cut the ridge away using a ridge reamer, exercising extreme care to avoid cutting too deeply. Remove the rag, and remove cuttings that remain on the piston.

If the ridge is not removed, and new rings are installed, damage to rings will result.

  1. Remove the connecting rod bearing caps and bearings.
  3. Install a section of rubber hose over the connecting rod bolts to prevent damage to the crankshaft.
  5. Slide the piston/connecting rod assembly through the top of the cylinder block.

To avoid damaging the rings, do not attempt to force the piston past the cylinder ridge (see above).


See Figure 3

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Fig. Fig. 3: Install the pistons with the notch facing the front of the engine and the oil bearing tang slots facing the opposite side of the camshaft

Most pistons are notched or marked to indicate which way they should be installed. If your pistons are not marked, mark the top front of the piston and the connecting rods with there respective cylinder number before removal. Then reinstall them in the proper position.


See Figures 4 through 8

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Fig. Fig. 4: Piston ring remover

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Fig. Fig. 5: Ring groove cleaner removes carbon deposits

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Fig. Fig. 6: Cylinder bore measuring points

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 broken or distorted, or the piston itself being damaged. When the rings are removed, clean the ring grooves using a ring groove cleaning tool, using care not to cut too deeply. Thoroughly clean all carbon and varnish from the piston with solvent.

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

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

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

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Fig. Fig. 8: Correct cylinder bore honing pattern

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 inch below the cylinder block deck (surface where the block mates with the heads). Then, with the micrometer, measure the piston perpendicular and a 1 / 2 inch below the 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 hone (or if necessary, bore) the cylinder to size. Generally, if the cylinder bore is tapered 0.005inch or more, or is out-of-round 0.003inch or more, it is advisable to re-bore for the smallest possible oversize piston and rings. After measuring, mark pistons with a felt-tip pen for reference and for assembly.

Cylinder block boring should be performed by a reputable machine shop with the proper equipment. In some cases, cleanup honing can be done with the cylinder block in the car, but most excessive honing and all cylinder boring must be done with the block stripped and removed from the car.


See Figure 9

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Fig. Fig. 9: Checking the piston ring end gap

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 inches of cylinder wall are coated. Using an inverted piston, press the rings approximately 1inch below the deck of the block. Measure the ring end gap with a feeler gauge, and compare to the Piston and Ring Specification chart in this Section. Carefully pull the ring out of the cylinder. If clearance is too tight, file the ends squarely with a fine file to obtain the proper clearance. If clearance is too large, try too obtain oversized rings.


See Figure 10

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Fig. Fig. 10: Piston ring end gaps in correct locations - V6 engine illustrated; V8 engine the same

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 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.


The piston pins are made of chromium steel. The fit within the piston is floating and the connecting rod is pressed on all except the 5.0L (VIN Y) engine. The fit within the piston and rod is floating on the 5.0L (VIN Y) engine.

Pin replacement should only be attempted by a qualified machine shop.