GM Cadillac 1967-1989 Repair Guide

Piston Ring and Wrist Pin

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See Figures 1, 2, 3, 4, 5, 6, 7 and 8REMOVAL

Some of the engines covered in this guide utilize pistons with pressed-in wrist pins; these must be removed by a special press designed for this purpose. Other pistons have their wrist pins secured by snaprings, which are easily removed with snapring pliers. Separate the piston from the connecting rod.

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 bent, scratched or distorted, or the piston itself being damaged. When the rings are removed, clean the ring 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.


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

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

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%) to the piston pin, 2 1 / 2 in. (63.5mm) 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 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 0.005 in. (0.13mm) or more or is out-of-round 0.003 in. (0.08mm) or more, it is advisable to rebore for the smallest possible oversize piston and rings.

After measuring, mark pistons with a felt-tip pen for reference and for assembly.

Cylinder honing and/or boring should be performed by a reputable, professional mechanic with the proper equipment. In some cases, clean-up 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.



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Fig. Fig. 1: Common piston rings and wrist pin



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Fig. Fig. 2: Remove the piston rings



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Fig. Fig. 3: Install the piston lock-rings, if used



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Fig. Fig. 4: Clean the piston ring grooves using a ring groove cleaner



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Fig. Fig. 5: Measuring the piston prior to fitting



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Fig. Fig. 6: Cylinder bore measuring points



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



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Fig. Fig. 8: Cylinder bore cross-hatching after honing

PISTON RING END GAP



See Figures 9, 10 and 11

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 in. (50mm) of cylinder wall are coated. Using an inverted piston, press the rings approximately 1 in. (25mm) below the deck of the block (on diesels, measure ring gap clearance with the ring positioned at the bottom of ring travel in the bore). Measure the ring end gap with a feeler gauge, and compare to the Ring Gap chart in this section. Carefully pull the ring out of the cylinder and file the ends squarely with a fine file to obtain the proper clearance.



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Fig. Fig. 9: Checking piston ring end gap with a feeler gauge



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Fig. Fig. 10: Piston ring gap location - 6-252 engine



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Fig. Fig. 11: Piston ring gap location - all except 6-252 engine

PISTON RING SIDE CLEARANCE CHECK & INSTALLATION



See Figure 12

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 gasoline engines, 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. On diesels, insert the gauge between the ring and the upper 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.



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Fig. Fig. 12: Checking ring side clearance

 
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