GM Cadillac 1967-1989 Repair Guide

Pistons and Connecting Rods

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REMOVAL & INSTALLATION



See Figures 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10

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 travelling 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.

  1. Remove intake manifold and cylinder head or heads.
  2.  
  3. Remove oil pan.
  4.  
  5. Remove oil pump assembly if necessary.
  6.  
  7. Matchmark the connecting rod cap to the connecting rod with a scribe; each cap must be reinstalled on its proper rod in the proper direction. Remove the connecting rod bearing cap and the rod bearing. Number the top of each piston with silver paint or a felt-tip pen for later assembly.
  8.  
  9. Cut lengths of 3 / 8 in. diameter hose to use as rod bolt guides. Install the hose over the threads of the rod bolts, to prevent the bolt threads from damaging the crankshaft journals and cylinder walls when the piston is removed.
  10.  
  11. Squirt some clean engine oil onto the cylinder wall from above, until the wall is coated. Carefully push the piston and rod assembly up and out of the cylinder by tapping on the bottom of the connecting rod with a wooden hammer handle.
  12.  
  13. Place the rod bearing and cap back on the connecting rod, and install the nuts temporarily. Using a number stamp or punch, stamp the cylinder number on the side of the connecting rod and cap this will help keep the proper piston and rod assembly on the proper cylinder.
  14.  

On V6 engines, starting at the front the cylinders are numbered 2-4-6 on the right bank and 1-3-5 on the left. On all 8-250, 273, 307, 350 and 429 engines, starting at the front the right bank cylinders are 2-4-6-8 and the left bank 1-3-5-7. On all other V8s, starting at the front the left bank cylinders are 2-4-6-8 and the right bank 1-3-5-7.

  1. Remove remaining pistons in similar manner.
  2.  

On all engines, the notch on the piston will face the front of the engine for assembly. The chamfered corners of the bearing caps should face toward the front of the left bank and toward the rear of the right bank, and the boss on the connecting rod should face toward the front of the engine for the right bank and to the rear of the engine of the left bank.

On various engines, the piston compression rings are marked with a dimple, a letter T , a letter O , GM or the word TOP to identify the side of the ring which must face toward the top of the piston.



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Fig. Fig. 1: Match the connecting rods to their caps with a scribe mark



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Fig. Fig. 2: Match the connecting rods to their cylinders with a number stamp



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Fig. Fig. 3: Connecting rod bolt guide



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Fig. Fig. 4: Ridge formed by piston rings at the top of their travel



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Fig. Fig. 5: Push the piston and rod out with a hammer handle



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Fig. Fig. 6: Piston to connecting rod positioning - 6-252 engine



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Fig. Fig. 7: Piston to connecting rod positioning - 8-250, 307, 350 and 429 engines



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Fig. Fig. 8: Piston to connecting rod positioning - 8-368, 425, 472 and 500 engines



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Fig. Fig. 9: Removing the connecting rod cap bolts



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Fig. Fig. 10: Removing the connection rod cap

CLEANING AND INSPECTING



See Figures 11, 12, 13, 14, 15, 16, 17, 18 and 19

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. Take 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 piston. Inspect the pistons for scuffing, scoring, cranks, 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 being 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 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.



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Fig. Fig. 11: Using a punch and hammer, the freeze plug can be loosened in the block



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Fig. Fig. 12: Once the freeze plug has been loosened, it can be removed from the block



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Fig. Fig. 13: Use a ring expander tool to remove the piston rings



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Fig. Fig. 14: Clean the piston grooves using a ring groove cleaner



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Fig. Fig. 15: A telescoping gauge may be used to measure the cylinder bore diameter



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Fig. Fig. 16: Measure the piston's outer diameter using a micrometer



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Fig. Fig. 17: Removing cylinder glazing using a flexible hone



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Fig. Fig. 18: A properly cross-hatched cylinder bore



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Fig. Fig. 19: Most rings are marked to show which side should face upward

 
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