GM Full Size Vans 1967-1986 Repair Guide

Pistons and Connecting Rods

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REMOVAL & INSTALLATION



See Figures 1 through 10



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Fig. Fig. 1: View of the ridge formed by the piston rings at the top of their travel



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Fig. Fig. 2: Place rubber hose over the connecting rod studs to protect the crank and bores from damage



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Fig. Fig. 3: Carefully tap the piston out of the bore using a wooden dowel



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Fig. Fig. 4: Match the connecting rods to their cylinders and caps with a number stamp



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Fig. Fig. 5: Most pistons are marked to indicate positioning in the engine



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Fig. Fig. 6: Piston and rod positioning-250 and 292 inline six cylinder engines



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Fig. Fig. 7: Piston and rod positioning-small block V8 engine



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Fig. Fig. 8: Installing the piston into the block using a ring compressor and the handle of a hammer



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Fig. Fig. 9: The notch on the the side of the bearing cap matches the groove on the bearing insert



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Fig. Fig. 10: Use a prytool to spread the connecting rods, then use a feeler gauge to check the connecting rod side clearance

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 traveling 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. (9.53mm) 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.
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  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 hand
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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.

On all V8s, starting at the front the right bank cylinders are 2-4-6-8 and the left bank 1-3-5-7. On the V6 engine even number cylinders 2-4-6 are in the right bank, odd number cylinders 1-3-5 are in the left bank, when viewed from the rear of the engine.

  1. Remove remaining pistons in similar manner.
  2.  

On all gasoline 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 on 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.

To install:

In 1985 GM introduced engines with silicone coated pistons. If your engine has these pistons, if replaced, they must be replaced with silicone coated pistons. Substituting another type of piston could reduce the life of the engine.

Install the connecting rod to the piston, making sure piston installation notches and any marks on the rod are in proper relation to one another. Lubricate the wrist pin with clean engine oil, and install the pin into the rod and piston assembly, either by hand or by using a wrist pin press as required. Install snaprings if equipped, and rotate them in their grooves to make sure they are seated. To install the piston and connecting rod assembly:

  1. Make sure connecting rod big end bearings (including end cap) are of the correct size and properly installed.
  2.  
  3. Fit rubber hoses over the connecting rod bolts to protect the crankshaft journals, as in the Piston Removal procedure. Coat the rod bearings with clean oil.
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  5. Using the proper ring compressor, insert the piston assembly into the cylinder so that the notch in the top of the piston faces the front of the engine (this assumes that the dimple(s) or other markings on the connecting rods are in correct relation to the piston notch(s).
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  7. From beneath the engine, coat each crank journal with clean oil. Pull the connecting rod, with the bearing shell in place, into position against the crank journal.
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  9. Remove the rubber hoses. Install the bearing cap and cap nuts and torque to specification.
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When more than one rod and piston assembly is being installed, the connecting rod cap attaching nuts should only be tightened enough to keep each rod in position until all have been installed. This will ease the installation of the remaining piston assemblies.

  1. 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 clearance is below the minimum tolerance, the rod may be machined to provide adequate clearance. If clearance is excessive, substitute an unworn rod, and recheck. If clearance is still outside specifications, the crankshaft must be welded and reground or replaced.
  2.  
  3. Replace the oil pump if removed and the oil pan.
  4.  
  5. Install the cylinder head(s) and intake manifold.
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