GM Full-Size Trucks 1980-1987 Repair Guide

Pistons and Connecting Rods

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PISTON REMOVAL



See Figures 1, 2, 3, 4, 5 and 6

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.
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  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 inch 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 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.
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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 V8s, starting at the front the right bank cylinders are 2-4-6-8 and the left bank 1-3-5-7.

  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 the 350 diesel, install each piston and rod in its respective cylinder bore so the valve depression in the top of the piston is towards the inner side of the engine. On the forward half of the engine (cylinders 1, 2, 3, 4) the large valve depression goes to the front. On the rear half, the large valve depression goes to the rear. On the 379 diesel, install the piston and rod assemblies with the rod bearing tang slots on the side opposite the camshaft.

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



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Fig. Fig. 2: Match the connecting rods to their caps with a scribe mark



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Fig. Fig. 3: Match the connecting rods to their cylinders with a number stamp



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Fig. Fig. 4: Connecting rod bolt guide



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



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Fig. Fig. 6: 350 diesel piston positioning

PISTON RING AND WRIST PIN REMOVAL



See Figures 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 and 17

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 degree) 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 to its wrist pin on the shirt. 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 inch or more or is out-of-round 0.003 inch 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. 7: 250 and 292 six cylinder piston and rod positioning



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Fig. Fig. 8: Small block V8 piston and rod positioning, V6 similar



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Fig. Fig. 9: Big block V8 piston and rod positioning



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Fig. Fig. 10: Piston ring and wrist pin assembly, 350 diesel shown. Gasoline engines similar



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Fig. Fig. 11: Use a ring expander to remove the piston rings



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Fig. Fig. 12: Install the piston lock-rings, if used



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Fig. Fig. 13: Clean the piston ring grooves using ring groove cleaner



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Fig. Fig. 14: Measuring the piston prior to fitting



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Fig. Fig. 15: Cylinder bore measuring points



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Fig. Fig. 16: Measuring cylinder bore with a dial gauge



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Fig. Fig. 17: Cylinder bore cross-hatching after honing

Piston Ring End Gap

See Figures 18, 19 and 20

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 inch of cylinder wall are coated. Using an inverted piston, press the rings approximately 1 inch 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. 18: Checking piston ring end gap with a feeler gauge



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Fig. Fig. 19: Ring gap location-all gasoline engines



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Fig. Fig. 20: V6 262 ring gap location

Piston Ring Side Clearance Check and Installation

See Figures 21 and 22

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. 21: Checking piston ring side clearance



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Fig. Fig. 22: 380 cu. in. diesel piston ring gap positioning

CONNECTING ROD BEARINGS



See Figures 23, 24 and 25

Connecting rod bearings for the engines covered in this guide consist of two halves or shells which are interchangeable in the rod and cap. When the shells are placed in position, the ends extend slightly beyond the rod and cap surfaces so that when the rod bolts are torqued the shells will be clamped tightly in place to insure positive seating and to prevent turning. A tang holds the shells in place.

The ends of the bearing shells must never be filed flush with the mating surface of the rod and cap.

If a rod bearing becomes noisy or is worn so that its clearance on the crank journal is sloppy, a new bearing of the correct undersize must be selected and installed since there is a provision for adjustment.


WARNING
Under no circumstances should the rod end or cap be filed to adjust the bearing clearance, nor should shims of any kind be used.

Inspect the rod bearings while the rod assemblies are out of the engine. If the shells are scored or show flaking, they should be replaced. If they are in good shape check for proper clearance on the crank journal (see below). Any scoring or ridges on the crank journal means the crankshaft must be replaced, or reground and fitted with undersized bearings.

Checking Bearing Clearance and Replacing Bearings

Make sure connecting rods and their caps are kept together, and that the caps are installed in the proper direction.

Replacement bearings are available in standard size, and in undersizes for reground crankshafts. Connecting rod-to-crankshaft bearing clearance is checked using Plastigage® at either the top or bottom of each crank journal. The Plastigage® has a range of 0.001-0.003 inch.

  1. Remove the rod cap with the bearing shell. Completely clean the bearing shell and the crank journal, and blow any oil from the oil hole in the crankshaft; Plastigage® is soluble in oil.
  2.  
  3. Place a piece of Plastigage® lengthwise along the center of the crank journal, then install the cap with shell and torque the bolt or nuts to specification. DO NOT turn the crankshaft with Plastigage® in the bearing.
  4.  



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Fig. Fig. 23: Apply a strip of Plastigage® or equivalent material across the crank journal



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Fig. Fig. 24: Install and torque the bearing to specification



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Fig. Fig. 25: Remove the bearing and compare the width of the line to the scale supplied in the kit

  1. Remove the bearing cap with the shell. The flattened Plastigage® will be found sticking to either the bearing shell or crank journal. Do not remove it yet.
  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 bearing clearance in thousandths of an inch.
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  5. Check the specifications chart in this section for the desired clearance. It is advisable to install a new bearing if clearance exceeds 0.003 inch; however, if the bearing is in good condition and is not being checked because of bearing noise, bearing replacement is not necessary.
  6.  
  7. If you are installing new bearings, try a standard size, then each undersize in order until one is found that is within the specified limits when checked for clearance with Plastigage®. Each undersize shell has its size stamped on it.
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  9. When the proper size shell is found, clean off the Plastigage®, oil the bearing thoroughly, reinstall the cap with its shell and torque the rod bolt nuts to specification.
  10.  

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.

PISTONS & CONNECTING ROD ASSEMBLY AND INSTALLATION



See Figures 26, 27 and 28

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 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. Coat the rod bearings with clean oil.
  4.  



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Fig. Fig. 26: Using a wooden hammer handle, tap the piston down through the ring compressor and into the cylinder

  1. 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(es).
  2.  
  3. 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.
  4.  
  5. Remove the rubber hoses. Install the bearing cap and cap nuts and torque to specification.
  6.  

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.  



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Fig. Fig. 27: Checking rod side clearance using a feeler gauge

  1. Replace the oil pump, if removed, and the oil pan.
  2.  
  3. Install the cylinder head(s) and intake manifold.
  4.  



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Fig. Fig. 28: Checking connecting rod side clearance with a feeler gauge. Use a small pry to carefully spread the connecting rods

 
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