GM Prizm/Nova 1985-1993 Repair Guide

Piston Pin (Wrist Pin)



See Figures 1, 2 and 3

The piston and pin are a matched set and must be kept together. Label everything and store parts in identified containers.

  1. Remove the pistons from the engine and remove the rings from the pistons.
  3. Remove the snapring at the ends of the piston pin. This may be done with either snapring pliers or needle-nosed pliers; don't try to lever it out with a screwdriver.

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Fig. Fig. 1: Remove the circlip

  1. Support the piston and rod on its side in a press. Make certain the piston is square to the motion of the press and that the rod is completely supported with blocks. Leave open space below the piston for the pin to emerge.
  3. Line up the press and insert a brass rod of the same or slightly smaller diameter as the piston pin. It is important that the rod press evenly on the entire face of the pin, but not on the piston itself.
  5. Using smooth and controlled motion, press the pin free of the piston. Do not use sudden or jerky motions; the piston may crack.
  7. When reassembling, identify the front of the piston by its small dot or cavity on the top. Identify the front of the piston rod by the small mark cast into one face of the rod. Make sure the marks on the piston and rod are both facing the same direction. Also insure that the correct piston pin is to be reinstalled - they are not interchangeable.

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Fig. Fig. 2: Align the piston and connecting rod front marks before reassembly

  1. For the all engines except 1988 VIN Code 5 engine, install 1 snapring in the piston and insert the piston under the press with the snap-ring down. Position the rod and support it. Coat the piston pin with clean oil and press it into place, using the same press set-up as removal. The piston pin will bottom onto the snap-ring; don't force it beyond the stopping point. Install the other snapring to lock the pin in place.
  3. For the 1988 VIN Code 5 engine, install 1 snapring. Place the piston in water and gradually heat the water to the boiling point. DO NOT drop the piston into already hot water. The minimum required temperature is 176°F (80°C); a little hotter makes it a little easier. This will expand the piston so that the pin will fit smoothly. While the water is heating, apply a coat of clean oil to the piston pin and have the pin at hand when the piston is removed from the water.

You are dealing with hot metal and boiling water. Tongs, thick heat-resistant gloves and towels are required.

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Fig. Fig. 3: The piston must be heated to install the piston pin. At 176 degrees F, the pin should push in with thumb pressure - 1988 VIN Code 5 engine

Remove the piston from the boiling water (carefully!) and hold it with gloves or several towels. Making sure the front marks align on the piston and connecting rod, hold the rod in position and press the piston pin into place with your thumb. The pin will bottom against the snapring. Allow the piston to air cool and when it is cool to the touch, install the other snapring. Check that the piston rocks freely on its pin without binding.


See Figures 4, 5 and 6

Although a piston ring can be reused if in good condition and carefully removed, it is recommended that the rings be replaced with new ones any time they are removed from the pistons.

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 becoming bent, scratched or broken. When the rings are removed, clean the grooves thoroughly with a bristle brush and solvent. Make sure that all traces of carbon and varnish are removed.

Wear goggles during this cleaning; the solvent is very strong and can cause eye damage. Do not use a wire brush or a caustic solvent on the pistons.

Check the piston condition and diameter following procedures outlined earlier in this section. Piston ring end gap should be checked when the rings are removed from the pistons. Incorrect end gap indicates that the wrong size rings are being used; ring breakage could occur.

Squirt some clean oil into the cylinder so that the top 2 or 3 inches of the wall is covered. Gently compress one of the rings to be used and insert it into the cylinder. Use and upside-down piston and push the ring down about an inch below the top of the cylinder. Using the piston to push the ring keeps the ring square in the cylinder; if it gets crooked, the next measurement may be inaccurate.

Using a feeler gauge, measure the end gap in the ring and compare it to the Piston and Ring Specifications chart at the beginning of this section. If the gap is excessive, either the ring is incorrect or the cylinder walls are worn beyond acceptable limits. If the measurement is too tight, the ends of the ring may be filed to enlarge the gap after the ring is removed form the cylinder. If filing is needed, make certain that the ends are kept square and that a fine file is used.

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Fig. Fig. 4: Checking the piston ring end gap

Check the pistons to see that the ring grooves and oil return holes have been properly cleaned. Slide each piston ring into its groove and check the side clearance with a feeler gauge. Make sure you insert the feeler gauge between the ring and its lower edge; any wear that develops forms a step at the inner portion of the lower land. If the piston grooves have worn to the extent that fairly high steps exist on the lower land, the piston must be replaced. Rings are not sold in oversize thicknesses to compensate for ring groove wear.

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Fig. Fig. 5: Checking the piston ring side clearance

Using the ring expander, install the rings on the piston, lowest ring first . There is a high risk of ring breakage or piston damage if the rings are installed by hand or without the expander. The correct spacing of the ring end gaps is critical to oil control. No 2 gaps should align, they should be evenly spaced around the piston with the gap in the oil ring expander facing the front of the piston (aligned with the mark on the top of the piston). Once the rings are installed, the pistons must be handled carefully and protected from dirt and impact.

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Fig. Fig. 6: Piston ring positioning


See Figures 7, 8, 9 and 10

Connecting rod bearings on all Nova and Prizm engines consist of 2 halves or shells which are not interchangeable in the rod and cap. When the shells are in position, the ends extend slightly beyond the rod and cap surfaces so that when the bolts are tightened, the shells will be clamped tightly in place. This insures a positive seating and prevents turning. A small tang holds the shells in place within the cap and rod housings.

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Fig. Fig. 7: Examples of upper and lower bearing shells. Note the position of the oil hole

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

If a rod becomes noisy or is worn so that its clearance on the crankshaft is sloppy, a new bearing of the correct undersize must be selected and installed. There is no provision for adjustment. Under no circumstances should the rod end or cap be filed to compensate for wear, nor should shims of any type be used.

Inspect the rod bearings while the rods are out of the engine. If the shells are scored or show flaking they should be replaced. ANY scoring or ridge on the crankshaft means the crankshaft must be replaced. Because of the metallurgy in the crankshaft, welding and/or regrinding the crankshaft is not recommended. The bearing faces of the crank may not be restored to their original condition causing premature bearing wear and possible failure.

Replacement bearings are available in 3 standard sizes marked either 1, 2 or 3 on the bearing shell and possibly on the rod cap. Do not confuse the mark on the bearing cap with the cylinder number. It is quite possible that No. 3 piston rod contains a number 1 size bearing. The rod cap may have a 1 marked on it. (You should have stamped a 3 or other identifying code on both halves of the rod before disassembly.)

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Fig. Fig. 8: Look for the manufacturer's codes to identify standard bearing sizes. Don't confuse the number on the rod end cap with its position (cylinder) number

Measuring the clearance between the connecting rod bearings and the crankshaft (oil clearance) is done with a plastic measuring material such as Plastigage® or similar product.

  1. Remove the rod cap with the bearing shell. Completely clean the cap, bearing shells and the journal on the crankshaft. Blow any oil from the oil hole in the crank. The plastic measuring material is soluble in oil and will begin to dissolve if the area is not totally free of oil.
  3. Place a piece of the measuring material lengthwise along the bottom center of the lower bearing shell. Install the cap and shell and tighten the bolts in 3 passes to 36 ft. lbs.

Do not turn the crankshaft with the measuring material installed.

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Fig. Fig. 9: Plastic measuring material installed on the lower bearing shell

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Fig. Fig. 10: Measure the compressed plastic to determine the bearing clearance

  1. Remove the bearing cap with the shell. The flattened plastic material will be found sticking to either the bearing shell or the crank journal. DO NOT remove it yet.
  3. Use the scale printed on the packaging for the measuring material to measure the flattened plastic at its widest point. The number within the scale which is closest to the width of the plastic indicates the bearing clearance in thousandths of an inch.
  5. Check the specifications chart in the beginning of this section for the proper clearance. If there is any measurement is approaching the maximum acceptable value, replace the bearing.
  7. When the correct bearing is determined, clean off the gauging material, oil the bearing thoroughly on its working face and install it in the cap. Install the other half of the bearing into the rod end and attach the cap to the rod. Tighten the nuts evenly, in 3 passes to 36 ft. lbs.
  9. With the proper bearing installed and the nuts properly tightened, it should be possible to move the connecting rod back and forth a bit on the crankshaft. If the rod cannot be moved, either the bearing is too small or the rod is misaligned.