GM Caprice 1990-1993 Repair Guide

Crankshaft and Main Bearings



Crankshaft servicing literally makes or breaks any engine; especially a high performance one. The most critical maintenance operation is the replacement of the crankshaft main bearings. These bearings are of the precision insert design and do not require adjustment through shims. They are offered in several undersizes.

Despite the advent of these inserts and accompanying precision machine work, it does happen that sizing mistakes are made and no crankshaft should be installed in a block without checking clearances. One of the simplest means of doing so is to use Plastigage®. This is a wax-like plastic material that is formed into precision threads. It will compress evenly between two surfaces, without damage, and when measured, will indicate the actual clearance.

It is easiest to check bearing clearance with the engine removed from the car and the block inverted. This ensures that the crank is resting against the upper bearing shells. If Plastigage® is to be used on an engine still in the vehicle, it will be necessary to support the crankshaft at both ends so that clearance between the crankshaft and the upper bearing shells is eliminated.


See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Support the connecting rods with rubber bands and install rubber hose over the cap bolts during crankshaft removal and installation

  1. Drain the engine oil and remove the engine from the car. Mount the engine on a work stand in a suitable working area. Invert the engine, so the oil pan is facing up.

The EPA warns that prolonged contact with used engine oil may cause a number of skin disorders, including cancer! You should make every effort to minimize your exposure to used engine oil. Protective gloves should be worn when changing the oil. Wash your hands and any other exposed skin areas as soon as possible after exposure to used engine oil. Soap and water, or waterless hand cleaner should be used.

  1. Remove the engine water pump and front (timing) cover.
  3. Remove the timing chain (if equipped) and gears.
  5. Remove the oil pan.
  7. Remove the oil pump.
  9. Stamp or mark the cylinder number on the machined surfaces of the bolt bosses of the connecting rods and caps for identification when reinstalling. If the pistons are to be removed from the connecting rod, mark the cylinder number on the pistons with silver paint or felt-tip pen for proper cylinder identification and cap-to-rod location.
  11. Remove the connecting rod caps. Install lengths of rubber hose on each of the connecting rod bolts, to protect the crank journals when the crank is removed.
  13. Mark the main bearing caps with a number punch or punch so that they can be reinstalled in their original positions.
  15. Remove all main bearing caps.
  17. Note the position of the keyway in the crankshaft so it can be installed in the same position.
  19. Install rubber bands between a bolt on each connecting rod and oil pan bolts that have been reinstalled in the block (see illustration). This will keep the rods from banging on the block when the crank is removed.
  21. Carefully lift the crankshaft out of the block. The rods will pivot to the center of the engine when the crank is removed.


See Figure 2

Click image to see an enlarged view

Fig. Fig. 2: Measure the main bearing clearance by comparing the flattened strip of Plastigage with the scale

Like connecting rod big-end bearings, the crankshaft main bearings are shell-type inserts that do not utilize shims and cannot be adjusted. The bearings are available in various standard and undersizes; if main bearing clearance is found to be excessive, a new bearing (both upper and lower halves) is required.

It is not uncommon for the factory to use undersize bearings and sometimes mix some standard and undersize bearings to achieve a perfect fit.

Generally, the lower half of the bearing shell (except No. 1 bearing) shows greater wear and fatigue. If the lower half only shows the effects of normal wear (no heavy scoring or discoloration), it can usually be assumed that the upper half is also in good shape; conversely, if the lower half is heavily worn or damaged, both halves should be replaced. Never replace one bearing half without replacing the other.


Main bearing clearance can be checked both with the crankshaft in the car and with the engine out of the car. If the engine block is still in the car, the crankshaft should be supported both front and rear (by the damper and the transmission) to remove clearance from the upper bearing. Total clearance can then be measured between the lower bearing and journal. If the block has been removed from the car, and is inverted, the crank will rest on the upper bearings and the total clearance can be measured between the lower bearing and journal. Clearance is checked in the same manner as the connecting rod bearings, with Plastigage®.

Crankshaft bearing caps and bearing shells should NEVER be filed flush with the cap-to-block mating surface to adjust for wear in the old bearings. Always install new bearings.

  1. If the crankshaft has been removed, install it (block removed from car). If the block is still in the car, remove the oil pan and oil pump. Starting with the rear bearing cap, remove the cap and wipe all oil from the crank journal and bearing cap.
  3. Place a strip of Plastigage® the full width of the bearing, (parallel to the crankshaft), on the journal.

Plastigage® is soluble in oil; therefore, oil on the journal or bearing could result in erroneous readings.

Do not rotate the crankshaft while the gaging material is between the bearing and the journal.

  1. Install the bearing cap and evenly torque the cap bolts to specification.
  3. Remove the bearing cap. The flattened Plastigage® will be sticking to either the bearing shell or the crank journal.
  5. Use the graduated scale on the Plastigage® envelope to measure the material at its widest point. If the flattened Plastigage® tapers toward the middle or ends, there is a difference in clearance indicating the bearing or journal has a taper, low spot or other irregularity. If this is indicated, measure the crank journal with a micrometer.
  7. If bearing clearance is within specifications, the bearing insert is in good shape. Replace the insert if the clearance is not within specifications. Always replace both upper and lower inserts as a unit.
  9. Standard, 0.001inch or 0.002inch undersize bearings should produce the proper clearance. If these sizes still produce too sloppy a fit, the crankshaft must be reground for use with the next undersize bearing. Recheck all clearances after installing new bearings.
  11. Replace the rest of the bearings in the same manner. After all bearings have been checked, rotate the crankshaft to make sure there is no excessive drag. When checking the No. 1 main bearing, loosen the accessory