Chevrolet Chevette/1000 1976-1988 Repair Guide

Main Bearings (Gasoline Engine)

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Main bearings are of the precision insert type and do not utilize shims for adjustment. If clearances are found to be excessive, a new bearing, both upper and lower halves, will be repaired. Service bearings are available in standard, 0.026mm, 0.050mm, 0.250mm and 0.500mm undersizes.

Selective fitting of main bearing inserts is necessary in production in order to obtain close tolerances. For this reason you may find one half of a standard insert with one half of a 0.026mm undersize insert which will decrease the clearance 0.013mm from using a full standard bearing.

When a production crankshaft cannot be precision fitted by this method, it is then ground 0.250mm undersize ON ONLY THOSE MAIN JOURNALS THAT CANNOT BE PROPERLY FITTED, ALL JOURNALS WILL NOT NECESSARILY BE GROUND. A 0.250mm undersize bearing will then be used for precision fitting in the same manner as previously described.

INSPECTION



In general, the lower half of the bearing, except #1 bearing shows a greater wear and the most distress from fatigue. If upon inspection the lower half is suitable for use, it can be assumed that the upper half is also satisfactory. If the lower half shows evidence of wear or damage, both upper and lower halves should be replaced. Never replace one half without replacing the other half.

CHECKING CLEARANCE



To obtain the most accurate results with Plastigage® (or its equivalent), a wax-like plastic material which will compress evenly without damaging either surface, certain precautions should be observed.

If the engine is out of the vehicle and upside down, the crankshaft will rest on the upper bearings and the total clearance can be measured between the lower bearing and journal. If the engine is to remain in the vehicle, the crankshaft should be supported both front and rear to remove the clearance from the upper bearing. The total clearance can then be measured between the lower bearing and journal.

To assure the proper seating of the crankshaft, all bearing cap bolts should be at their specified torque. In addition, preparatory to checking fit of bearings, the surface of the crankshaft journal and bearing should be wiped clean of oil.

  1. Starting with the rear main bearing, remove bearing cap and wipe oil from journal and bearing cap.
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  3. Place a piece of gauging plastic the full width of the bearing (parallel to the crankshaft) on the journal. Do not rotate the crankshaft while the gauging plastic is between the bearing and the journal.
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  5. Install the bearing cap and evenly torque the retaining bolts to specifications. Bearing cap MUST be torqued to specifications in order to assure proper reading. Variations in torque affect the compression of the plastic gauge.
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  7. Remove the bearing cap. The flattened gauging plastic will be found adhering to either the bearing shell or journal.
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  9. On the edge of gauging plastic envelope there is a graduated scale which is correlated in thousandths of a millimeter. Without removing the gauging plastic, measure its compressed width (at the widest point) with the graduations on the gauging plastic envelope.
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Normally, main bearing journals wear evenly and are not out-of-round. However, if a bearing is being fitted to an out-of-round journal 0.025mm max, be sure to fir to the maximum diameter of the journal: If the bearing is fitted to the minimum diameter and the journal is out-of-round 0.025mm, interference between the bearing and journal will result in rapid bearing failure.

If the flattened gauging plastic tapers toward the middle or ends, there is a difference in clearance indicating taper, low spot or other irregularity of the bearing or journal. Be sure to measure the journal with a micrometer if the flattened gauging plastic indicates more than 0.025mm difference.

  1. If the bearing clearance is within specifications, the bearing insert is satisfactory. If the clearance is not within specifications, replace the insert. Always replace both upper and lower insert as a unit. If a new bearing cap is being installed and clearance is less than 0.025mm, inspect for burrs or nicks, if none are found then install shims as required.
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  3. A standard 0.025mm or 0.050mm undersize bearing may produce the proper clearance. If not, it will be necessary to regrind the crankshaft journal for use with the next undersize bearing. After selecting new bearing, recheck clearance.
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  5. Proceed to the next bearing. After all bearings have been checked, rotate the crankshaft to see that there is no excessive drag. When checking #1 main bearing, loosen accessory drive belts and timing belt so as to prevent tapered reading with plastic gauge.
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  7. Measure crankshaft end-play (see specifications) by forcing the crankshaft to the extreme front position. Measure at the front end of the #5 bearing with a feeler gauge.
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  9. Install a new rear main bearing oil seal.
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REPLACEMENT



Main bearings may be replaced with or without removing the crankshaft.

With Crankshaft Removal
  1. Remove and inspect the crankshaft.
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  3. Remove the main bearings from the cylinder block and main bearing caps.
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  5. Coat bearing surfaces of new, correct size main bearings with oil and install in the cylinder block and main bearing caps.
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  7. Install crankshaft.
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Without Crankshaft Removal
  1. With oil pan, oil pump and spark plugs removed, remove cap on main bearing requiring replacement and remove bearing from cap.
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  3. Install a main bearing removing and installing tool in oil hole in crankshaft journal. If such a tool is not available, a cotter pin may be bent as required to do the job.
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  5. Rotate the crankshaft clockwise as viewed from the front of the engine. This will roll the upper bearing out of the block.
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  7. Oil new selected size upper bearing and insert plain (unnotched) end between crankshaft and indented, or notched, side of block. Rotate the bearing into place and remove the tool from oil hole in crankshaft journal.
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  9. Oil new lower bearing and install in bearing cap.
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  11. Install main bearing cap with arrows pointing toward front of engine.
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  13. Torque all main bearing caps, EXCEPT THE REAR MAIN CAP, to specifications. Torque rear main bearing cap to 14-16 Nm (10-12 ft. lbs.) then tap end of crankshaft, first rearward, then forward with a lead hammer. This will line up rear main bearing and crankshaft thrust surfaces. Retorque all main bearing caps to specification.
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Click image to see an enlarged view

Fig. Fig. 1The notch on the the side of the bearing cap matches the groove on the bearing insert

 
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