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    2005 GMC Truck Yukon Denali AWD 6.0L SFI 8cyl

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    Repair Pontiac Mid-size 1974-1983 Repair Guide

    Crankshaft and Main Bearings



    Click image to see an enlarged view

    Fig. Fig. 1 Install pieces of rubber hose over the connecting rod bolts and hold them with rubber bands to prevent the crank journals from being during crankshaft removal

    Click image to see an enlarged view

    Fig. Fig. 2 Exploded view of the crankshaft and bearings-diesel engine shown

    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.
    3. Remove the engine front (timing) cover.
    5. Remove the timing chain and gears.
    7. Remove the oil pan.
    9. Remove the oil pump.
    11. Stamp 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 eventually 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.
    13. 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.
    15. Mark the main bearing caps with a number punch or punch so that they can be reinstalled in their original positions.
    17. Remove all main bearing caps.
    19. Note the position of the keyway in the crankshaft so it can be installed in the same position.
    21. 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.
    23. Carefully lift the crankshaft out of the block. The rods will pivot to the center of the engine when the crank is removed.


    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 too sloppy, a new bearing (both upper and lower halves) is required.

    Factory-undersized crankshafts are marked, sometimes with a "9" and/or a large spot of light green paint; the bearing caps also will have the paint on each side of the undersized journal.

    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 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.

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

    1. Install the bearing cap and evenly tighten 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 towards 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.

    1. 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.
    3. Standard, .001 in. or .002 in. 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.
    5. 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 drive belts (engine in car) to prevent a tapered reading with the Plastigage®.


    Engine Removed
    1. Remove and inspect the crankshaft.
    3. Remove the main bearings from the bearing saddles in the cylinder block and main bearing caps.
    5. Coat the bearing surfaces of the new, correct size main bearings with clean engine oil and install them in the bearing saddles in the block and in the main bearing caps.
    7. Install the crankshaft. See "Crankshaft Installation."

    Engine in Vehicle

    Click image to see an enlarged view

    Fig. Fig. 3 Fabricate a home-made bearing roll-out pin to remove the main bearing when the engine is in the vehicle

    1. With the oil pan, oil pump and spark plugs removed, remove the cap from the main bearing needing replacement and remove the bearing from the cap.
    3. Make a bearing roll-out pin, using a bent cotter pin as shown in the illustration. Install the end of the pin in the oil hole in the crankshaft journal.
    5. Rotate the crankshaft clockwise as viewed from the front of the engine. This will roll the upper bearing out of the block.
    7. Lube the new upper bearing with clean engine oil and insert the plain (unnotched) end between the crankshaft and the indented or notched side of the block. Roll the bearing into place, making sure that the oil holes are aligned. Remove the roll pin from the oil hole.
    9. Lube the new lower bearing and install the main bearing cap. Install the main bearing cap, making sure it is positioned in proper direction with the matchmarks in alignment.
    11. Tighten the main bearing cap bolts to specification.

    See "Crankshaft Installation" for thrust bearing alignment.


    Click image to see an enlarged view

    Fig. Fig. 4 Aligning the crankshaft thrust bearing

    Click image to see an enlarged view

    Fig. Fig. 5 Mounting a dial gauge to read crankshaft run-out

    Click image to see an enlarged view

    Fig. Fig. 6 Turn the crankshaft slowly by hand while checking the gauge

    When main bearing clearance has been checked, bearings examined and/or replaced, the crankshaft can be installed. Thoroughly clean the upper and lower bearing surfaces, and lube them with clean engine oil. Install the crankshaft and main bearing caps.

    Dip all main bearing cap bolts in clean oil, and torque all main bearing caps, excluding the thrust bearing cap, to specifications (see the "Crankshaft and Connecting Rod" chart in this section to determine which bearing is the thrust bearing). Tighten the thrust bearing bolts finger-tight. To align the thrust bearing, pry the crankshaft the extent of its axial travel several times, holding the last movement toward the front of the engine. Add thrust washers if required for proper alignment. Tighten the thrust bearing cap to specifications.

    To check crankshaft end-play, pry the crankshaft to the extreme rear of its axial travel, then to the extreme front of its travel. Using a feeler gauge, measure the end-play at the front of the rear main bearing. End-play may also be measured at the thrust bearing. Install a new rear main bearing oil seal in the cylinder block and main bearing cap. Continue to reassemble the engine.

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