BMW Coupes and Sedans 1970-1988 Repair Guide

Crankshaft and Main Bearings

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REMOVAL



  1. Drain the engine oil and remove the engine from the car. Mount the engine on a workstand in a suitable working area. Invert the engine, so the oil pan is facing up.
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CAUTION
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 front (timing) cover.
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  3. Remove the timing chain/belt and gears.
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  5. Remove the oil pan.
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  7. Remove the oil pump.
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  9. 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.
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  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.
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  13. Mark the main bearings caps with a number punch or punch so that they can be reinstalled in their original positions.
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  15. Remove all main bearing caps.
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  17. Carefully lift the crankshaft out of the engine block.
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MAIN BEARING INSPECTION



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 sizes; if main bearing clearance is found to be too sloppy, a new bearing (both upper and lower halves) is required.

Checking Clearance

See Figures 1, 2 and 3

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. Remove the bearing cap. Wipe all oil from the crank journal and bearing cap.
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  3. Place a strip of Plastigage® the full width of the bearing, (parallel to the crankshaft), on the journal.
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Click image to see an enlarged view

Fig. Fig. 1: A dial indicator can be used to check crankshaft end-play



Click image to see an enlarged view

Fig. Fig. 2: Using a prytool and great care, pry back and forth to for end-play



Click image to see an enlarged view

Fig. Fig. 3: Crankshaft run-out is measured using a dial gauge

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.
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  3. Remove the bearing cap. The flattened Plastigage® will be sticking to either the bearing shell or the crank journal.
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  5. Use the graduated scale on the Plastigage® envelope to measure the material at its widest point.
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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 or show signs of damage. Always replace both upper and lower inserts as a unit.
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  3. Standard, 0.010 inch or 0.020 inch (0.025mm or 0.050mm) undersize bearings should produce the proper clearance. If these sizes still produce a sloppy fit, the crankshaft must be replaced. Recheck all clearances after installing new bearings.
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  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.
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INSTALLATION



If the crankshaft is being replaced with factory reground unit, it will be supplied with matching bearings. Check the bearing clearance if the original crankshaft is being reused. It is a good idea to check the bearing clearances on both new and used crankshafts.

heck the crankshaft for scoring or burn marks. The crankshaft is surface hardened at the factory and can not be reground without the surface treatment being completed.

  1. Remove and inspect the crankshaft.
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  3. Remove the main bearings from the bearing saddles in the cylinder block and main bearing caps.
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  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
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  7. Inspect the oil spray jets in the bearing webs on equipped engines. Install the crankshaft and bearing caps.
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  9. Clean and lubricate the bolts with oil. Tighten the bolts on the 2.5L , 2.7L, 2.8L, 3.0L and 3.2L engines to 42-46 ft. lbs. (58-63 Nm). On the 1.8L 16 valve, M3 and M5 engines, tighten the bolts to 14-18 ft. lbs. (20-25 Nm), then turn the bolts an additional 47-53 degrees.
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  11. Check the end-play of the crankshaft by prying in one direction, then to the other side. Insert a feeler gauge between the crankshaft and a main bearing cap to measure the play. A dial indicator can also be used to measure the end-play. If the end-play is not correct, loosen the thrust bearing, tap the crankshaft in both directions and retighten. Recheck the end-play.
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  13. Install a new pilot shaft bearing in the end of the crankshaft.
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