Main Bearing


Main bearings are replaced with the crankshaft in the engine using a tool installed in the oil feed hole in the journal. The bearings must be rolled out on the side opposite the bearing locating lug, or tang.

Main bearings can be rolled out and new bearings rolled back in. If the special tools are not available, you can make one out of a cotter pin. (Figure a, Courtesy of General Motors Corporation, Service Technology Group).

When selecting new main bearings, make sure they match the crankshaft journal diameters and main bearing bores. If the crankshaft has been ground undersize, the main bearings will also have to be undersize. Similarly, if the housing bores have been machined oversize by align boring or align honing, the bearings must take up this space. Bearing size is usually marked on the bearing box and on the back of the bearing.

When the bearings are ready to be installed in the main bearing bores, make sure the bore is clean and dry before installing the bearing halves into place. Use a clean, lint-free cloth to wipe the bearing back and bore surface.

Put the new main bearing inserts into each of the main bearing caps and into the bearing bores in the cylinder block housings.

Place the bearing inserts into the bore; make sure the locating lugs fit into their recess. Courtesy of Federal-Mogul Corporation.

Make sure all holes align. The backs of the main bearing inserts should never be oiled or greased. Place the crankshaft in the block on the main bearing inserts and arrange the main bearing caps in the correct order and direction over the crankshaft. Follow the factory markings or use those made during disassembly.

The next step is to measure the oil clearance between the crankshaft and the main bearing. Proper lubrication and cooling of the bearing depend on correct crankshaft oil clearances. Scored bearings, worn crankshaft, excessive cylinder wear, stuck piston rings, and worn pistons can result from too small an oil clearance. If the oil clearance is too great, the crankshaft might pound up and down, overheat, and weld itself to the insert bearings.

Plastigage is fine, plastic string used to measure the oil clearance between the bearing and the crankshaft. One side of the plastigage's package has stripes for inch measurements, the other side has stripes for metric measurements. The string can be purchased to measure different clearance ranges. Usually, only the smallest clearance range is necessary for reassembly work.

Tighten the Main Caps

For a five-main bearing block, the torque sequence is 1-4-3-2-5.

As each main cap is torqued down, check to see that the crank continues to turn easily.

After the rear cap is removed, check the rear seal drag. Some manufacturers give a torque specification for the amount of effort required to turn the crank with the damper bolt in an assembled engine.

Align the Thrust Bearing Halves

Torque all bearing caps except the thrust main. Its halves should be aligned before torquing.

Misaligned thrust halves could eliminate end play. This is done by prying on the crankshaft while the thrust main is still loose.

(b) Checking crankshaft end play: (a) With a feeler gauge. (b) With a dial indicator. (Figure a, Courtesy of Federal-Mogul Corporation; Figure b, Courtesy of Ford Motor Company).