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Re: crank bearings? (archive)

[ Follow Ups ] [ 5-series (E39) Message Board ] [ Msg. Board FAQ ]

Posted by Steve D on July 09, 2000 at 11:34:53:

In Reply to: crank bearings? posted by Ku on July 08, 2000 at 12:55:59:

Crankshaft(main) bearings support the crankshaft in the engine block. They are soft metal poured into steel backed inserts that fit into bearing saddles and caps in the block. The crankshaft has highly polished hardened journals on which it spins in the bearings.

The clearance between the journals and the bearing surface is 1-3 thousands of an inch so the journals ride on a 0.5 - 1.5 thousand of an inch film of oil provided by the oil pump. Bearing wear usually occurs on cold startup and over time the clearance increases.

When this clearance becomes excessive oil pressure will drop since the volume of oil flow increases as the cube of the bearing clearance. So oil pressure is a good way of monitoring the condition of the bearings. The same type of bearings are used in the connecting rods and camshafts.

The number of main bearings is a function of the length of the crankshaft, the journal diameter (journal diameters are sometimes staggered from front to rear), and the design max. RPM. Too few bearings cause crankshaft whip at high speeds and resonance points and too many bearings cause high internal engine friction. Most V8 engines use 5 main bearings.

Sturdy blocks for high RPM operation use 4 or 6 bolts to fasten the bearing caps instead of the normal 2 and are sometimes crossbolted.

Hope this was not too technical and answers your question.

Steve D

: Hi, can someone tell me what are crank bearings. Also, does number of crank bearings have anything to do with the studiness of the engine?

: cheers,

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