Main Bearing


Bearings are used to carry the critical loads created by crankshaft movement. They are a major wear item in the engine and require close inspection. Main bearings support the crankshaft journals. Connecting rod bearings are installed between the crankshaft and connecting rods.

Modern crankshaft bearings are known as insert bearings. There are two basic designs of insert bearings.

Full-round and split insert bearings.

A full round (one-piece) bearing is used in bores that allow the shaft's journals to be inserted into the bearing, such as a camshaft. A split (two halves) bearing is used where the bearing must be assembled around the journal with the bearing housing being of two parts also, including a cap that holds the assembly together. Crankshaft bearings are typically the split type.

Many crankshafts are fitted with a main bearing that has flanged sides. This type bearing is typically called a thrust bearing and is used to control any horizontal movement or endplay of the shaft. The flange bearing is used in the thrust position of the block. Most thrust main bearings are doubled flanged.

Some late-model engines do not use separate main bearing caps; instead they are fitted with a lower engine block assembly.

A lower cylinder block assembly. Courtesy of Ford Motor Company.

This assembly works like a bridge and contains the lower half of the bore for the main bearings. The assembly is torqued to the engine block and holds the crankshaft in place.

Bearing Spread

Most main and connecting rod bearings are manufactured with spread. Bearing spread means that the distance across the outside parting edges of the bearing insert is slightly greater than the diameter of the housing bore. To position a bearing half that has spread, it must be snapped into place by a light forcing action.

Spread requires a bearing to be lightly snapped into place.

This assures positive positioning against the inside of the bore and helps to keep the bearings in place during assembly.

Bearing Crush

Each half of a split bearing is made so that it is slightly greater than an exact half. This can be seen quite easily when a half is snapped into place in its housing. The parting faces extend a little beyond the seat.

Crush assures good contact between the bearing and the housing.

This extension is called crush. When the two bearing halves are assembled and the housing cap tightened, the crush sets up a radial pressure on the bearing halves so they are forced tightly into the housing bore.

Bearing Locating Devices

Engine bearings must be provided with some means to keep them from rotating or shifting sideways in their housings. Many different methods have been used by manufacturers to keep the bearings in place. The most common way is the use of a locating lug. As shown below, this consists of a protrusion at the parting face of the bearing. The lug fits into a slot in the bearing's bore.

The locating lug fits into the slot in the housing.

Oil Grooves

Providing an adequate oil supply to all parts of the bearing surface, particularly in the load area, is an absolute necessity. In many cases, this is accomplished by the oil flow through the bearing oil clearance. In other cases, however, engine operating conditions are such that this oil distribution method is inadequate. When this occurs, some type of oil groove must be added to the bearing. Some oil grooves are used to assure an adequate supply of oil to adjacent engine parts by means of oil throw-off.

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