sleeve that fits around the rotating journal or shaft (fig.
3-68). Connecting rod or camshaft (main) bearings
are of the split or half type (fig. 3-69). On main
bearings, the upper half is installed in the counterbore in
the cylinder block. The lower-bearing half is held in
place by the bearing cap (fig. 3-70). On connecting rod
bearings, the upper-bearing half is installed in the rod
and the lower half is placed in the rod cap. The piston
pin bearing in the connecting rod is of the full round or
The lubrication of bearings is very important to
engine service life because it forces oil to high friction
points within the engine. Without lubrication between
parts, bearings overheat and score from friction.
The journal or shaft must be smaller in diameter
than the bearing (fig. 3-71), so there is clearance (called
oil clearance) between the two parts; oil circulates
through the clearance. The oil enters through the oil
hole (fig. 3-66) and fills the oil groove in the bearing.
From there, the rotating journal carries the oil around to
all moving parts of the bearing. The oil works its way to
the outer edges of the bearing. From there, it is thrown
off and drops back into the oil pan. The oil thrown off
helps to lubricate other engine parts, such as the
cylinder walls, the pistons, and the piston rings.
Figure 3-69.Crankshaft main bearings.
Figure 3-70.Connecting rod bearings.
As the oil moves across the faces of the bearings, it
not only lubricates them but also helps keep them cool.
The oil is relatively cool, as it leaves the oil pan. It picks
up heat in its passage through the bearing. This heat is
Figure 3-71.Oil clearance between bearing and shaft.