The relationship between the camshaft and the
crankshaft determines the valve timing. Gears,
drive chains, and reinforced neoprene belts are
used to drive the camshafts that open and allow
the valves to close in relation to the position of
the pistons in the cylinders. The gears, drive
sprockets, or cogs, as the case may be, of the
camshaft and crankshaft are keyed in position so
they cannot slip.
With directly driven timing gears (fig. 3-24),
one gear usually has a mark on two adjacent teeth
and the other, a mark on only one tooth. To time
the valves properly, you need to mesh the gears
so that the two marked teeth of the one gear
straddle the single marked tooth of the other gear.
In chain-driven sprockets, you can obtain
correct timing by having a certain number of chain
teeth between the marks or by lining up the marks
with a straightedge, as shown in figure 3-24.
Engines using a continuous neoprene belt have
sprockets, or cogs, attached to the camshaft and
crankshaft. The belt has square-shaped internal
teeth that mesh with the teeth on the sprockets.
All engines with this system use a timing belt
tensioner. Timing marks on this system vary with
Before setting the valve timing on any engine
that you are overhauling, always check the
manufacturers specifications and instructions.
Most modern engines have main and connect-
ing rod bearings of the precision-insert type, which
can be replaced without removing the crankshaft.
However, if oil passages are blocked, journals are
tapered out of round, or the crankshaft is bent,
simply replacing the bearings will not correct the
If the bearings appear to have worn uniformly,
probable the only requirements are crankshaft
journal checks and bearing replacement. If
bearing wear appears uneven, then the safest
procedure is to remove the crankshaft from the
engine and check it.
BEARING CAPS REMOVAL
When removing bearing caps, if they are not
already marked, be sure to mark them so they will
be replaced on the same journal from which they
were removed. If bearing caps stick, carefully
work them loose by using a soft-faced hammer,
Figure 3-24.Driving the camshaft.