Figure 3-67.Methods of valve timing with a chain drive.
In a gear drive setup (fig. 3-66), the timing gear on
the crankshaft meshes directly with the gear on the
camshaft. Timing gears are commonly used on heavy-
duty applications due to their dependability; however,
they are noisier than a chain or belt drive. Since they are
keyed to their respective shafts, they can be replaced if
they become worn. With directly driven gears, one gear
usually has a mark on two adjacent teeth and the other
mark on only one tooth. To time the valve properly,
mesh the gears so the two marked teeth of one gear
straddle the single marked tooth of the other.
A timing chain and sprockets can also be used to
turn the camshaft (fig. 3-66). This is the most common
type of gear train. There are two types of timing chains.
One is a silent link type that is used in standard and
light-duty applications. The other is the roller-link
chain, which is used in heavy-duty applications. Like
timing gears, the chain sprockets have timing marks.
The correct timing may be obtained by hating a certain
number of chain-link teeth between the marks or by
lining up the marks with a straightedge, as shown in
In a belt drive gear train, the sprockets on the
crankshaft and the camshaft are linked by a continuous
neoprene belt (fig. 3-66). The belt has square-shaped
internal teeth that mesh with the teeth on the sprockets.
The timing belt is reinforced with nylon or fiber glass to
give it strength and prevent it from stretching. This
drive configuration is limited to overhead camshaft
Most engines with a chain drive and all belt-driven
engines use a tensioner. The tensioner pushes against
the belt or chain to keep it tight. This serves to keep it
from slipping on the sprockets. This provides more
precise valve timing and compensates for component
wear and stretch. Engines with a belt drive usually use a
spring-loaded idler wheel. Engines with a chain drive
use a fiber-rubbing block that is either spring loaded or
Always check the manufacturers service
manual when you are in doubt about the
method of timing used for the engine you are
Bearings are installed in an engine where there is
relative motion between parts. Engine bearings are
called sleeve bearings because they are in the shape of a
Figure 3-68.Typical sleeve-type bearing half.