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Figure 3-65.—Correct valve-to-seat contact after narrowing.
When wear is indicated inside the rocker bore, you can
measure it with a telescoping gauge and a micrometer or
a bore gauge. Rocker arms with bushings can be
rebushed if the old bushing is worn On some rocker
arms, worn valve ends can be ground down on the valve
grinding machine. Excessively worn rocker arms
should be replaced.
Also, inspect the rocker arm shaft for wear. A worn
rocker arm shaft has indentions where the rocker arms
swivel on the shaft. Wear on the shaft is usually greater
on the bottom. Using a micrometer, check the shaft to
determine whether wear is within the manufacturer’s
specifications.
When reinstalling rocker arms and shafts in the
cylinder head, make sure that the oil holes (in the shaft if
so equipped) are on the underside, so they can feed oil to
the rocker arms. Ensure that all spring and rocker arms
are restored to their original positions as you attach the
shafts to the head.
Valve Spring Service
After prolonged use, valve springs tend to weaken,
lose tension, or even break. During engine service,
always test valve springs to make sure they are usable.
Valve springs should be tested for uniformity and
strength. The three characteristics to check are valve
spring squareness, valve spring free height, and valve
spring tension.
Valve spring squareness is easily checked with a
combination square. Place each spring next to the
square on a flat surface. Rotate the spring while
checking for a gap between the side of the spring and the
square. Replace any spring that is not square.
Valve spring free height can also be measured with
a combination square or a valve spring tester. Simply
measure the length of each spring in normal
uncompressed condition. If it is too long or too short,
replace the spring.
Valve spring tension, or pressure, is measured by
using a spring tester. Compress the spring to
specification height and read the scale on the tester.
Spring pressure must be within specifications. If the
reading is too low, the spring has weakened and must be
replaced.
TIMING GEARS (GEAR TRAINS)
Because the crankshaft must rotate twice as fast as
the camshaft, the drive member on the crankshaft must
be exactly one half as large as the driven member on the
camshaft So for the camshaft and crankshaft to work
together, they must be in time with each other. This
initial position between the two shafts is designated by
marks that are called timing marks. To obtain the
correct initial relationship of the components, align the
corresponding marks at the time of assembly. Timing
gears keep the crankshaft and the camshaft turning in
proper relation to one another, so the valves open and
close at the proper time. This is accomplished by gear-
drive, chain-drive, or belt-drive gear trains (fig. 3-66).
Figure 3-66.—Timing gear trains.
3-39
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