Chevrolet Light Operator's Manual page 34

If a tension wrench is used when installing the plugs, the'
proper tension is 20 to 25 foot pounds maximum. If a tension
wrench
is
not available, screw each plug in "finger tight" and
then with a wrench tighten each plug lh to % turn beyond
this.
Manifold Heat Control Valve. The manifold heat control valve
is
located on the inside of the exhaust manifold and is oper-
ated by the thermostatic spring, the center of which
is
attached
to a ;Iot in the valve shaft and
the outer end bears against
a stop pin on the manifold.
When the engine is cold
the valve is
in
the "heat-on"
position as shown
in
the left
half of Figure 39, and the hot
exhaust gases are directed
against the center of the
in-
take manifold. As the engine
warms up, the thermostatic
spring moves the valve to the "heat-off" position as shown
in
the right half of Figure 39 and directs t,he exhaust gases away
from the center of the intake manifold.
This thermostatic control results in maintaining the proper
temperature of the incoming gases under all operating condi-
tions.
The tension of the thermostatic spring is very important.
When it
is
too tight the heat will not be turned off the intake
heat riser as the engine warms up, with the result that the
incoming gases will be expanded several times greater in vol-
ume than in normai operation and it will be impossible to get
a full charge of gas and air
into the cylinders. This
con~
dition reduces engine power
and maximum speed;
it
will
also cause detonation and
overheating at higher speeds.
Therefore, it is important
that the thermostatic spring
be wound up just enough to
slip its outer end over the
anchor pin in the manifold
fig. 40-Ma"i/old H.a'
Val~.
TI •• ,moda'ic Co"'rol