figure 3-11, have a gauge indicating a percentage
of air loss. The gauge is connected to a spring-
loaded diaphragm. The source of air is connected
to the instrument and counterbalances the action
of the spring against the diaphragm. By adjusting
the spring tension, you can calibrate the gauge
properly against a variety of air pressure sources
within a given tolerance.
The tachometer is a speed-indicating instru-
ment that measures the rpms of a rotating shaft.
It maybe either manually or electrically operated.
A manual tachometer (fig. 3-12) is held by its
tip against the end of an exposed rotating shaft.
Make sure the end of the shaft is clean and there
is no slippage between the tip of the tachometer
and the shaft. Read the speed directly on the
tachometer dial, which is calibrated in revolutions
per minute. No timing is necessary, as variations
in speed will be reflected by movement of the
pointer on the dial during the test.
When using the manual tachometer on a shaft,
make sure that that shaft turns at the same speed
as the crankshaft or you will not get an accurate
reading of engine rpms. In many instances, it is
easy to take manual tachometer readings from a
Figure 3-11.Cylinder leakage tester.
Figure 3-12.Manually operated tachometer.
camshaft or fuel pump shaft. On four-cycle
engines, this shaft runs at one-half engine speed.
Consequently, any manual tachometer reading
taken from this shaft must be doubled to get the
true engine speed.
The electric tachometer is connected to the
ignition primary circuit to measure the number
of times per minute the primary circuit is
interrupted. It then translates this information
into engine speed.
The electric tachometer may have a selector
switch on it that can be turned to correspond with
the number of lobes on the distributor cam. The
number of lobes will be the same as the number
of cylinders in the engine. For the proper method
of hooking up and using the electric tachometer,
check the manufacturers instructions for the
tachometer you are using.
GAUGE CARE AND MAINTENANCE
As a CM1, you will probably be responsible
for the care and maintenance of the engine testing
equipment, such as cylinder compression tester,
vacuum gauge, cylinder leakage tester, and
tachometer. You, as the supervisor, must impress
upon the mechanics that these gauges and testers
are fragile instruments that can be damaged
through improper use or rough handling. They
should be kept in a safe place in the toolroom and