FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 8. Engines, Fuel, Exhaust, and Propellers | 1. Engines | 8-15. Spark Plugs
AC 43.13-1B
9/8/98
(a) Perform the compression test as
soon as possible after the engine is shut down
to ensure that the piston rings, cylinder walls,
and other engine parts are well-lubricated.
(b) Remove the most accessible
spark plug from each cylinder.
(c) With the air valve closed, apply
an external source of clean air (approximately
100 to 120 psi) to the tester.
(d) Install an adapter in the spark
plug bushing and connect the compression
tester to the cylinder.
(e) Adjust the pressure regulator to
obtain a reading of 20 psi on the regulator
pressure gauge. At this time, the cylinder
pressure gauge should also register 20 psi.
(f) Turn the crankshaft, by hand, in
the direction of rotation until the piston (in the
cylinder being checked) is coming up on its
compression stroke. Slowly open the air valve
and pressurize the cylinder to 80 psi.
CAUTION: Care must be exercised
in opening the air valve since suffi-
cient air pressure will have built up in
the cylinder to cause it to rapidly ro-
tate the propeller if the piston is not at
top dead center (TDC).
(g) Continue rotating the engine
against this pressure until the piston reaches
TDC. Reaching TDC is indicated by a flat
spot or sudden decrease in force required to
turn the crankshaft. If the crankshaft is rotated
too far, back up at least one-half revolution
and start over again to eliminate the effect of
backlash in the valve operating mechanism and
to keep piston rings seated on the lower ring
lands.
(h) Open the air valve completely.
Check the regulated pressure and readjust, if
necessary, to read 80 psi.
(i) Observe the pressure indication of
the cylinder pressure gauge. The difference
between this pressure and the pressure shown
by the regulator pressure gauge is the amount
of leakage through the cylinder. A loss in ex-
cess of 25 percent of the input air pressure is
cause to suspect the cylinder of being defec-
tive; however, recheck the readings after oper-
ating the engine for at least 3 minutes to allow
for sealing of the rings with oil.
(j) If leakage is still occurring after a
recheck, it may be possible to correct a low
reading. This is accomplished by placing a fi-
ber drift on the rocker arm directly over the
valve stem and tapping the drift several times
with a hammer to dislodge any foreign mate-
rial between the valve face and seat.
NOTE: When correcting a low read-
ing in this manner, rotate the propel-
ler so the piston will not be at TDC.
This is necessary to prevent the valve
from striking the top of the piston in
some engines. Rotate the engine be-
fore rechecking compression to reseat
the valves in the normal manner.
8-15. SPARK PLUGS. The spark plug pro-
vides the high-voltage electrical spark to ignite
the fuel/air mixture in the cylinder. The types
of spark plugs used in different engines will
vary with regard to heat range, reach, thread
size, and other characteristics required by the
particular installation.
a. Heat Range. The heat range of a spark
plug is the principal factor governing aircraft
performance under various service conditions.
The term “heat range” refers to the
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