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
c. Installation Procedures. When in-
stalling spark plugs, observe the following
procedure.
(1) Visually inspect the plug for clean-
liness and condition of the threads, ceramic,
and electrodes.
FIGURE 8-3. Hot and cold spark plugs.
b. Reach. The spark plug reach (see fig-
ure 8-4) is the threaded portion which is in-
serted into the spark plug bushing of the cylin-
der. A plug with the proper reach will ensure
that the electrode end inside the cylinder is in
the best position to achieve ignition. Spark
plug seizure or improper combustion within
the cylinder will probably occur if a plug with
the wrong reach is used. Shell threads of spark
plugs are classified as 14- or 18-mm spark
plug diameter, long reach or short reach, thus:
NOTE: Never install a spark plug
which has been dropped and always
use new gaskets every time you install
a spark plug.
(2) Check the plug for the proper gap
setting, using a round wire feeler gauge as
shown in figure 8-5. In the case of used plugs,
procedures for cleaning and regapping are usu-
ally contained in the various manufacturers’
manuals.
Diameter
14 mm
18 mm
Long reach
Short reach
1/2 in (12.7 mm) 3/8 in (9.53 mm)
13/16 in. (20.64 mm) 1/2 in (12.7 mm)
FIGURE 8-4. Spark plug reach.
Page 8-10
FIGURE 8-5. Method of checking spark plug gap.
(3) Check the plug and cylinder bushing
to ascertain that only one gasket is used per
spark plug. When a thermocouple-type gasket
is used, no other gasket is required.
(4) Apply anti-seize compound spar-
ingly to the shell threads, but do not allow the
compound to contact the electrodes since the
material is conductive and will short out the
Par 8-15