Piper Navajo Chieftain PA-31-350 Service Manual page 789

NAVAJO CHIEEFTAIN SERVICE MANUAL
b. Inspection and Servicing (Spark Plug).
1. If the spark plug appears to be in good condition, except for a mild coating of oxide on the
porcelain and electrodes, it may be cleaned and reused. Cleaning is accomplished on a conventional airplane
type spark plug cleaner, except that it will be necessary to use two or more adapters in order to raise the
long extension of the plug far enough out of the cleaner nozzle opening to perform an effective job. Plug
the ceramic insert cavity at the terminal end of the plug with a piece of paper or cloth to keep out any of
the cleaning sand. Wipe this cavity out thoroughly with a cloth wet with carbon tetrachloride. If after
cleaning the spark plug porcelain is white and the electrodes are not eroded, proceed to check the ground
electrode in the heater and adjust the spark gap in accordance with Step c of this paragraph.
NOTE
If the spark plug fails to clean up properly and/or the electrodes
are badly eroded, it should be replaced.
c. Spark Gap Check and Adjustment. (See Figures 13-11 and 13-12.) A spark gap of 0.156 to 0.188
inches must be maintained on the P/ N 39D18 spark plug. This gap should be checked any time a plug is
replaced or at the time of heater overhaul. A spark gap greater than that specified can shorten the life of the
ignition assembly. There are several ways to check spark gap on this heater. Method I is recommended when
the heater is being overhauled and before the installation of the fuel nozzle. Method II is suitable forchecking
the gap through the spark plug well when the heater is not disassembled, and Method III is for heaters which
have the ground electrode welded within the combustion head assembly as shown in Figure 13-12.
Method 1:
1. Using a 5/32 inch drill (0.156) or a piece of 5/32 inch rod, reach through the opening in the
combustion head (fuel nozzle location) and find the ground electrode. (It is welded inside the head.)
2. Move the drill along the side of the electrode on the spark plug side. (Movement should be
from the outer edge towards the center.) The drill or rod should just pass through the spark plug gap
opening. Should the drill fail to pass through this opening, the gap is too narrow. If it passes through too
freely, the gap is too wide. In either case, it will be necessary to bend the ground electrode in the direction
required. This may be done by removing the spark plug and reaching through the opening.
3. Recheck the gap after repositioning of the ground electrode.
Method II:
1. Measure the distance between the seating surface of the spark plug with a new gasket
installed to the end of the plug electrode.
2. Using a depth gauge, measure the distance between the ground electrode in the heater to the
spark plug seating surface in the heater jacket and check this measurement against the measurement
obtained in Step 1. The difference should be between 0.156 to 0.188 of an inch.
3. The ground electrode can be bent to obtain the required gap.
Method III. (Refer to Figure 13-12.)
1. Fabricate the simple checking tool per Figure 13-16.
2. Insert the checking tool into the spark plug hole, being certain that the edge of the tool
marked "pipe" faces the combustion air blower pipe and the edge marked "label" faces the label on the
heater.
3. Using a small flashlight and mirror, sight down along the scribe line of the tool to check the
ground electrode setting. The electrode should touch the side of the tool.
4. The ground electrode in the heater may be bent to obtain this setting
Reissued: 9/23/80
HEATING AND
VENTILATING
SYSTEM
4B21