Piper Navajo PA-31 Service Manual page 931

PIPER NAVAJO SERVICE MANUAL
Method III: (39D 18 spark plugs only)
1. Fabricate or purchase from Piper the special tool from dimensions or information given
in Figure 13-20.
2. Install the threaded end of the tool into the spark plug hole.
3. Slide the rod of the tool into the combustion head until it contacts the ground
electrode.
4. Check that the indicator ring on the rod lines up with the end of the tool. The ground
electrode may be bent to obtain the required gap.
NOTE
Inspect the ground electrode for erosion. If it is eroded to
approximately half of its original 1/8 inch diameter, it should be
replaced. This can be done as follows:
(a) Grind off the head of the rivet where it projects through
the combustion head and remove the electrode.
(b) Install a new CRES rivet AN125452 which is 1.500
inches in length.
(c) Heliarc tack weld the rivet head to hold it in place.
(d) Check spark gap as noted in Method II or III.
d. Installation:
1. If a new spark plug is being installed, be sure to adjust the spark gap as outlined in Paragraph
13-27, Step c. Do not bend the center electrode on the spark plug.
2. Place a new spark plug gasket on the threads. A small drop of Aviation Permatex, or similar
material may be used on the gasket to help hold it to the spark plug shell during installation.
3. Screw the spark plug into the heater with a deep socket wrench. Tighten to a torque of 28
foot-pounds. Install the grommet in heater jacket opening.
4. Carefully insert the spring connector on the high-voltage lead into the spark plug shell; press
down gently and start the nut on the threads. Tighten the nut to 20 foot-pounds.
5. Operate the heater to check dependability and replace the access panels.
13-28. IGNITION UNIT. This unit converts 28-volt DC to high voltage, oscillating current capable of
producing a continuous spark in the combustion chamber of the heater. This unit remains energized and
produces a continuous spark during heater operation. It contains a condenser, resistor, radio noise filter,
vibrator and ignition coil.
13-29. IGNITION UNIT REMOVAL AND INSTALLATION.
a. Removal:
NOTE
Make sure the heater electrical circuits are de-energized.
1. Disconnect the primary wire from the primary terminal of the ignition assembly.
2. Carefully unscrew and disconnect the high-voltage ignition cable at the spark plug. Exercise
care to avoid fouling or damaging the connector.
3. Remove the four attaching screws and lift the ignition assembly off the heater jacket.
Reissued: 10/12/79
HEATING AND VENTILATING SYSTEM
5C1