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C.  Remove the grommet.
D.  Using a 7/8 inch deep hex socket, unscrew and remove the spark plug.  Make sure the spark plug gasket is removed with the spark plug.  It will normally stick on the spark plug threads, but if gasket should drop into the ventilating air passages of the heater, remove with a wire hook.
2.  Inspection and Servicing (Spark Plug):
A.  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 3 of this paragraph.
— Note —
If the spark plug fails to clean up properly and/or if the electrodes are badly eroded, it should be replaced.
3.  Spark Gap Check and Adjustment: (See Figure 21-12.)  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 methods in which the spark gap of this heater may be checked.  Method I is recommended when the heater is being overhauled and before the installation of the fuel nozzle. Methods II and III are suitable for checking the gap through the spark plug well when the heater is not disassembled.
Method I.
A.  Using a 5/32 inch drill (0.156) or a piece of 5/32 rod, reach through the small opening in the combustion head and find the ground electrode. (It is welded inside the head.)
B.  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 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.
C. Recheck the gap after repositioning of the ground electrode. Method II.
A.  Measure the distance between the seating surface of the spark plug with a new gasket installed to the end of the plug electrode.
B.  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 A.  The difference should be between 0.156 to 0.188 of an inch.
C.  The ground electrode can be bent to obtain the required gap.
21-40-00 Page 21-29 Revised: February 28, 2002
Method III.
A.  Fabricate or purchase from Piper the special tool from dimensions given in Figure 21-22.
B.  Install the threaded end of the tool into the spark plug hole.
C.  Slide the rod of the tool into the combustion head until it contacts the ground electrode.
D.  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 has of its original 1/8 inch diameter, it should be replaced.  This can be done as follows:
(1)  
Grind off the head of the rivet where it projects through the combustion head and remove the electrode.

(2)  
Install a new CRES rivet AN125452 which is 1.500 inches in length.

(3)  
Heliarc tack weld the rivet head to hold it in place.

(4)  
Check spark gap as noted in Methods I or II.

4.  Installation: (Refer to Figure 21-12)
A.  If a new spark plug is being installed, be sure to adjust the spark gap.  Do not bend the electrode on the spark plug.
B.  Place a new spark plug gasket on the threads.  If the gasket does not hold on the threads and would be likely to fall off during installation, place a small drop of Aviation Permatex or similar material on the gasket to stick it temporarily to the plug shell.
C.  Screw the spark plug into the heater with a deep socket wrench.  Tighten to a torque of 28 foot-pounds.
　
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