IO-390-D Series Engine Maintenance Manual
72-30
© 2020 Avco Corporation. All Rights Reserved
Page 230
March 2020
IO-390-D Series Engine Maintenance Manual
NOTICE:
Always install a larger oversized 0.010 in. (0.254 mm) Heli-Coil
®
insert in the spark
plug hole on the cylinder head to replace a standard sized Heli-Coil
®
insert. Never
replace a standard sized Heli-Coil
®
insert with another standard sized Heli-Coil
®
insert.
The oversize Heli-Coil
®
inserts are identified by three marks on the tang of the Heli-
Coil
®
insert as shown Figure 41.
Figure 41
Heli-Coil
®
Inserts for Long Reach Spark Plugs
(1)
Disable all power to the engine to prevent propeller rotation and engine start. Disconnect
ignition leads from all spark plugs.
(2)
If not already done, remove and discard the Heli-Coil
®
insert from the spark plug hole.
(3)
Apply of coat of grease liberally to the oversize tap.
(4)
If the Heli-Coil
®
insert is replaced while the cylinder is installed on the engine, take
precautions to prevent metal shavings from falling into the combustion chamber.
CAUTION
IF METAL SHAVINGS FALL INTO THE COMBUSTION CHAMBER
OF THE CYLINDER, STOP AND REMOVE ALL SHAVINGS AND
DEBRIS. THE COMBUSTION CHAMBER MUST BE CLEAN.
(5)
Turn the crankshaft to the start of the compression stroke.
(6)
Put 8 ft. (2.4 m) of 3/8 in. (9.5 mm) nylon rope through the opposite spark plug hole.
(7)
Turn the crankshaft to force the rope against the bottom of the spark plug hole that is to
be tapped.
(8)
Use the 0.010 in. oversize tap, to drill a larger oversize hole for the oversize insert.
(9)
Install the new oversize 0.010 in. Heli-Coil
®
insert in the spark plug hole. Refer to Figure
41.
(10)
Remove all chips and shavings to prevent contamination from foreign object debris.
(11)
Remove the rope from the spark plug hole.
(12)
Record replacement, in the engine logbook, of the standard sized Heli-Coil
®
insert with
a new oversized Heli-Coil
®
insert for the applicable cylinder number.
