Cylinder Borescope Inspection - Lycoming IO-390-D Series Maintenance Manual

5. Cylinder Borescope Inspection

WARNING
DURING A CYLINDER BORESCOPE INSPECTION, MAKE SURE THAT
THE IGNITION SWITCH IS TURNED OFF AND THAT POWER TO THE
ENGINE IS DISCONNECTED.
AS A PRECAUTION, DO NOT STAND OR ALLOW ANYONE TO STAND
WITHIN THE ROTATIONAL ARC RADIUS OF THE PROPELLER. MAKE
SURE THE ENGINE IS COOL.
A. The cylinder borescope inspection is done to examine the inner walls of the engine cylinders
for rust, deposits and unusual wear patterns of the combustion chamber, valve, piston top,
and the cylinder barrel.
B. When to complete the Cylinder Borescope Inspection:
(1) Before engine cylinder removal
(2) If oil consumption is excessive
(3) 100-hour or annual inspection
(4) After an engine overspeed
(5) Low compression
C. Cylinder Borescope Inspection Procedure
(1) Remove a spark plug from each cylinder per the "Spark Plug Removal" procedure in
Chapter 74-20.
(2) Put the piston at bottom dead center on the power stroke.
(3) Install the borescope through the vacant spark plug hole on the engine cylinder and
examine the combustion chamber, the top of the piston, the internal surfaces of each
cylinder, including the exhaust valve and exhaust valve seat. Complete inspection steps in
Table 3.
(4) Remove the borescope from the cylinder.
(5) Put the piston at bottom dead center at the end of the intake stroke.
(6) Install the borescope through the vacant spark plug hole and examine the intake valve and
intake valve seat. Complete inspection steps in Table 3. Corrective action in Table 3
applies to procedures in this chapter.
(7) Reinstall the spark plug in the cylinder per the "Spark Plug Installation" procedure in
Chapter 74-20.
(8) Record all results and corrective action in the 100-Hour or Annual Inspection Checklist in
Chapter 05-20.
72-30
Page 198
IO-390-D Series Engine Maintenance Manual
IO-390-D Series Engine Maintenance Manual
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March 2020