IO-390-D Series Engine Maintenance Manual
© 2020 Avco Corporation. All Rights Reserved
72-30
March 2020
Page 217
IO-390-D Series Engine Maintenance Manual
11.
Piston Ring Replacement
CAUTION
DO NOT
UNDER ANY CIRCUMSTANCES
INSTALL CHROME-PLATED
PISTON RINGS IN AN ENGINE HAVING CHROME-PLATED CYLINDER
BARRELS. IF YOU ARE UNSURE OF THE CORRECT COMBINATION OF
PISTON RINGS TO BE USED, REFER TO THE LATEST REVISION OF
SERVICE INSSTRUCTION NO. SI-1037.
DURING REMOVAL OF THE THREE PISTON RINGS IN THE NEXT STEP,
USE CARE NOT TO SCRATCH OR SCORE THE PISTON.
A.
Start from the top down, use the commercially available piston ring expander tool to remove
the two top compression rings, the oil regulating piston expander ring, and the inner expander
ring (Figure 22).
NOTICE:
New cylinders made by Lycoming Engines will have the correct piston ring finish
and do not need further honing. Otherwise, hone the cylinder per the latest revision
of Service Instruction No. SI-1047 to ensure correct seating of the new piston rings.
For new piston rings, refer to the applicable parts catalog to identify the correct new
piston rings to be installed on the piston.
New piston rings are shipped from Lycoming Engines with the oil regulating piston
expander ring and the inner expander ring assembled. The inner expander ring must
be separated from the oil regulating piston expander ring before installing them on
the piston.
B.
Apply a generous coating of a mixture of 15% pre-lubricant (STP or equivalent) and 85%
SAE 50 mineral-base aviation-grade lubricating oil (unless otherwise directed per the latest
revision of Service Instruction No. SI-1059) to the piston rings.
C.
With the piston top side up on a workbench, install the inner expander ring in the first groove
above the piston hole (Figure 22).
Figure 22
Piston Ring Positions
D.
Assemble the oil regulating expander ring over the expander with its gap 180° opposite the
inner expander ring gap. Compress the assembly several times with the fingers to assure that
the ring lies free and loose in the groove. Both the regulating rings and the expander are
symmetrical, but the oil regulating expander ring is to be installed with the part number
toward the top of the piston.
E.
Install the two top compression rings (Figure 22) with the word "Top" toward the top of the
piston in the remaining top grooves.
