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TM 10-3930-633-34
the piston pin holes, can be caused by a bent
connecting rod. Replace pistons that show signs
of excessive wear, wavy ring lands or fractures, or
damage from detonation or pre-ignition.
(3) Check the piston cylinder bore clearance
by measuring the piston and bore diameters.
Refer to table 6-5 for the proper clearance. Refer
to paragraph 6-23 for the bore measurement
procedure. Measure the OD of the piston with
micrometers at the centerline of the piston bore
and at 90 degrees to the pin bore axis. Check the
ring side clearance following the procedure under
Fitting Piston Rings in this section.
(4) Replace piston pins showing signs of
Figure 6-25. Cleaning Ring Grooves.
fracture, etching or wear. Check the piston pin fit
b. Inspection.
in the piston and rod. Refer to table 6-5.
(1) Carefully inspect the pistons for fractures
(5) Check the OD of the piston pin and the ID
of the pin bore in the piston. Replace any piston
at the ring lands, skirts, and pin bosses, and for
pin or piston that is not within specifications
scuffed, rough, or cored skirts. If the lower inner
portion of the ring grooves have high steps,
replace the piston. The step will interfere with
(6) Replace all rings that are scored, chipped
ring operation and cause excessive ring side
or cracked. Check the end gap and side clearance.
clearance.
It is good practice to always install new rings
when overhauling an engine. Rings should not be
(2) Spongy, eroded areas near the edge of the
piston top are usually caused by detonation or
transferred from one piston to another regardless
pre-ignition. A shiny surface on the thrust surface
of mileage.
of the piston, offset from the centerline between
Table 6-5. Piston, Pin, and Ring Fits
ITEM
INCHES
PISTON
. . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .
Diameter
(red
code)....
. . . . . . . . . . . . . ...3.9984-3.9990
Diameter
(blue
code)
.
.
.
.
.
.
.
.
.
.
.
.
.
.
..
..
.
.
.
.
.
.
.
..
.
.
.
.
..
.3.9996-4.0002
. . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . .
Diameter
(0.003
oversize).
. . . . ...4.0008-4.0014
. . . . . .. . . . . . . . .. . . . . . . . . . . .. .
Piston-to-bore
clearance.
.
.0.0014-0.0022
. . . . . . . . . ..  . . . . . . . . . . . . . . . . . . . . . . . ..
Pin
bore
diameter.
.
.0.9753
-0.9756
Ring groove width:
. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .
Top.
.0.0800
-0.0810
. . . . . . . . . . . . . . . . . . . .. . . . . ..
Bottom.
.
0.0800-0.0810
............................. .   .   .
Oil
ring.
.
.
..0.1880-0.1890
PISTON
PIN
3.150-3.170
Length
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
..
..
.
.
..
.
.
.
.
.
.
.
.
.
.
.
..
.
.
..
.
.
.
.
.
........
..
.
. . . .. . . . . .
.
. . . . . . . 0.9750-0.9753
Diameter
(std).
Diameter  (.001). . . . . . . . . . .
..
.
..........
.
. . . . . . . . 0.9760-0.9763
(.002). . . . . . . .
..
.
. . .. . . . . . .
.
. . . . . . . 0,9770-0.9773
Diameter
C l e a r a n c e  ( i n  p i s t o n ) . . . . . . . .. . .
.. .
.
............
.
. . . . . . . . . 0.0002-0.0004
Clearance  (in  rod  bushing). . . . . . . .
...
.
.............
.
. . .. . . . . . . . .i n t e r f e r e n c e
PISTON
RINGS
. . . . . . . .. . . . .. . . . . . . . .. . . . . . . .. . . . 0 . 0 7 7 0 - 0 . 0 7 8 0
Width
(compression
rings).
. . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .  0 . 0 0 2 - 0 . 0 0 4
Side clearance (topping).
S i d e  c l e a r a n c e  ( b o t t o m  r i n g ) . . . . . . . . .. . . .. . . . . . . . . ... .
. . . . . . . . . .. 0 . 0 0 2 5 - 0 . 0 0 4 0
S i d e c l e a r a n c e ( o i l r i n g ) . . . . . . . . . . . . ... . . . . .. . . . . . . . . . . . . . . . . .. . . snug  fit
R i n g  g a p  ( t o p  r i n g ) . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . 0 . 0 1 0 - 0 . 0 2 0
R i n g  g a p  ( b o t t o m  r i n g ) . . . . . . . . . . . . ., .. . . . . . . . . .. .. . . . . .  0 . 0 1 0 - 0 . 0 2 0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.015-0.055
Ring  gap  (oil  ring).

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