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10.3
Section 10
Inspection and Reconditioning
10
Check the cylinder bore wall for scoring. In severe
cases, unburned fuel can cause scuffing and scoring
of the cylinder wall. It washes the necessary lubricating
oils off the piston and cylinder wall. As raw fuel seeps
down the cylinder wall, the piston rings make metal to
metal contact with the wall. Scoring of the cylinder wall
can also be caused by localized hot spots resulting
from blocked cooling fins or from inadequate or
contaminated lubrication.
If the cylinder bore is badly scored, excessively worn,
tapered, or out-of-round, resizing is necessary. Use an
inside micrometer to determine the amount of wear
(refer to the “Specifications, Tolerances, and Special
Torque Values”, in Section 1), then select the nearest
suitable oversize of either 0.25 mm (0.010 in.) or 0.50
mm (0.020 in.). Resizing to one of these oversizes will
allow usage of the available oversize piston and ring
assemblies. Initially, resize using a boring bar, then use
the following procedures for honing the cylinder.
NOTE: Some CH25-26 engines feature POWER-
BORE
™
cylinders a special pantented nickel-
silicone plating process for increased power,
superior oil control, reduced exhaust
emission, and virtually permanent cylinder life.
POWER-BORE™ cylinders cannot be resized
or honed as described in the following
procedure. If a plated cylinder bore is
damaged or out of specification, use a new
miniblock or short block to repair the engine.
Use the following procedure for crankcases
with a cast iron sleeve.
Honing
While most commercially available cylinder hones can
be used with either portable drills or drill presses, the
use of a low speed drill press is preferred as it
facilitates more accurate alignment of the bore in
relation to the crankshaft crossbore. Honing is best
accomplished at a drill speed of about 250 RPM and
60 strokes per minute. After installing coarse stones in
hone, proceed as follows:
1. Lower hone into bore and after centering, adjust
so the stones are in contact with the cylinder wall.
Use of a commercial cutting-cooling agent is
recommended.
2. With the lower edge of each stone positioned
even with the lowest edge of the bore, start drill
and honing process. Move the hone up and down
while resizing to prevent the formation of cutting
ridges. Check the size frequently.
Figure 10-3. Cylinder Bore Crosshatch after
Honing.
4. After resizing, check the bore for roundness,
taper, and size. Use an inside micrometer,
telescoping gauge, or bore gauge to take
measurements. The measurements should be
taken at three locations in the cylinder – at the
top, middle, and bottom. Two measurements
should be taken (perpendicular to each other) at
each of the three locations.
Clean Cylinder Bore After Honing
Proper cleaning of the cylinder walls following boring
and/or honing is very critical to a successful overhaul.
Machining grit left in the cylinder bore can destroy an
engine in less than one hour of operation after a
rebuild.
NOTE: Kohler pistons are custom-machined to
exacting tolerances. When oversizing a
cylinder, it should be machined exactly 0.25
mm (0.010 in.) or 0.50 mm (0.020 in.) over
the new diameter (Section 1). The
corresponding oversize Kohler replacement
piston will then fit correctly.
3. When the bore is within 0.064 mm (0.0025 in.) of
the desired size, remove the coarse stones and
replace them with burnishing stones. Continue
with the burnishing stones until the bore is within
0.013 mm (0.0005 in.) of the desired size and
then use finish stones (220-280 grit) and polish
the bore to its final size. A crosshatch should be
observed if honing is done correctly. The
crosshatch should intersect at approximately 23°-
33° off the horizontal. Too flat an angle could
cause the rings to skip and wear excessively, and
too steep an angle will result in high oil
consumption. See Figure 10-3.