Kohler K91 Service Manual page 105

CYLINDER BLOCK
Inspection and Recond itioning
Check all gasket surfaces to make sure they are
free of gasket fragments . Gasket surfaces must
also be free of deep scratches or nicks .
Scoring of the Cylinder Wall : U nburned fuel , in se­
vere cases, can cause scuffing and scoring of the
cylinder wall . As raw fuel seeps down the cylinder
wall, it washes the necessary lubricating oils off
the piston and cylinder wall so that the piston rings
make metal to metal contact with the wal l . Scoring
of the cylinder wall can also be caused by local­
ized hot spots resulting from blocked cooling fins
o r from inadequate or contaminated lubrication .
If the cylinder bore i s badly scored , excessively
worn , tapered , or out of round, resizing is neces­
sary. Use an inside micrometer to determine
amount of wear (see " General I nformation " , Sec­
tion 1 ) , then select the nearest suitable oversize
of either . 003 " , . 0 1 0 " . 0 20 " or .030 " . Resizing to
one of these oversizes will allow usage of the
available oversize piston and ring assemblies. I n­
itially. resize using a boring bar, then use the fol­
lowing procedures for honing the cylinder:
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 pre­
ferred 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 fol­
lows :
Lower hone into bore and after centering , ad­
1 .
just so that stones are in contact with the cyl­
inder wall . Use of a commercial cutting-cool­
ing agent is reco mmended .
2 .
With the lower edge of each stone positioned
even with the lowest edge of the bore , start
drill and honing process . Move hone up and
down while resizing to prevent formation of
cutting ridges . Check size frequently.
NOTE : Keep in mind the temperatures caused by
honing may cause inaccurate measurements. Make
sure the block is cool when measuring .
3 . When bore is within . 0025 " of desired size.
remove coarse stones and replace with bur­
nishing stones . Continue with burnishing stones
until within . 0005 " of desired size and then use
1 1 . 2
finish stones ( 220-280 grit) and polish to final
size . A crosshatch should be observed if hon­
ing is done correctly. The crosshatch should
intersect at approximately 23 0-330 off the
horizontal . Too flat an a ngle could cause the
rings to skip and wear excessively, too high an
angle will result in high oil consumption . Refer
to Figure 1 1 -2 .
Figu re 1 1 - 2 . 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 . Refer to Figure 1 1 -3 . The
measurements should be taken at three loca­
tions in the cylinder - at the top, middle , and
bottom . Two measurements should be taken
(perpendicular to each other) at each of the
three locations .
Figure 1 1 -3 . Measu ring Cylinder Bore.
Thoroughly clean cylinder wall with soap and
5 .
hot water. Use a scrub brush to remove all
traces of boring/honing process . Dry thor­
oughly and apply a light coat of SAE 1 0 oil to
prevent rust .