Honing; Clean Cylinder Bore After Honing; Measuring Piston-To-Bore Clearance - Kohler CH940-CH1000 Service Manual

Honing

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 new diameter.
corresponding oversize Kohler replacement
piston will then fi t correctly.
While most commercially available cylinder hones can
be used with either portable drills or drill presses, use
of a low speed drill press is preferred as it facilitates
more accurate alignment of bore in relation to 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
stones are in contact with cylinder wall. Use of a
commercial cutting-cooling agent is recommended.
2. With lower edge of each stone positioned even with
lowest edge of bore, start drill and honing process.
Move hone up and down while resizing to prevent
formation of cutting ridges. Check size frequently.
3. When bore is within 0.064 mm (0.0025 in.) of
desired size, remove coarse stones and replace
them with burnishing stones. Continue with
burnishing stones until bore is within 0.013 mm
(0.0005 in.) of desired size and then use fi nish
stones (220-280 grit) and polish bore to its fi nal size.
A crosshatch should be observed if honing is done
correctly. Crosshatch should intersect at
approximately 23°-33° off horizontal. Too fl at an
angle could cause rings to skip and wear
excessively, and too steep an angle will result in high
oil consumption.
4. After resizing, check bore for roundness, taper, and
size. Use an inside micrometer, telescoping gauge,
or bore gauge to take measurements.
Measurements should be taken at three locations in
cylinder – at top, middle, and bottom. Two
measurements should be taken
(perpendicular to each other) at each of three
locations.
62 690 01 Rev. C
23°-33°
Crosshatch
KohlerEngines.com

Clean Cylinder Bore After Honing

Proper cleaning of cylinder walls following boring and/
or honing is very critical to a successful overhaul.
Machining grit left in cylinder bore can destroy an engine
in less than one hour of operation after a rebuild.
Final cleaning operation should always be a thorough
scrubbing with a brush and hot, soapy water. Use a
strong detergent that is capable of breaking down
machining oil while maintaining a good level of suds. If
suds break down during cleaning, discard dirty water and
start again with more hot water and detergent. Following
scrubbing, rinse cylinder with very hot, clear water, dry
it completely, and apply a light coating of engine oil to
prevent rusting.

Measuring Piston-to-Bore Clearance

Measure 11 mm above
Bottom of Piston Skirt at
Right Angles to Piston Pin
Before installing piston into cylinder bore, it is necessary
that clearance be accurately checked. This step is
often overlooked, and if clearances are not within
specifi cations, engine failure will usually result.
NOTE: Do not use a feeler gauge to measure piston-to-
bore clearance – it will yield inaccurate
measurements. Always use a micrometer.
Use following procedure to accurately measure piston-
to-bore clearance:
1. Use a micrometer and measure diameter of piston
11 mm (0.433 in.) above bottom of piston skirt and
perpendicular to piston pin.
2. Use an inside micrometer, telescoping gauge, or
bore gauge and measure cylinder bore. Take
measurement approximately 63.5 mm (2.5 in.) below
top of bore and perpendicular to piston pin.
3. Piston-to-bore clearance is difference between bore
diameter and piston diameter (step 2 minus Step 1).
Disassembly
11 mm (0.433 in.)
55