Engine Service Manual

76
Disassembly/Inspection and Service
KohlerEngines.com 32 690 03 Rev. I
Ring failure is usually indicated by excessive oil
consumption and blue exhaust smoke. When rings fail,
oil is allowed to enter combustion chamber where it is
burned along with fuel. High oil consumption can also
occur when piston ring end gap is incorrect because
ring cannot properly conform to cylinder wall under this
condition. Oil control is also lost when ring gaps are not
staggered during installation.
When cylinder temperatures get too high, lacquer and
varnish collect on pistons causing rings to stick, which
results in rapid wear. A worn ring usually takes on a
shiny or bright appearance.
Scratches on rings and pistons are caused by abrasive
material such as carbon, dirt, or pieces of hard metal.
Detonation damage occurs when a portion of fuel charge
ignites spontaneously from heat and pressure shortly
after ignition. This creates 2 fl ame fronts, which meet
and explode to create extreme hammering pressures on
a specifi c area of piston. Detonation generally occurs
from using low octane fuels.
Pre-ignition or ignition of fuel charge before timed spark
can cause damage similar to detonation. Pre-ignition
damage is often more severe than detonation damage.
Pre-ignition is caused by a hot spot in combustion
chamber such as glowing carbon deposits, blocked
cooling fi ns, an improperly seated valve, or wrong spark
plug(s).
Replacement pistons are available in STD bore size,
and 0.25 mm (0.010 in.) oversize. Replacement pistons
include new piston ring sets and new piston pins.
Replacement ring sets are also available separately for
STD, and 0.25 mm (0.010 in.) oversize pistons. Always
use new piston rings when installing pistons. Never use
old rings.
Some important points to remember when servicing
piston rings:
1. Cylinder bore must be de-glazed before service ring
sets are used.
2. If cylinder bore does not need re-boring and if old
piston is within wear limits and free of score or scuff
marks, old piston may be reused.
3. Remove old rings and clean up grooves. Never
reuse old rings.
4. Before installing new rings on piston, place top 2
rings, each in turn, in its running area in cylinder
bore and check end gap. Top compression ring end
gap clearance is 0.189/0.277 mm (0.0074/0.0109 in.)
for a new bore, or 0.531 mm (0.0209 in.) for a used
bore. Middle compression ring end cap is
1.519/1.797 mm (0.0598/0.0708 in.) for a new bore,
or 2.051 mm (0.0808 in.) for a used bore.
5. After installing new compression (top and middle)
rings on piston, make sure top compression ring-to-
groove side clearance and is 0.030/0.070 mm
(0.0010/0.0026 in.) and middle compression ring-to-
groove side clearance 0.030/0.070 mm
(0.0010/0.026 in.). If side clearance is greater than
specifi ed, a new piston must be used.
Inspection and Service
Check bearing area (big end) for excessive wear, score
marks, running and side clearances (See Specifi cations).
Replace rod and cap if scored or excessively worn.
Service replacement connecting rods are available in
STD size and 0.25 mm (0.010 in.) undersize. Undersized
rods, 0.25 mm (0.010 in.), have an identifi cation marking
on lower end of rod shank. Always refer to appropriate
parts information to ensure correct replacements are
used.
Piston and Rings
Inspection
Piston and Rings Components and Details
A
C
B
F
E
D
H
G
A Piston Ring B End Gap
C Identifi cation Mark D Piston
E
Top Compression
Ring
F
Middle Compression
Ring
G Rails H Expander
Scuffi ng and scoring of pistons and cylinder walls occurs
when internal engine temperatures approach welding
point of piston. Temperatures high enough to do this are
created by friction, which is usually attributed to improper
lubrication and/or overheating of engine.
Normally, very little wear takes place in piston boss-
piston pin area. If original piston and connecting rod can
be reused after new rings are installed, original pin can
also be reused but new piston pin retainers are required.
A piston pin is included as part of piston assembly – if
pin boss in piston or pin are worn or damaged, a new
piston assembly is required.