Piston And Rings Components And Details; Install New Piston Rings - Kohler ECV940 Service Manual

Disassembly/Inspection and Service
Piston and Rings
Inspection

Piston and Rings Components and Details

A
A Piston Ring
C Identifi cation Mark
Top Compression
E
Ring
G Rails
Oil Control Ring
I
(3 Piece)
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.
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.
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.
76
B
C
D
E
F
G
I
H
B End Gap
D Piston
Center
F
Compression Ring
H Expander
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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.
Preignition or ignition of fuel charge before timed spark
can cause damage similar to detonation. Preignition
damage is often more severe than detonation damage.
Preignition is caused by a hot spot in combustion
chamber from sources 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
in 0.25 mm (0.010 in.), and 0.50 mm (0.020 in.) oversize.
Replacement pistons include new piston ring sets and
new piston pins.
Replacement ring sets are also available separately
for STD, 0.25 mm (0.010 in.), and 0.50 mm (0.020 in.)
oversize pistons. Always use new piston rings when
installing pistons. Never reuse old rings.
Some important points to remember when servicing
piston rings:
1. Cylinder bore must be deglazed before service ring
sets are used.
2. If cylinder bore does not need reboring 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 and center
compression ring end gap clearance is 0.30/0.55
mm (0.011/0.021 in.) with maximum wear limit of
0.94 mm (0.037 in.).
5. After installing new compression (top and middle)
rings on piston, make sure ring-to-groove side
clearance is 0.04/0.08 mm (0.0015/0.0031 in.). If
side clearance is greater than specifi ed, a new
piston must be used.

Install New Piston Rings

NOTE: Rings must be installed correctly. Ring
installation instructions are usually included with
new ring sets. Follow instructions carefully. Use
a piston ring expander to install rings. Install
bottom (oil control) ring fi rst and top
compression ring last.
To install new piston rings, proceed as follows:
1. Oil control ring (bottom groove): Install expander and
then rails. Make sure ends of expander are not
overlapped.
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