Kohler Command PRO CS Service Manual page 122

Section 10
Internal Components
Pistons and Rings
Inspection
Scuffing and scoring of pistons and cylinder walls
occurs when internal temperatures approach the
melting point of the piston. Temperatures high
enough to do this are created by friction, which is
usually attributed to improper lubrication, and/or
overheating of the engine.
Normally, very little wear takes place in the piston
boss-piston pin area. If the original piston and
connecting rod can be reused after new rings are
installed, the original pin can also be reused but new
piston pin retainers are required. The piston pin is
included as part of the piston assembly–if the pin boss
in the piston or the pin are worn or damaged, a new
piston assembly is required.
Ring wear/failure is usually indicated by excessive oil
consumption and blue exhaust smoke. When rings fail,
oil is allowed to enter the combustion chamber where
it is burned along with the fuel. High oil consumption
can also occur when the piston ring end gap is
incorrect because the ring cannot properly conform to
the 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 the piston causing the 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 the fuel
charge ignites spontaneously from heat and pressure
shortly after ignition. This creates two flame fronts
which meet and explode to create extreme hammering
pressures on a specific area of the piston. Detonation
generally occurs from using low octane fuels.
Preignition or ignition of the fuel charge before the
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 the combustion chamber from sources such as:
glowing carbon deposits, blocked fins, improperly
seated valve, or wrong spark plug.
See Figure 10-13 for common types of piston and ring
damage.
10.10
Stuck, Broken Rings
Abrasive Scratched Rings
Abrasive Worn Rings
Scored Piston and Rings
Figure 10-13. Common Types of Piston and Ring
Damage.