Kohler COMMAND CH18, Service Manual

Page: 192 / 232

10.9
Section 10
Inspection and Reconditioning
10
Ring 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 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 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 cooling fins, an
improperly seated valve, or wrong spark plug(s).
See Figure 10-8 for some common types of piston and
ring damage.
Overheated or Deteriorated Oil Scored Piston and Rings
Abrasive Scratched Rings Stuck, Broken Rings
Figure 10-8. Common Types of Piston Damage.