9.9
Section 9
Inspection and Reconditioning
9
Overheated or Deteriorated Oil
Scored Piston and Rings
Abrasive Scratched RingsStuck, Broken Rings
Figure 9-8. Common Types of Piston Damage.
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 a er ignition. This creates two fl ame fronts
which meet and explode to create extreme hammering
pressures on a specifi c 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 o en 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 fi ns, an
improperly seated valve, or wrong spark plug(s).
See Figure 9-8 for some common types of piston and
ring damage.
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