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10.8
Section 10
Inspection and Reconditioning
Pistons and Rings
Inspection
Scuffing and scoring of pistons and cylinder walls
occurs when internal temperatures approach the
welding 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
bosses or the 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 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. 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, improperly seated valve, or
wrong spark plug. See Figure 10-7 for some common
types of piston and ring damage.
Figure 10-7. Common Types of Piston and Ring Damage.
Abrasive Scratched RingsStuck, Broken Rings
Overheated or Deteriorated Oil
Scored Piston and Rings