Husqvarna Kohler SV470, Workshop Manual

Page: 89 / 116

9.10
Section 9
Inspection and Reconditioning
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 that
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 9-12 for
some common types of piston and ring damage.
Replacement pistons are available in STD and 0.08
mm (0.003 in.) oversize, which include new rings and
piston pins. Service replacement piston ring sets are
also available separately. Always use new piston rings
when installing pistons. Never reuse old rings.
The cylinder bore must be deglazed before service
ring sets are used.
Abrasive Scratched Rings Stuck, Broken Rings
Overheated or Deteriorated Oil Scored Piston and Rings
Figure 9-12. Common Types of Piston and Ring Damage.