Kohler LH630-775 Service Manual page 137

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 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
Stuck, Broken Rings
Overheated or Deteriorated Oil
Figure 10-8. Common Types of Piston and Ring Damage.
Inspection and Reconditioning
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 10-8 for some common
types of piston and ring damage.
Abrasive Scratched Rings
Scored Piston and Rings
Section 10
10
10.9