Husqvarna 555FX Workshop Manual page 49

The guide pin for the piston ring has been
pushed up through the top of piston.
Piston ring guide pin
vibrated loose
A too high engine speed can cause the
ends of the piston ring to hammer against
the guide pin when the piston ring moves
in its groove. The intensive hammering
can drive out the pin through the top of the
piston causing serious damage also to the
cylinder.
Small score marks and a matt, grey surface
on the piston's inlet side caused by fi ne dust
particles.
Cylinder and piston
Deep, irregular grooves caused by a loose
circlip. Shown here on the piston's inlet side.
Damage caused by
gudgeon pin circlips
A too high engine speed can cause
the gudgeon pin circlips to vibrate. The
circlips are drawn out of their grooves due
to the vibrations, which in turn reduces
the circlips' tensioning power. The rings
can then become loose and damage the
piston.
Foreign objects
Everything other than clean air and pure fuel that enters the engine's inlet port
causes some type of abnormal wear or damage to the cylinder and piston.
This type of increased wear shows on the piston's inlet side starting at the lower
edge of the piston skirt.
The damage is caused by badly fi ltered air that passes through the carburettor and
into the engine.
Irregular grooves on the piston's inlet side
caused by a broken roller retainer.
Bearing failure
Failure of the crankshaft bearing or on the
connecting rod bearing is usually caused
by a too high engine speed, resulting in the
bearing being overloaded or overheating.
This in turn can cause the bearing rollers
or ball to glide instead of rotate, which can
cause the roller or ball retainer to break.
The broken debris can be trapped between
the piston and cylinder wall, damaging the
piston skirt.
Debris can also pass up through the
cylinder's transfer channels and cause
damage to the top and sides of the piston
as well as to the cylinder's combustion
chamber.
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