Husqvarna 129C Workshop Manual page 47

Piston damage caused by a too high
engine speed.
Typical damage associated with a too high engine
speed includes broken piston rings, broken circlip
on the gudgeon pin, faulty bearings or that the
guide pin for the piston ring has become loose.
Piston ring breakage
A too "lean" carburettor setting results in a too
high speed and a high piston temperature. If the
piston temperature rises above the normal working
temperature the piston ring can seize in its groove,
consequently it will not sit deep enough in its
groove. The edges of the piston ring can then hit
the top edge of the exhaust port and be smashed
and also cause piston damage.
A too high engine speed can also cause rapid wear
to the piston ring and play in the piston ring groove
primarily in front of the exhaust port. The ring is
weakened by the wear and can be caught in the
port causing serious piston damage.
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.
Gudgeon pin circlips vibrated loose
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.
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.
Exhaust side damaged by a broken piston
ring. The piston ring parts damage the top
section and cause score marks.
Piston ring giude pin has been pushed up
through the top of piston.
Gudgeon pin circlip became loose and
caused deep, irregular grooves. Shown here
on the piston's inlet side.
Irregular grooves on the piston's inlet side
caused by a broken roller retainer.
Cylinder and piston
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