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Cylinder and piston
Exhaust side damaged by a broken piston
ring. The piston ring parts damage the top
section and cause score marks.
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.
The guide pin for the piston ring has been
pushed up through the top of piston.
Irregular grooves on the piston’s inlet side
caused by a broken roller retainer.
Deep, irregular grooves caused by a loose
circlip. Shown here on the piston’s inlet
side.
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.
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.
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 overheat-
ing. 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 be-
tween 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.