Husqvarna 343R, 345RX, 343F, 345FX, 345FXT manual Piston damage caused by a too high engine speed

Cylinder and piston 6

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.

Exhaust side damaged by a broken piston ring. The piston ring parts damage the top section and cause score marks.

The guide pin for the piston ring has been	Deep, irregular grooves caused by a loose	Irregular grooves on the piston’s inlet side
pushed up through the top of piston.	circlip. Shown here on the piston’s inlet	caused by a broken roller retainer.
	side.

Piston ring guide pin	Damage caused by	Bearing failure
vibrated loose	gudgeon pin circlips	Failure of the crankshaft bearing or on the
A too high engine speed can cause the	A too high engine speed can cause the	connecting rod bearing is usually caused
		by a too high engine speed, resulting in
ends of the piston ring to hammer against	gudgeon pin circlips to vibrate. The
		the bearing being overloaded or overheat-
the guide pin when the piston ring moves	circlips are drawn out of their grooves due
in its groove. The intensive hammering	to the vibrations, which in turn reduces	ing. This in turn can cause the bearing
		rollers or ball to glide instead of rotate,
can drive out the pin through the top of	the circlips’ tensioning power. The rings
		which can cause the roller or ball retainer
the piston causing serious damage also	can then become loose and damage the
		to break.
to the cylinder.	piston.
		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.

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