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. |
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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- | |||
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| tween the piston and cylinder wall, | |
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| damaging the piston skirt. | |
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| Debris can also pass up through the | |
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| cylinder’s transfer channels and cause | |
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| damage to the top and sides of the piston | |
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| as well as to the cylinder’s combustion | |
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| chamber. |
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