Saturday, June 25, 2011

Scored piston

Scored piston

Fig 7.27
Scoring is an advanced stage of scuffing (Figure 7.27). As well as the original cause of scuffing, continued operation of the engine with scuffed pistons and cylinders will generate excessive heat and friction. This will exaggerate the problem, so that a large area of the piston becomes scored.
Deep scoring in a cylinder can be the result of a mechanical failure, such as a broken piston ring, a broken circlip, or a loose piston pin. This type of damage could be confined to only one cylinder and the cause is easy to see.
Abrasion is a type of light scoring that could be caused by dust and dirt entering the engine through a faulty or badly serviced air cleaner.
There will be abrasive wear to the valve sterns and guides as well as to the pistons, rings and cylinders. The scoring will not be deep, but will be in the form of a mat finish of very fine score marks covering all the rubbing surfaces. All the cylinders would be affected.

Fig 7.28
Seized piston
With a seized piston, the entire thrust surfaces would be marked (Figure 7.28). A likely cause is insufficient lubrication. If this is the correct diagnosis, the bearings and journals will also be affected and the engine oil will be contaminated.
Other factors that could cause a breakdown in the oil film between the piston and cylinder must also be considered, such as cooling-system problems, or blowby due to sticking rings.

·              Scuffing, scoring and seizing are conditions that are closely related. In some cases, they could be three stages of damage from the same initial cause.

Fig 7.29
Corrosion can affect a piston as shown in Figure 7.29. This can be the result of coolant leaking into the combustion chamber and finding its way down past the piston rings. This would affect cylinder-wall lubrication and could cause scuffing.
A coolant leak can contaminate the engine oil and form sludge in the oil pan. This will further affect cylinder lubrication. Blowby can also contaminate the engine oil, and make the additives less effective.
Blowby will occur if the piston rings do not seal properly. The hot gases that blow past the piston will increase the temperature of the piston, piston rings and cylinder walls.
The tension of the piston rings could be lost because of overheating, and so the problem of blowby increased. Where the engine is continued in operation, this can destroy the piston lands and rings as shown in Figure 7.30.
Fig 7.30
Piston damage

Figure 7.31 shows damage to the piston-pin boss area of a piston. This has been caused by a loose circlip. Circlips must be a good fit in their grooves and should be renewed whenever an engine is dismantled.
Misalignment of the connecting rod can cause side thrusts on the piston pin which are transmitted to the circlip. This could break the circlip, or damage the piston. If the circlip breaks, particles could score the piston and cylinder, or jam the piston rings, to eventually produce damage such as that illustrated. 

Fig 7.31

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