Piston knock or slap is a loud metallic knocking sound produced when the piston flops back and forth inside its cylinder.
It is caused by excessive piston skirt or cylinder wear, clearance and possible damage.
Piston slap is normally louder when engine is cold and tends to quiet down as the engine reaches operating temperature.
Piston pin knock occurs when too much clearance exists between the piston pin and piston pin bore or connecting rod bushing. Excessive clearance allows the pin to hammer against the rod or piston as the piston changes direction in cylinder.
Piston pin knock will usually make a double knock (two rapid knocks and then a short pause). II does not change much with engine load. Worn piston rings and cylinders result in blow-by, blue gray engine smokes low engine power, spark plug fouling and other problems caused by poor ring sealing, burned piston is often a result of precognition or detonation damage. Abnormal combustion excessive pressure and heat actually melt and blow a hole in the piston crown or area around the ring lands. The engine may smoke, knock, have excessive blow-by or other symptoms.
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