PISTON ASSEMBLY

1. Piston Malfunctions

The piston assembly is a common cause of engine malfunctions. Let's start with the defect thatcould be most injurious to the engine due to broken fragments, a cracked piston head. If the piston head has a hole or crack in it, how would that affect the intake, compression, and power  strokes? If you knew that a cracked piston would allow pressure to pass through it, you are on the right track. When a piston is cracked or has a hole in it, a partial vacuum cannot be created on the intake stroke; therefore, compression and power cannot take place. Another factor that will help you with your diagnosis is the sound usually produced by a cracked piston head. Remove a spark plug from an engine and let it run for a moment or two. The "clacking" sound produced by the missing spark plug is very similar to the sound that may be produced by a cracked piston.Before pulling the cylinder head off to verify a misfire you suspect is a cracked piston, test the cylinder in case the malfunction is caused by another source. What tests are you going to perform on the cylinder?

2. Piston Troubleshooting and Remedies

Complete a "dry" compression test to ensure that this is the only defective cylinder. To make repairs on one cylinder and reassemble the engine only to find that other cylinders are defective due to worn rings is a waste of effort and money. Upon completion of the "dry" test, a "wet" test must be performed on the known defective cylinder and on any others that are found defective during the "dry" test. If a piston is cracked, the "dry" test and the "wet" test should produce the

about same readings. If the "wet" test reading rises above that of the "dry" test and you do not suspect a scored cylinder wall, what might be a cause of the rise in pressure? By injecting oil into the cylinder for the "wet" test, you have improved the seal between the piston rings and the cylinder walls. Therefore, if the readings noted on the "wet" test are substantially higher than those of the "dry" test, you may safely assume that the piston rings are worn and must be replaced. In some instances, however, the crack in the piston head may be so slight that the oil will form a temporary seal. If the piston rings are worn to the extent that the engine is losing compression, a coating of black soot from burned oil should be apparent on the inner wall of the tailpipe. Additionally, when the engine is "revved" or "gunned," blue smoke should emit from the tailpipe. This smoke will not dissipate as fast as steam does. When the spark plugs are removed, they will also be coated with black soot on the electrode end (firing end). The only remedy for worn rings is to replace them.