Why piston cracked

The piston-to-wall clearance is a measure of the space between the outer piston surface and the cylinder wall. Even a small crack in the piston wall can change the shape of the piston slightly, increasing the resistance between the cylinder wall and the piston. This will affect the smoothness of the engine and can cause excess heat and a decrease in engine power. If the piston to wall clearance increases then this can cause the piston rings to leak, allowing oil into the cylinder.

As outlined above, engine oil in the cylinder will affect the performance of the engine causing a decrease in engine power. An increase in the gap between the piston and cylinder wall will also affect the internal pressure of the cylinder. A decrease in compression can cause engine misfires and a decrease in engine power. If you regularly see grey or blue smoke coming from the exhaust on your car, this can be a symptom of a damaged or cracked piston.

In most cases, the grey or blue smoke is caused by burning oil. A piston with a cracked wall may leak oil into the cylinder. When this oil burns, then it creates a blue or dark grey smoke from the exhaust. Dark smoke from the exhaust can also be caused by a bad head gasket or by too much fuel in the fuel mix. A cracked piston can also make a distinct slapping noise, especially when the vehicle is accelerating.

It is often described as a bell-like rattling or knocking sound and is commonly referred to as piston slap. Piston slap is caused by damage or excessive wear to the piston. The banging or clapping noise is the piston hitting against the cylinder wall as it moves up and down. Damage and wear to a piston is what ultimately leads to piston slap. A cracked piston that has changed shape or causes piston ring failure can also lead to piston slap.

The cracked piston may cause damage to the cylinder wall, the connecting rods, and the piston rings and this, in turn, can further compound the issue. Another common symptom of a cracked piston is a drop in cylinder compression. Similar to piston ring failure, a cracked piston may affect the seal between the cylinder and the engine block, allowing oil in and air out.

A simple cylinder leak-down test will confirm if there is a problem with the piston seal and its ability to maintain the proper pressure. To confirm a cracked piston, the engine will need to be stripped down and the pistons removed for inspection. Excessive deposits of carbon or hot spots in the combustion chambers sharp edges that are thin enough to glow red-hot and cause pre ignition.

Diesel engines suffer the same type of damage and is usually caused by faulty injectors, injector pump or pump timing. If the damage is concentrated on the crown of the piston it is usually foreign material entering the combustion chamber or a failure of a cylinder head component such as valve or valve seat.

Ample evidence can usually be gathered by close examination of the head, carburettor and intake system. Often the failed part may belong to another cylinder but finds its way via the intake manifold. Detonation or pre-ignition is the other cause. In petrol engines the causes for detonation are usually, over advanced ignition timing base or total advance , compression too high or incorrect octane rating of the fuel.

Primary Menu Skip to content. Secondary Menu Skip to content. Piston Engine Problem Please select the menu for your specific piston problem. Picked Up. Burnt Out. Worn Out. Piston engine problem - CRACKED Cracks in the top of a piston crown in petrol engines are usually the result of excessive combustion pressure caused by excessive compression or over advanced ignition timing. These conditions cause excessive combustion pressure, which in turn causes the piston crown to crack as the piston is operating outside the pressure it was designed to work under.

In diesel engines this damage can be caused by a condition called thermal fatigue. Back to search. Information on diagnostics. Overstressing can occur due to: knocking combustion, severe vibrations of the piston, for example if the piston head has mechanical contact with the cylinder head, material defects, excessive skirt clearance.

Formation of a cleavage fracture that leads to the piston crown. This results in the piston being split into two parts Fig. Boss fatigue crack in the centre axis of the piston pin bore Fig.

Abnormal combustion, in particular spontaneous combustion caused by ignition delay. Excessive or inappropriate use of starting aids during cold starts.

The cylinder has filled up with water, fuel or oil whilst the engine is stopped hydraulic lock. Performance enhancements e. Incorrect or weight-reduced piston pins. The piston pin is deformed to an oval shape, placing excessive loads on the piston pin bed in the process. Impact marks on the piston crown Fig. Fracture in the direction of the piston pin due to vibrations and the effects of violent impact.

Piston skirt has broken off in the lower oil ring groove, fracture surfaces display the characteristics of a fatigue fracture Fig. Excessive clearances in the connecting rod bearings or a worn-out connecting rod bearing, particularly in conjunction with over-revving when driving downhill. The gap dimension the minimum distance between the piston crown and the cylinder head was too small at TDC of the piston.

The following scenarios may have caused this: Pistons with an incorrect compression height. During engine reconditioning, the mating surface of the cylinder block is often reworked. If pistons with the original compression height are then refitted after machining, the piston protrusion may be too large. This is why pistons with a reduced compression height are available for repairs, enabling the piston protrusion to be kept within the tolerance range specified by the engine manufacturer.

Many manufacturers provide cylinder head gaskets with different thicknesses for the same engine. This is necessary to compensate for component tolerances during production, and it also allows adaptation of the piston protrusion during repairs.

For this reason, when carrying out repairs ensure that only cylinder head gaskets with the prescribed material thickness are used. This ensures that the specified gap dimension will be achieved after the repair. Is it brown and mucky? A local garage will be able to tell you if it has the correct amount of antifreeze. Finally, check when the spark plugs on a petrol engine were last changed. Most will last 60, miles with no problem, but consult your handbook because some cars differ. Go to front page.

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