istons play an essential role at the core of an internal combustion engine.
Made up from a series of moving cylinders fitted with rings, pistons are designed to create an air-tight seal once fitted within the engine cylinder.
The power in an internal combustion engine is generated by downward force continuously pushing down on the pistons.
Piston force is calculated by taking the area of the piston multiplied by the pressure of the gas pushing down on them. The larger the pistons and the higher the gas pressure, the more power generated.
Bridging the gap between the pistons and the cylinder wall in an area are the piston rings.
Termed the piston belt, the spaces between the openings are known as ring lands.
PEach piston sits in a short hollow tube known as the wrist pin, attached to the connecting rods. The wrist pin is designed to carry the full force of combustion.
Because it needs to stand up to constant pressure, the piston's side surface must be exceptionally smooth to keep it vertically upright. These side surfaces are known as the piston skirt.
The piston rings fit around the piston, bridging the small clearance between the piston and cylinder wall. Typically, there are three piston rings on each piston, all of them performing a separate function.
The uppermost pair of piston rings are the compression rings whose principal role is to prevent gases, known as blowby, from escaping from between the cylinder wall and the piston to maintain maximum compression.
Typically produced from solid cast iron, compression rings generate pressure driven by the natural resilience of the rings on the cylinder wall.
The lowest position is the oil control ring. The engine is continually sprayed with oil during the compression process, coming from minute holes in the connecting rods and onto the cylinder walls.
In order to minimise friction, a thin film builds up on the wall of the piston. The role of the oil control ring is to prevent the buildup of excess oil, with the remaining level of oil at the optimum level so that the compression rings and piston skirt can glide over them smoothly.
When considering a classic car for purchase, it is challenging to assess any mechanical damage to the pistons, as they are situated inside the combustion chamber.
If the piston can be viewed and are showing damage, such as their tops appear to have melted or have been severely burned, it generally means that a condition known as preignition has caused severe damage to the piston through excess heat being generated in the combustion chamber.
If the engine turns over, the first thing to look for is a loss of compression which will cause the engine to misfire and a consequent loss of power.
A common problem that can arise with pistons goes by the exotic title of "piston slap."
Piston slap occurs when wear in either the cylinder wall or the piston skirt leads to a buildup in the clearances between the piston and the cylinder wall causing excessive sideways movement of the piston.
As the piston changes its direction at either the top and bottom of its stroke, it will knock against the cylinder wall causing a noise known as piston slap or false knock.
Piston slap occurs when wear in either the cylinder wall or the piston skirt leads to a buildup in the clearances between the piston and the cylinder wall causing excessive sideways movement of the piston.
As the piston changes its direction at either the top and bottom of its stroke, it will knock against the cylinder wall causing a noise known as piston slap or false knock.
Piston slap can be cured by having the cylinder machined or by fitting a piston that is slightly larger in diameter.
If the engine appears to be underpowered or is stalling intermittently, it may be worthwhile to carry out a compression test.
Signs of reduced compression in a cylinder will indicate that it is more or less likely that the piston is badly cracked.
The only way is to open the cylinder head and remove the faulty part to enable a thorough inspection.
If faults are discovered the chances are that the pistons will need to be sent for repair or be replaced.
Any hint of blue smoke spewing from the exhaust, is an almost sure sign that the valve guides have become worn or the valve guide seals are damaged.
Other issues that can occur inside pistons include broken or worn piston rings or a cracked piston's worst scenario.
Replacing damaged piston rings is an expensive process and best left to experts as it entails some intricate machining to return the pistons to the necessary high levels of exactitude they need to work at.
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