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Your Piston worn out? » Read more about common malfunctions • Repair • Replacement manual
The heart of the engine: The piston The piston is the central component in an internal combustion engine. It turns the force of the explosion of the air-fuel mixture into a linear motion, which is then converted to a circular motion at the connecting rod. Although subjected to enormous pressure, the piston usually outlives the life of the car. The piston is also sometimes called a piston rod. Function and position of the piston rod The piston rod is located deep inside the engine's inner workings. It is connected to the crankshaft via the bottom side of the connecting rod. The top side forms the combustion chamber along with the closed valves. When the fuel is injected through the injectors, the exhaust valve is closed and the spark is triggered by the spark plug, the fuel-air mixture explodes. This explosion pushes the piston towards the crank shaft. The connecting rod converts the piston's linear motion inside the cylinder liner to the circular motion of the crank shaft. This circular motion is the primal power driving all other units and force paths in the vehicle. The piston rod consists of a monolithic cast part subsequently finished by turning and milling. Its exterior features three lateral grooves. Its bottom side features two openings, to which the connecting rod is connected via a bolt. The bolt is held in place with the help of spring washers. The piston rings and the oil scraper ring are inserted in the lateral grooves. The piston rings serve to seal the combustion chamber. The oil scraper ring ensures that the oil fed by the crankshaft does not reach the combustion chamber. Piston rod defects The piston rod may be damaged by mechanical, tribological and thermal influences. A defect on the piston rod is considered major damage to the engine, which is very expensive to fix in any case. Usually, many additional components must be repaired in the course of repairing piston damage. Mechanical damage to the piston occur if something makes its way between the piston and its free run. Among other things, this can be the result of installing a spark plug that is too long. Piston are made of high-strength aluminum, which, however, is easily pierced. Another common cause of piston damage is breakage of the timing belt. In this case, the piston will hit the open outlet valve and is also immediately destroyed. Tribological damage occurs when the piston is no longer sufficiently lubricated. If there is too little oil in the lubrication system, the lubrication film on the inner wall of the cylinder liner rips off. Consequently, the piston ring will wear off material from the wall until the compression pressure can escape into the crankshaft's housing. This results in abrupt loss of compression, causing the car to run slower and slower. The most common damage to the piston rod is of thermal origin. If the engine overheats due to a failure of the cooling system, all moving parts expand heavily. In less serious cases, this will result in the so-called "piston jamming". This just means that the piston rod is stuck inside the liner. However, in most cases, an overheated engine leads to the much more serious "piston seizure". In this case, the piston's inner wall is damaged, just as is the case with insufficient lubrication. Repairing piston damage If an engine has suffered piston damage, it must be removed and completely disassembled. In most cases, the old piston rods can't simply be replaced by new ones. Typically, the damage in the liner is so significant, that one must either replace the short motor or drill out the cylinder liner and rehone it. Then, only a so-called "oversize piston" will fit inside the now enlarged cylinder liner. Quality of original pistons Due to the enormous effort indicated for the repair of a vehicle with piston damage, care should be taken to always opt for maximum quality replacement parts. Oversize replacement pistons are precision-machined components, which come at a price: A new piston rod by MAHLE costs upwards of 150 euro. A high-performance product can easily cost twice that. Prevention is always the best way of piston repair: Ensuring there's always enough oil and coolant, and closely adhering to the timing belt's servicing intervals makes the need for repairing piston rods very unlikely.