Medium speed four-stroke diesel engines are internal combustion engines with a nominal speed of 250 min⁻¹ to 750 min⁻¹. They are widely used diesel-generator driving machines for the electricity production (high-speed diesel engines and slow-speed two-stroke diesel engines can be used), and they are common as propulsion machines on both river and sea transport ships. The general overhaul of this diesel engines is mainly planned maintenance of the machine. In the opposite case, it is a failure (breakdown), what makes the repair very expensive. Preventive and planned maintenance between two general overhauls are of crucial to prevent machine failure. When and in what quantity the overhaul will be carried out depends first of all of the condition, engine performance and the time interval between two overhauls prescribed by the maker. A high-quality and professionally performed engine overhaul ensures safe exploitation and long-term operation of the engine until the next planned general overhaul. It is often case that, in the desire for financial savings, parts that are not original or with dubious quality are installed, what of course should be avoided. Nowadays, the installation of parts that affect air pollution or the emission of exhaust gases (NOx) is prohibited. Disassembly, inspection of the components of the piston mechanism and other parts, measurement are performed according to the instructions from the manufacturer's instruction manual for the specific engine type.