This paper presents changes in the tightness of a combustion chamber during a 1200-hour bench durability test. The study was conducted on three four-cylinder turbocharged DI diesel engines with a capacity of 1.3 dm3. The changes in the tightness were determined on the basis of the results ofperiodical measurements of the blow-by flow rate under dif­ferent conditions of engine operation and measurements of maximum compression pressure in the cylinders. The results showed that the maximum compression pressure remained at the same level during a long period of engine operation, corresponding to a distance of150 000 km covered by a car. After that period, the pressure started to decrease (after a distanceof200 000 km, it had dropped by 10 %). The blow-by flowrate decreased in the initial period of engine operation and then it started to rise linearly atfull engine load. The rate of increase depended on engine speed - it was the smallest at low speeds (20 % increase in the time range of100-1200 hours, which corresponded to a distance of16 000-192 000 km covered by a car - at 1000 rpm), and the highest at 4000 rpm (an increase of 70% in the same time range).