Figure 102. Location of marks when the piston of the first cylinder is at TDC
The valve timing cannot be adjusted. This is why it is necessary to follow all the instructions in this manual if it becomes necessary to remove the camshaft drive chain, drive sprockets and other parts that affect the valve timing system. Alignment and centering of the valve timing mechanism parts according to the drawing 102 are not sufficient for perfect valve control. The engine may only be started if the indicated marks match.
High mileage of the vehicle causes wear on the camshaft cams. In this case, only a service station can determine the condition of the gas distribution mechanism and the degree of its wear.
(Information copied from the website MERCEDESMAN.ru)