Timing chains will stretch over time and will require replacing. The benefits of chains is that they usually last a lot longer than belts. Mercedes used to (not sure of they still do) called for periodic measurement of stretch and replacement when it exceeds a certain amount. Luckily, most of the engines were also designed for easy chain replacement - actually costing less than most belts. As I'd driven some of my MBs over 400K, I'd replaced a few chains - and even at the dealer is was only about $200.