Chains wear. A 130 link chain has 130 wear points. If each wears by .002", the chain stretches by over a quarter inch! As a chain stretches, the valve timing changes and performance/economy decrease. If it stretches too much, the tensioner can't compensate, the chain jumps a tooth or two and the valves meet the piston heads. You now have a $4000 plus repair bill - rebuild heads, replace pistons, new crank, etc.
The chain stretch can be measured by comparing the index mark on the valve cams with the timing mark on the crank. When it gets out of limit - REPLACE the chain, guides, and tensioner. Or just replace every 100,000 miles as a PM step. Guides seldom wear out, but the plastic gets brittle and is prone to break after twelve or so years.
Have replaced several timing chains in various MB engines and consider it cheap insurance if you intend to run the car a long time. Had only one engine that the chain jumped a tooth. The previous owner had the "chain replaced" at a dealer 25K miles before and the recepts to prove it, so i did not measure the chain stretch or inspect the guide rails. Dropped stone dead on I-495 and cost me over $1100 in parts and a lot of my time. Could have done several chain jobs for the time and money.
Replace the timing chain, guide rails, and tensioners in gassers at 100-125K or pay the price. That's why.
