Thanks ILUVMILS. There are many people who know that they must change timing BELTS as a preventive maintenance item at a specified mileage to be safe. From that, it is natural for them to think the same is true for a timing CHAIN. Stretch is a key and close inspection of other components such as tensioner and rails may even be more important.
Even a stretched chain usually won't break unless something causes it to break such as a rail breaking off and hanging up the works.
To check for stretch with the valve cover off I turn the engine with the harmonic balancer bolt until the cam mark is nearing the bearing stand mark then start turning the engine as slowly as possible until the cam mark is aligned, then look at the harmonic balancer mark, if it is NOT at TDC, or extremely close, I am comfortable.
This is NOT the correct chain stretch measurement method, however. To do it properly you need a dial indicator and the exact procedure for YOUR particular engine. It involves measuring valve lift and associating that with crankshaft position. This is much more precise.
If it is on my own engine that I am personally inspecting over the years, then it is all relative. I know that the marks aligned properly when it was a young engine, so I can tell if there is any noticable stretch by aligning the marks.
As engatwork pointed out, if you change oil and filter frequently your timing chain wear will in all likelihood be minimal to nonexistent. If you go by MB recommended oil change interval, there will be microscopic particulate in the oil that will wear the chain.
BTW when we talk about stretch, this is a misleading term. The steel links of the chain do not STRETCH. It is the rollers and pins that wear. Since there are so many links in the chain, even a half thousandth of wear on each link would add up to significant "stretch" of the chain.
If an engine ran with 10 degrees of chain stretch and did not break, there would be no damage to the engine, it just wouldn't run well. It would be time to change the chain, probably the tensioner, depending on the engine model and certainly inspect the rails.
Have a great day,