My '83 300SD has 242K on the clock and is still on the original chain. I have a new one plus rails and the tensioner which I am going to change at the 250K interval. A 300D I used to own ('83) was at 295K when I sold it and also was still on the original chain. Just check for stretch at valve adjustment intervals and if needed, replace it at that point. Since MB really didn't prescribe an interval for diesel chain replacement, I recommend to some people that 150K is the "safe" mileage to replace them (if they are new customers--because I don't know the maintenance history of their car, etc..) but what I will generally do is check the stretch and go from there. Many of my customers will just go ahead and do it at 150K just to be safe, I think I have pounded that "preventative maintenance" thing into their heads well!

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Regards,
Aaron