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To check stretch, you line up the TDC mark on the cam with the TDC mark on the #1 cam bearing, then read off the crank indicator. You need to do this several times because of "eyeball" error, when you get consistent results it's probably close. There is a more accurate procedure in the manual to use a dial gauge on the #1 intake valve. I did this AND did the eyeball method. The eyeball method was within 1/2-degree of the dial indicator.
Note: The TDC indicator on the crank is adjsutable over a 3 degree range. The chain stretch check assumes your pointer is still accurate. It's tempting to loosen it when replacing the water pump b/c it's in the way. Just an FYI. It's a MAJOR pain to re-set, you need to get a dial indicator on the piston top. Much easier with the head off, a real challenge with the head on.
My TDC indicator was 1 degree off. After setting that, my chain stretch is 4 degrees - right at the limit allowed by the manual. Dr. Booth on the mbz.org diesel email list (also MBCA Diesel tech advisor) says any less than ~4-5 degrees has little, if any, effect on engine performance. I haven't replaced my chain yet so I'm not sure.
A new chain was ~$70. MB makes a neat tool to make replacement a 1-man job, it's ~$20. The hassle is the rivet. The tool is ~$200, or you can rent it for ~$40. Only the valve cover needs to be removed (well, maybe the fan clutch too so you can access the crank bolt). I've been told you can use a sledgehammer as an anvil, and a ball peen hammer to set the rivet pins, but I'm real nervous about trying that on an interference engine that costs $7500 for a rebuilt, $8000 new (wholesale), and $11,000 from the dealer. (And that's a bare longblock!)
HTH,
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