My local indy (MB trained!) say the following on timing chains:

Like a lot of other things, I usually check chain stretch every 30000 miles on customers cars. The only ones that seem to break without warning are the 380 single chains and the ones on 420s.

When the stretch gets as large as 8 degrees (cam at TDC, read pointer on crank), replace chain. More stretch and the 420 chain slaps on startup cold, and bam, it's rework time. Less than 8 degrees, doesn't matter if the guides are old, they don't break. He does replace the guides when he puts a new chain in, though, as the usually don't look to good.

He says he has never had any problems with this method -- he's been in the US since 1983 and has worked on MBs since he was a kid -- his dad had a taxi company.

Regular oil changes make a big difference -- those metal chains wear really fast if the oil is dirty or lacks lubricating qualities.

The best buy in the US is a 420 that has just had a broken chain replaced and all the related work done -- almost a new engine!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!