These are my comments. I think a lot of the times the timing chain snaps when something got in the way; vacuum pump failure, guide, tensioner etc. So it is a crap shoot to predict when to replace them. I think all my cars have original chains, they are at 360K, 370K, 315K etc. The one with the lowest mile is about 260K. In theory they are long overdue but they look in pretty good shape. I never did any chain stretch measurement as I doubt how accurate a prediction it is. I had one oil pump chain snapped on me at 305K on a SDL. I could never understand why, may be a piece of the plastic guide broke off. So I always replaced the oil pump chain if I took the pan off.

But don't get me wrong, I bought timing chains already but is waiting for an opportune time to roll it in. The bottom line is that roll a new one in if the OP is concern. However, there is always a risk that some other things broke and take the new one out. Who knows?

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Remember on these high mileage engines that the oil pump on the turbo motors is chain driven and is located just under the main cam chain. it gets a lot more revolutions than the cam chain and may possibly fail when the cam chain is fine.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I wonder if some chain failure is a combination of:

Chain stretch allowing the chain to run loose on the sprockets (pull is only on the last tooth to contact the chain, which would be high already as it rode around loose and high

Worn guides allowing more chain movement

Low oil pressure allowing low tensioner pressure (plus possibly a worn tensioner)

I think we'd all like to know the answer, but in my mind this is kind of like the when to replace a vacuum-pump (or oil-pump chain) debate, ... can you afford to be wrong?

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Gone to the dark side

- Jeff

You cannot. You need to replace everything to be 100% sure so I take my chances.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

On the OM671.952 I have not read any evidence that the Timing Chains pop all by themselves due to stretch.

Of coures if the stretch gets to be enough the Camshaft Timing would be off enoungh that Valves might contact Pistons but I have not read of anyone that far off.

Since Mercedes supplies several Off-set Woodruff Keys and you have not even reached the point of the first off-set I can't see reason to be worried.
You might be better off pulling your Vacuum Pump (which I have done 3 times to inspect in 7 years) and inspect the Bearing, Arm and parts of the Piston (for cracks where the bolt goes through it). And, more important check the end play of the Timer Gear (actually called the intermediat shaft bushing).

See a pic of the worn intermediate (timer) shaft bushing in post #3.
Vacuum Pump Failure, picking up the pieces?

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84 300D, 82 Volvo 244Gl Diesel

There is a tiny Snap-ring that has to do with the Oil Pump Chain Tensioner Spring and the Spring itself that ought to be inspected.
The little Snap-ring wears out and the Tensioner gets a little free.

See post #37 for a diagram.
1985 300TD wagon broken camshaft

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84 300D, 82 Volvo 244Gl Diesel