Ok, this is probably gonna start some "interesting" discussion. My question is, has anyone ever known the timing chain on an OM617 to actually break? Ok, I can understand one or two here and there due to manufacturing issues, but despite all the dire warnings, has anyone ever heard of widespread timing chains breaking or had timing chain issues more severe than chain stretch.

I tend to think the engineers at M/B used a double row chain for durability and longevity to match the rest of the vehicle. After all, the vehicles were built when quality and design meant something.

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

I’d say with timely oil changes these things last indefinitely.
Probably why there are keys to compensate for stretch.
Check your stretch.
And perhaps change out the tensioner and rail.

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1983 300CD Turbodiesel

One data point;

240d, 150000mi, well cared for. Steady speed-70mph, flat ground (no load), timing chain broke. Had been checked a month before for stretch-OK. No obvious reason for break.

I'll say it again; these things are timing out, not wearing out. The myth of lasting forever is just that, a myth.

MY Question on Timing Chain Failure
Who has or knows someone that has experienced a timing chain breakage/failure? - PeachParts Mercedes-Benz Forum

My question What in particular causes vacuum pump failure?
What in particular causes vacuum pump failure?

Vacuum Pump Failure with pic of worn Intermediate (timer) shaft bushing
Vacuum Pump Failure, picking up the pieces?

2 Vacuum Pumps destroyed timer Bushing Identified as the problem
http://www.mbca.org/forum/2013-12-29/why-are-these-vacuum-pumps-being-destroyed

On the 617.952 there is a tiny snap ring on the Oil Pump Chain Tensioner Shoe Shaft. If that wears through the Shoe jitters around and can cause that chain to fail and the parts getting into the timing gears can cause the Timing Chain to fail.
In fact anything getting between the Timing Chan and the Gear Teeth can cause the Chain to fail.

I am not sure of other issues like Timing Chain Guide Rail wear and Timing Chain Tensioner issues. However, the non-turbo 300D’s and 240D’s have a chain tensioner that is to a large extent dependent on Oil Pressure. If you loose enough Oil pressure the Tensioner plunger might move backwards and the Timing Chain might skip teeth.
The 617.952 has a Timing Chain Tensioner that that has sort of a one way plunger and it cannot easily be pushed backwards even of the Oil pressure falls.

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84 300D, 82 Volvo 244Gl Diesel

Some of us bottom-feeders who own these cars now scratch our heads at some of the "exquisite German engineering", such as a vacuum pump which can send parts sliding into the timing chain to jam and ruin the engine, the aux-water-pump (unneeded) which can stall and cause a dash fire, the oil cooler which can break a hose (evil p.s. belt wants to bite it too) or have its wax valve fail to lose or block oil flow, etc. Depends on your engine's mileage and history. I just used the offset key to compensate for chain wear in my 1984 & 1985. I don't know the mileage of either since 1984 has a dash cluster w/ apparent PicNPull writing and 1985 has a used engine (ca 1982) after its engine failed at 330K miles. Don't expect 1M miles. That was probably earlier and simpler non-turbo engines. The chain will definitely last longer than the "bicycle chains" used in many Japanese and Euro DOHC V engines or the rubber belts used in most 4 cyl engines.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I've seen broken timing chains on OM 616's.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Same, seen one 240D in Pick-n-pull with a broken chain, that's it.

Never personally heard of one breaking. I don't remember the exact number but the FSM speaks of installing keys that would take up WAY more stretch than most people seem to be comfortable with. Which would seem to indicate that, while 7 degrees or more stretch needs addressed to retain the engine's performance, it doesn't mean the chain's strength is compromised.

Is it possible the tensioner style of the NA engines, harsher vibes of the OM616, and that it spins so much faster (greater stress and more cycles) all contribute to more failures on the 4 cyls.?

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".

A couple years ago there was an '85 300D in the Pick'N'Pull with a broken timing chain. I don't know if a vacuum pump failure precipitated it. The chunk missing out of the head was scary.

I've seen a broken one at a pick and pull many years ago (617) snapped the camshaft in half. Pretty sure the car had well over 300k and looked like it had been abused for a long time.

A common cause of timing chain failure is something else along the path failing and causing a problem, like a worn chain guide.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Timing chain wear or the amount of it is dependant on oil changes. The most oil sensitive component in an engine. As it stretches with wear it no longer handles power the same to the sprockets. More loading is occurring on fewer teeth and chain links. The sprocket teeth wear to it. Just not enough to really compensate. The chain is growing longer.

At some point the combination is running at more than designed loading. Simply put the power transfer or loading from sprocket to chain is the same but the transfer of power interface area is effectively reduced.

The longer worn chain transfers more and more power on a less and less tooth area of the sprockets.

I think this is one component contributing to failure. A stretched chain has to work harder. Or is under greater sprocket contact area stress.

Fatigue that results from the decreased loading probably increases to the point of failure. If the sprockets could increase their teeth spacing I suspect there would be no issue. I guess a simpler expression is the fit for transfer of power between teeth and sprocket is changing to some extent with chain wear. Or the relationship between them has deteriorated.

Anyways give me engines with timing chains over rubber bands that are not that easy to change on some engines anymore. One reason I like Toyota over Honda is they have chains. If the oil is changed on schedule they can go the life of the car. Or at least far longer than the rubber bands.

If you change a chain before the stretch is really excessive. The chain sprockets will remain in better condition. Putting a new chain on sprockets that have been servicing a really stretched chain for a long time. Is not the best thing to do. Still better than not doing it. The cam sprocket is the most likely to have suffered the most wear I suspect.

As I accumulated a few of these cars I tried to do a quick check on chain stretch as soon as practical after their purchase. Dealing with the aftermath of a broken chain is far from desirable.