I have engine manuals and repair microfiche pruduced by MB. I have manuals for the 4.5, 3.8, 4.2, and 5.6 and none of them specify a number for excess chain stretch. They all say that you can use offset keys to adjust timing unless the chain is stretched beyond specs, but they never state how many degrees is too much. I'm looking for documentation from some factory bulletin or something I haven't seen. We've already had this discussion over in the diesel site. There are no specs for the OM617 engine either although the OM603 manual does give specs for that engine. Please don't respond with theories, and I know how to do the adjustments with the offset keys. The first time I used offset keys to adjust valve timing was at the dealer in the early 70s. I'm looking for cold hard documention if it exists. I have 3 cars right now that I bought cheap, because the chain in each of them apparently passed that magical point of stretch. I picked up an 82 380SEL, a 91 420SEL, and an 83 380SEC. I'm going to do valve jobs on all of them, because they all suffer from the classic broken rail slipped timing on the left bank syndrom.


Thanks in advance,
Peter

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No spec listed because you replace the chain if a 6 degree offset key won't restore it. MB manuals are a bit thin on "normal mechanics" because German mechanics already know all this, they aren't OJT over there!

Chain stretch measured by aligning the mark on the cam tower with the notch in the thrust washer on the cam sprocket and reading the crank degree wheel must not exceed 8 degrees -- if it gets as high as 12, the valves will touch the pistons.

If more than 8 degrees late by the valve lift method (as described in the manuals), you cannot correct with the 6 degree offset, so you replace the chain -- not stated because you are suppsed to know it, I guess!

If you have a single row chain on the 380s, check every oil change for safety, they are a bit frail. The double row replacement is as good as the one in the 420, but it's a bit iffy, check at least every 30,000 miles.

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!

The term 'stretch' is not strictly correct. The actual mechanism is wear on the pins and link holes. The practical limit is the 6 deg offset key, and is that because any more offset would result in too weak a key and potential for a sheared key and resulting catastrophic engine damage. The chain (double row) is plenty strong to handle LOTS more wear before weakening enough to be of concern. However, a worn chain will also cause abnormal wear to the sprockets. If the new chain does not fit into the bottoms of the teeth smugly, the sprockets must be changed. Also, if the sprocket teeth have any 'hook' or concave wear on the driving side tooth, they must be replaced or rapid chain wear/failure can occur.

Peter

I cant help you with the hard numbers but I would be interested in knowing how much milage is on each of these vehicals.

John Roncallo

I'd assume that if a variance of more than 10 degrees was acceptable that MB would make an offset key available for this amount. Conversely, why would MB even make an offset key available for 10 degrees if a variance of more than 6 degrees would make the valves hit the pistons? HOW would the car get to the shop with 10 degrees stretch? I would say on these engines that over 10 degrees is the limit, and be aware that maybe it already has an offset key!
Gilly

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Peter,
I'd go with your theory if you had some documentation. Gilly's thoughts on the matter make a lot of sense, because MB does make a 10 degree key. IIRC the OM603 manual says that 10 degrees is the limit.

Roncallo,
Mileage doesn't mean anything on a car. What is important is how the car is driven and how it is serviced. To answer your question, the 380SEC has 130k on the clock, and the 420SEL has 385k.

Kebowers,
For the sake of brevity I used the term chain stretch. I am well aware of the fact that the chain elongates because of wear on the pins and links. Actualy MB has determined that a 10 degree key is still strong enough, because they use one. I agree that the double row chain is very strong and won't fail, but it is not the chain that fails. What happens is the chain gets so long that tha tensioner can't control the whip. The whipping chain beats the crap out of the inner chain rail on the left bank until it breaks off and climbs up the chain to make the cam stop and bend the valves. At least that is how I see it until someone can convince me with a different theory. A few years ago I received a call from someone 50 miles away telling me he heard a sound and then the engine died on his 420SEL. I told him that most of the valves were bent on the left bank and hee might as well get it towed in. That is exactly what the problem was. I do appreciate your input. Thanks.

I'm open to all the help I can get on this one.

Peter

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More than 8 degrees and I suspect the chain is going to slap pretty bad on startup since the tensioner will be "flat".

I've been told 8 degrees by someone who has worked on these cars (and had a Master Mechanic certificate from Dusseldorf) since 1959, so I will take his word as gospel.

There is absolutely no reason not to replace a chain at 8 degrees of stretch, they aren't that expensive and it's not exactly a difficult job. A broken chain and bent valves is a big PITA on a 380 as you are likely to have to install steel inserts in order to re-install the head, this is both a bother and relatively expensive.

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!

when the question is why...money will always be the answer.

Peter has it right as to why not to replace the chain. it's 60 bucks and if you are your own wrench then there is only the 60 dollar cost. but if you're having it done, i for one would be hard pressed to pay some one to fix it for 1200 to 1500 bucks on a 3 to 5k dollar car. i also feel that when you are in there [engine] replace everything you can get your hands on.

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Craig

1972 350sl Red/Blk 117k
1988 420sel charcoal/Blk 140k
1987 420sel gold/tan 128k
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Peter and Gilly,

In the absence of cold hard facts I'm going to go with both of your arguments of 8 to 10 degrees. I would of course do my own chains, because the chain is cheap. I was looking for a difinitive answer to give to clients when I do the inspection on their cars. I will strongly urge that they replace the chain so I don't end up doing a valve job on their car. Peter, I agree with you; even though I have the 1 degree offset jig that bolts to the block to guide the drill, it is a PITA drilling and tapping 36 holes on the early engines without the special bolts. Thanks for all the input guys.

Peter

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