Valve (cam) cover is coming off tonight for a valve adjustment and while I'm in there I'd like to see what my chain is off by. I read something about rotating the cam to TDC via the mark and lining it up accordingly then something about taking half of what's seen on the timing marks?

Does this seem about right or is it totally off base?

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

The Big Timing Chain Question

Thanks!

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

Hey,
Here's some more reading for you:

http://www.peachparts.com/shopforum/showthread.php?t=96572

Q on timing chain stretch...

W123 Timing Chain Stretch

All interesting reading. Personally I determined 4-5 deg stretch and put in a 4deg offset key. I used the cam timing marks because I couldn't match my cam code to any of the ones listed in the shop manuals instructions.
Click here for the manual instructions

If I could have used the 2mm valve lift method I would have but I did what I did and I don't regret it. The difference is very noticeable.

Either was as long as the valve cover is off you can line up the marks and do a cursory inspection at least.

Also, notice Mercedes lists the keys for "correction at crank".
I just read your post again. You line up the cam marks first then check timing at crank.
It's all in the manual..good luck.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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Just my $.01 worth. The only time an offset key should be used is to set cam timing when everything is new (chain. guides, sprockets, or any other thing that will effect cam timing.) Lets say, someone checked the timing on their engine and found the cam timing to be off by 5 deg. They put in an offset key to correct the timing. Later they sell the car to you and forget to mention the offset key. You check the timing and find it less than 5 deg so say, "All is OK". A few months the chain lets go and distroyes the engine. You say "But it was less than 5 deg a few months ago." In reality, it was a little less than 10 deg.
Being a mechanic, I see so many cars that someone covered up a problem rather than fixing it correctly the first time. Now the problem requires not only finding the current problem which is covered up by the previous, but finding the previous problem also. Boy this can be expensive to the customer.
So, NEVER cover up a problem. ALWAYS fix it correctly.

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84 500 SEL (307,xxx miles)

Using an offset key is not covering up anything. The chain will wear more in the first 10,000 miles than during any of the next 10,000 mile increments. Using it to get the valve timing matched up to the crank position is perfectly good physics.... Most double roller chains fail due to stuff around them like slide rails wearing through, etc....
The Factory Shop Manual is not the instructions for building the engine at the Factory... it is the manual for the mechanics in the field...and it specifies this as a way to correct for a certain amount of chain wear. And if you find it on an engine it is a sign it was well maintained... as compared to someone trying to cover up something.

That's true to a point. If you do decide to use a woodruff key and find one allready in place you would have to start from scratch with a normal key to check the chain. Then you would know exactly how much stretch you have.
I guess if someone put in a 9 deg key without you knowing about it you could get in some trouble. Otherwise there is really no guesswork involved.

Also, it would be foolish to assume anything on a 20 year old car without service records.

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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Greg, my only concern with returning the timing to spec using the offset keys is the potential wear on the sprockets. As the chain gets longer, the distance between the teeth will also get longer. So, the chain won't properly sit on the sprockets and the load will not be distributed across all the teeth on the sprocket. This will cause undue wear on the teeth as they are loaded to a greater degree than originally designed.

I don't know the limit where this starts to become a significant issue, but, I would think, intuitively, that you would not want to use offset keys to correct chain stretch that exceeds 6 degrees or so.

Please note that this only applies to actual chain stretch causing the late timing. If you install a new chain and the timing remains at 6 degrees late, then the chain was not stretched, at all, and the timing is late for other reasons. In this situation, we are agreed that using the offset keys is definitely the proper way to go.

The entire problem is the inability to determine how much the chain has actually stretched, from brand new. The late timing could be caused by many other issues rather than the chain. Several members have spent time and money to install a new chain and the result was timing that remained about 4-5 degrees late.

Actually you don't need to worry about that ' new' comparison stuff... since the Mercedes Engineers give the parameters of what they want the running condition to be... IE, the position of the number one intake valve at a corresponding crank position.
A double roller chain puts less load on each sprocket tooth than a regular chain... so the side wear on each tooth, given proper oiling , may be less than you are visualizing...

What I was referring to is the difference between the length of a new chain and the existing chain. In your scenario, whereby the keys can fix all problems, then there is no issue.

However, if you determine that you wish to replace the chain after it has stretched six degrees, then the use of a new chain would be necessary for comparison. There is no way to know whether late timing of six degrees is caused by chain stretch, or tolerance stackup in the height of the head, or both.

If one were worried about getting past some arbitrary number like 6 degrees... I see nothing wrong with preventitive maintenance of putting in a new chain at each 200K miles...
However, to do it correctly one would need to install new sprockets on the cam and the crank .... a Lot of work compared to just rolling in a new chain.
In other words, using a worn sprocket will accelerate the wear on a new chain all the time it is in contact... as compared to the new chain wearing more just upon initial use under load.

Brian,

Good post. I would elaborate just a little bit on the part above, adding that we'd also want a specification against which to compare such a measurement. You may have meant that to be implied in your post; I've just stated it explicitly.

There's a post about this from Beagle that I rather liked, and it mentioned taking just such a measurement. This has some intellectual appeal to me, but I'm not going to go out and collect the stuff to do it if I don't have a specification against which to compare the result.

I think that you and I are in agreement that for most of us, it's practical and cost effective, when in doubt, to simply replace the chain (along with any other components in the neighborhood that are ready for attention), and then to determine by the valve lift method if an offset key is required to get the valve timing right.

An individual with sufficient experience might be inclined to declare, upon visual inspection, that the sprockets are "just right", that the chain must therefore not be excessively elongated, and that an offset key would therefore be the right course. I'm not that individual!

I will agree with you on one condition. When explaining to someone how to check for chain stretch, the first thing that needs to be said is "remove the cam gear and make sure it doesn't have an offset key installed." Otherwise there is no way to know if the chain is stretched or not. My whole point was if there is an offset key and you don't know it you might as well leave the valve cover on. By the way, I just set the cam timing back on my 500SEL to factory specs and it came back to about 1 deg of stretch with about 20,000 miles on it (and it has NO offset keys in it). Of course, I take Larry Bible's advice about oil changes "Change it hot and change it often" which is the best advice on the site.

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84 500 SEL (307,xxx miles)

I don't agree.

The worn sprockets won't cause any additional wear on the chain. The chain doesn't care if the sprockets are worn. It must suffer a certain amount of tension from the load no matter if two teeth provide it or if 15 teeth provide it.

The chain won't suffer additional stretch due to the use of worn sprockets.

However, the sprockets will suffer additional wear as caused by the new chain. The pitch of a new chain will differ significantly from the pitch of worn sprockets. So, the wear on the sprockets will be accelerated when a new chain is installed.

Of course, this is all theory because you might be right in the fact that the sprockets suffer very little wear because the chain is double row and the sprockets don't have much load to deal with.

So, there is some method to determine the actual chain stretch versus the total amount of the late timing?

I'm not so sure that starting with a zero offset key is the answer. I recall reading about M/B using some 4 degree offset keys, from the factory, to fix a certain manufacturing problem. If you have one of these engines, your entire database is flawed if you assume that a zero offset key is the starting point.