My 83 300SD has 180,000 miles and I do not have a record of the chain being replaced. I've been trying to wrap my head around the whole timing chain/offset keys system. I assume that the offset keys are to compensate for the slight differences in machining and resulting differences in cam timing even with a new chain. I imagine that usually an offset key needs to be used to set the cam precisely. I recently adjusted the pump timing which was retarded by 8° at the crank. I got concerned about the timing chain. I then looked into how to check the timing chain stretch and did so. The cam was retarded by
1 1/2-2°. At the time I thought that was fine as 5° is the point I have heard to replace the chain. Then I got to thinking about the possibility that someone had already compensated for chain stretch by using an offset key. Is there any way to tell?

Andrew

You must have found Beagle's post. Very interesting article.

What method did you use to determine chain stretch?

The only way to tell if there is an offset key installed is to get in there and check it. If you remove the sprocket bolt you can probably see if the sprocket keyway is offset from the cam keyway.
If that doesn't work you will have to remove the sprocket.

I think it would be a safe bet to assume there is no offset installed. Especially since your IP was off so much.

Danny

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There is no requirement to replace the chain.......at any mileage or degree of stretch. There are offset keys that can manage just about any amount of stretch. However, once the chain gets to a certain point of elongation, the wear on the sprockets is accelerated because the pitch of the chain doesn't match the pitch of the sprockets.

A chain with 250-300K on it will likely need it's guide rails and tensioning rail replaced. Once you make the effort to change those items, then the chain should probably be done as well.........but.........there is still no requirement to do it.

But, they eventually break...That's something I would like to avoid.

Andrew

Nope, didn't read Beagle's post. Can you point me in that direction?

I'm not concerned as to whether or not an ofset key is installed, more concerned about determining the amount that the chain has actually stretched in it's life. I propose that it cannot be easily determined due to the fact that one cannot tell when or why an offset key was used.

I lined up the marks of the washer and cam tower. I did not use the dial indicator, but have one and could easily enough.

Why would my injection pump being off have anything to do with whether or not the cam timing has been adjusted to compensate for chain stretch?

Andrew

They usually bread when a portion of the tensioning rail or the guide rail gets caught in the chain. I don't believe that a chain will "break" eventually, simply due to mileage.

But, others will disagree.

Here's Beagle's thread:
http://www.peachparts.com/shopforum/showthread.php?t=96572

I stated that it would be safe to assume there is no offset key simply by the fact that it doesn't appear to ever have been addressed. I used your IP timing being off by so much as proof.

Look here for timing inspection guide:
http://www.peachparts.com/Wikka/OM617TimingChainStretch

Danny

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I assumed that they came from the factory with offset keys installed for precise cam timing. According to Beagle's post that is correct and so knowing to what extent the cam timing has been adjusted by offset keys would require documentation or clairvoyance. Thanks for the links.
I had previously read the chain stretch DIY article. I have not yet checked the cam timing using a dial indicator, but given that the pump was out by 8° I would have assumed the cam out by more than that due to the fact that it is "later" on the chain. That the cam was out by 1/4 the amount I have to assume that the pump was set incorrectly or something other than chain stretch caused it to be so significantly retarded. There are quite a few things on the car that were done incorrectly (valve lash was more than 2X the gap on all intakes) and so the posibility of incorrect setting of the timing is not far fetched. Now having read Beagle's post I wonder how to actually measure chain stretch rather than the cam timing (which is what the dial indicator on the cam lobe will tell you).

Andrew

You got it. The dial indicator on the cam is to accurately set cam timing and nothing more. I don't know how to measure chain stretch but one guess is to see how it rides on the cam gear. A severly stretched chain wont fit proporly but thats just a guess.

Well, wouldn't cam timing and chain stretch be directly related?

It's true you may have an offset key already installed. If it was my car I would find out because I would want to know.
If therer is a key the only way would be to install a zero offset key and take a reading.

If the cam timing is on then there is no chain stretch. If you do find stretch now you can pick the appropriate key.

Danny

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Yes, cam timing and chain strectch would be directly related, but there *will* be differences in alignment of various engine components. The align bore of the crank bearings on one block vs another will be slightly different as will the align bore of one set of cam bearings vs another. In Beagle's post he goes on to mention how even various head gasket compressions, and on and on. Those differences may be slight, but cumulatively can end up being a fair amount. Therefore, even on a brand spankin new engine, head, chain, etc. the cam might be off by a couple degrees with the "standard" key. Soo, in production, to compensate for those various differences these engines will come with different offset keys from the get-go. The various methods of checking "chain stretch" (either aligning the mark on the cam washer and tower or using the dial indicator) really tell you nothing about how much the chain has stretched. They merely tell you how far off your valve timing is from correct. The way I see it there are only two ways to know chain stretch (I'm open to others...). One would be to measure the distance between links. Measuring more links would be more accurate. The other would be to *know* what the cam timing was when the chain was new and compare that to the current reading. Without knowing whether or not someone has swapped keys to compensate for chain stretch you cannoth know what the chain stretch is without measuring the chain itself. That's my opinion, we welcome yours.

Andrew