So, I get around to checking the timing chain stretch in a W123 240D I recently acquired and the chain stretch is zero. I have no evidence that the chain was replaced and the previous owner never mentioned it. It has 250K on it, so I am perplexed.

How could I verify it has a new chain? There is no clip style master link on the chain.

M.

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He's wild and free and so close to it only Gunk will clean his fingernails!

The Factory Shop manual suggests in a total rebuild that a chain with no master link be used... so not having one is not a necessarily mean it is the original chain.
How about describing as completely as possible what you did to ' check ' the chain stretch.... we will see if we can figure out what is happening....
Do you have a Factory Shop manual ... and have you read it if you do ?

Well, I have two manuals, one printed (Genuine MBZ) and a knock off CD manual.

I rotated the engine in the correct direction, using a socket on the power steering pulley, until the groove on the camshaft lined up with the mark on the cam bearing tower. Now I lined them up so the tower mark was dead center in the groove. And, I did this four times. I read the stretch from the vibration dampener and timing mark--after cleaning it.

This was done as part of a valve adjustment; the intake valves had zero clearance. It sure starts better-BTW. I don’t think this engine has been rebuilt as there is some scuffing on the cam lobes that would had caused it to be replaced, at least I would have replaced the camshaft or had it reground. Although, the camshaft compartment was fairly clean for an engine with 250K on it. (no scum on anything)

If I grab the chain and pull it up I can lift it off the sprocket, but just barely.

M.

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He's wild and free and so close to it only Gunk will clean his fingernails!

It just occurred to me, could some one have put in an offset key? Could you tell by removing the bolt that holds the cam sprocket on?

M.

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He's wild and free and so close to it only Gunk will clean his fingernails!

You should be able to see that key if you back off that bolt.. but I think you need to use the other method to check the position of the cam with relation to the crank ( and thus chain stretch).
I think it also says not to use the power steering pulley nut to turn the engine.... and if it does not say that I WILL.... can really be a problem if you ever need to get that nut off... that is a tapered shaft you are cranking that nut down on...
You said you had the manual... but did not say if you have read it...
Did you perform a search on this issue... there is a lot of good info already posted on this... including what I have said above....

I just did the offset key thing and I can tell you that I couldn't determine if an offset key had been used on Marlene until I removed the cam sprocket.

The MBZ offset keys are half-moon shaped and sit in a half-moon concave which is machined into the cam. Consequently, the slot does not extend out to the front surface of the camshaft and is not visable for comparison with the slot in the sprocket which is.

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Never a dull moment at Berry Hill Farm.

I guess its down to getting a degree wheel and checking the cam timing. Any good sources for a degree wheel?

M.

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He's wild and free and so close to it only Gunk will clean his fingernails!

Oh yes, I have read the manual. (RTFM)

M.

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He's wild and free and so close to it only Gunk will clean his fingernails!

There's a perfectly adequate device for this integrated into the front of your engine.

Read on:
Replace Timing Chain?? When??

Timing everything! Cam timing that is...

http://www.pindelski.com/cars/W126/Valve_adjustment/valve_adjustment.html

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Never a dull moment at Berry Hill Farm.