Well about 3 yrs. ago when I got the 85 300 SD a mechanic told me I should replace the timing chain because when he adjusted the valves he could see the chain tensioner was near the end of its adjustment to keep tension on the chain.I did some research on chain replacement and no place did it mention by looking at the tensioner that would dictate chain replacement every where said to measure stretch by either of 2 ways 1st lining up marks on cam tower I got 0 or 1 degree. The more precise way to measure by taking lash out of 1 intake valve then measureing the reading when the valve drops 2 mm or .078 which I did and again came up same reading 11 right on or 12 degrees . All the rails looked good the spring in the tensioner did show some wear and was about 1/4" shorter then the new one so I did replace that but think I am not going to replace chain as I do not see how I can better the readings I got. Looking from top of engine down the adjuster is sticking out about 1/4" against the tensioner rail. The car has about 300,000 on it but dont know the history on it. Like to hear some thoughts on matter. Oh and I really do'nt know why you would have to pull whole tensioner out when all you have to do is take center bolt out to take spring out and release tension when rolling a new chain in?

Some would view the replacement of the chain as insurance against the catastrophic ramifications of chain breakage. With 300,000 miles, I would probably side with that group.

You indeed have to remove the tensioner as the tensioning pin has grooves in the slide mechanism that only allows the tensioner to slide in and not out. There will be a couple milimeters of slide action allowed that you can see when the spring is released, but you will be unable to reset the length unless you remove the tensioner and reinsert the pin from the outside.

Note: when the tensioner is removed, ensure the gasket mating surface is clean and shiny prior to installing the new gasket. Kinda difficult to simply scrape it off; I ended up using a small die grinder with a scotchbrite attachment to polish it. If I recall, (but verify) sealant should be applied on the gasket to tensioner side but not the gasket to cylinder head. Others please chime in if this is not correct.

You indicate that the measurements show that the chain is not elongated, but also you don't know the history of the motor. It is possible that a previous owner went the route of an offset key on the top timing gear to bring the timing back to spec. This could explain the measurement being correct, but the tensioner being at its max insertion limit. Just a possibility, but personally I would have just rolled in a new chain as opposed to inserting an offest key. The offset key will bring the valve train back into time, but will still leave the IP slightly out of sycnh with specifications if the chain is elongated.

Mike

Since the valve timing measures great, I wouldn't touch it (ain't broke, don't fix it). I am guessing the mechanic could see the tensioner by viewing down the chain cavity. I don't know how to do that. I don't think there is anything external on the engine to view the adjustment. I thought it is just a spring, with no screw adjustment. A lot of people here say "just do it", "good insurance", ... but you must do the work, search & pay for the parts, and risk making things worse.

If worried about an offset key, you could remove the bolt & washer on the camshaft end to see if there is one. Regardless, if "start of fuel delivery" is off, you can adjust that separately. It is trickier to loosen one of the 3 mounting bolts on the IP on your 85 with the silly "rack position sensor" on the side (used only for EGR controls).

The chain itself seldom breaks.... it is stuff in contact with the chain which gives up the ghost and causes huge problems by blocking the path..... so I would suggest rolling in a new chain and replacing those things in contact with the chain....including at your mileage inspecting the sprockets which the chain runs on.

since the IP is set by adjusting the fuel flow after installing the new chain or offset key... or periodically just for fun.... I don't think that is something to worry about.... it is a 'bottom line' setting which automatically compensates for wear in the system... or the renewing of the chain.

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

There are more than one type of ' ratcheting' tensioners .... but those called that obviously have a ratchet mechanism inside them which moves towards the chain as chain wear occurs... and lessens the distance which the OIL pressure needs to further move the tensioner at each startup. So it is a progressive one way, at rest ,slack reducer.

In this risk reward computation.... 'guessing ' that the mechanic had some way to do something which you and I both do not know to be possible.. is putting HIS money and time at risk ... with no risk to YOU.... I don't think that is fair.... to have that kind of blind trust in that mechanic you do not even know...

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

It might be worth while to see if you already have an Offset Woodruff Key installed.

Also at 300K you might need an new Timing Chain Tensioner Rail. I think the Crank Damper needs to be pulled to get at the lower Pin.

I have also seen pictures where the Timing Chain Tensioner spring has the sides of the Coils worn. You would need to remove the whole Timing Chain Tensioner assembly to change the Tensioner Rail.

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84 300D, 82 Volvo 244Gl Diesel

Comment on Mechanics. When I was in Tradeschool one of the Instructors told us that when you have the Vehicle in you Shop look for other work that you can do.

The tone of the Instruction was leaning towards that you sould make the Customer doubt the condition of they Vehicle. Like if you saw a tiny crack on the Fan Belt you would tell the Customer something like it is possible for that Belt to fail hoping the Customer would opt for a new Belt.

In the past a lot of US Cars that had Timing Chaings had Timing Chain Gear Teet on the large Gear that were made out of Nylon or Plastic. They were good for about 150-200K so Mechanics got used to telling People to repalce the chain at the earlist Mileage.

Along with the above some Timing Chain setups have no tensioner.

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84 300D, 82 Volvo 244Gl Diesel

Iam no expert on replacing the Chain, but here is a good thread that Cell did a while back. May be helpful.


9 degrees of chain stretch


Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

Have decided not to replace chain at this time checked stretch both ways and come up with 1 degree figure I will not replace till it is 4 or 5 degrees even if the tensioner is at the end of its travel will see how many miles it takes to go beyond stretch limit. As I look down the top of engine at tensioner pin it sticks out about 1/4" from block do not know what its total limit is but did not seem to move a bit in last 2 yrs. What I will get done at this time is valve adjustment, new valve cover gasket to try to eliminate leak at rear of engine, and wam throwing the egr valve off for less crap on engine. Thanks for replys always something two do to keep these 30 yr. old antiques on the road but sure do give a good ride.