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  #1  
Old 03-09-2008, 12:23 AM
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Cool Replace 420 lower timing chain guides

I have a 420SEL with a little over 320K on it. Runs like a top except for some tranny problems.

I read a thread about replacing the timing chain and upper guides. I took a look. The guides need it judging by the dark brown color. Chain is tight. Did not check TDC.

So I am ready to replace the uppers, chain and tensioner. But what about the lowers? I saw one post that said they were good till 350K. Another said 500K. If it is a job I can do I would rather err on the side of early replacement and save my engine!

I can't seem to find anything on replacing the lowers. Does anyone have anything on that procedure? My factory service manual doesn't seem to have anything in it about timing guide replacement.

Many thanks in advance to all you wonderful people. I have already learned so much from this forum. (Made me decide to keep the ol' girl)

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  #2  
Old 03-09-2008, 10:47 AM
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I wouldn't trust them that long... They are plastic (20 or so year old plastic @ that now) that has been heat cycled a multitude of times. I like to see the lower rails done around 300K.

You'll be pulling the front cover...which can, by some account, be done with the heads on but I'd recommend doing so while the heads are off for machining etc..

Also a great time to replace the water pump and the timing gears which are Sure to be worn @ 350K.

Jonathan
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  #3  
Old 03-09-2008, 11:00 AM
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Replacing the lower right rail requires just about the same job whether done for replacement or after a failure. Back when we were converting lots of single row 380 chains to double row we did the job by removing the pan and pulling the front cover. This requires reusing the part of the head gasket that seals the top of the cover and caused a number of future leaks. We were much happier about the job when the chain system failed and we were able to remove the heads and do a complete job.

If your engine is virgin it has a better chance of having bad valve guides than it does having a bad lower rail. Do the upper three rails for maintenance and either throw it away or bite the bullet when the lower fails. or if you just wish to restore, do the whole job now from the top with a thorough valve job and a cover removal.

BTW: we charge 25 hours for a complete valve job and 40 with a front cover removal. Just to give you an idea of the work involved.
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Old 03-09-2008, 12:42 PM
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To slightly sidetrack here: if you have an engine with ~160k/mi on it, sitting on an engine stand, would you take the time to tear it all the way down and work it over, including the new rails? Or would you just do the upper rails and tensioner, replace the chain, and put the engine back together?

I've been trying to figure this one out, on my first v8 MB engine swap, so this thread caught my eye.

Sorry, that's the end of my hijacking of this thread!
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  #5  
Old 03-09-2008, 02:15 PM
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It looks like the experts are divided. The valve guides are worn a bit (just a little smoke at start up). Everything else looks pretty good on the upper end. We put around 5k on it each year so I am not thinking highly of a head job right now. I would like to address that when it is a critical issue. I hate to rip open the chain cover to replace the lower guides and cause a leaking head gasket. So let me try another approach:

If an upper guide goes out it sounds like a virtual guarantee for bent valves. What about the lower guides? If one goes out does it cause a death sentence or will it just fall into the pan? What are the signs of a broken lower? I assume a loose chain, but if the tensioner is working correctly it will take up the slack, right?

The more I think the more I want to just do the uppers, chain and tensioner :-)

And thank you folks for taking the time and care to answer. I do appreciate it.
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  #6  
Old 03-09-2008, 02:30 PM
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Out of the hundreds of chain/timing failures my shop has done over the last 30 years only 2-3 were caused by the lower rail. When it wears through the chain rides on the steel retaining pins and the chain is too long to be tightened by the tentioner as a result. It probably made noise, but by the time I saw them they were broke. The results of the failure looked just like most of the others.

The upper rails are relatively simple to replace and are dirt cheap. They are good maintenance. I see no point in doing a job as maintenance that involves more than 70% of the cost of the repair it is intended to prevent.
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  #7  
Old 03-09-2008, 05:40 PM
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Question

I am with you. I think I will take my chances on the lowers. However I am looking at the tensioner and how far out it is riding and I am wondering if I have already lost the lowers. Can one of you take a look at these pictures and tell me if this is normal.

http://picasaweb.google.com/damon1172/420SELTensioner

Also am noting that the timing is about as good as can be. Just a little off on the passenger side. This surprises me given the dark color of the guides. I would assume the chain has been replaced but not the guides. Why would someone do that?

http://picasaweb.google.com/damon1172/420SELChainTiming

Again, thanks so much for allowing me to pick your brain. I would let you pick mine but there ain't much there
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  #8  
Old 03-09-2008, 06:07 PM
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Those upper rails probably have less than 100k on them. Thye are only half way to the darkness that is disaster. Those rails have nothing to do with the length of the chain though as they do not touch the chain when it is tight.

It is hard to tell about the extent the tentioner is out though with pictures, maybe even in real life. I usually depress the tentioner to release the oil and then see how far it must extend to hit the chain. It is still ambiguous. I'd just put new upper rails in and leave the rest. The chains never fail on their own and replcing it won't help the lower rail if it is about to wear through.
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  #9  
Old 03-09-2008, 08:30 PM
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So if I've got you: The lower guide fails and causes slop in the chain which the tensioner is unable to adjust. So the loose chain slips and wham-o, top end Cap'n Crunch.

For my current examination if the tensioner is keeping the chain tight then the lower guide is still intact.

You recommend only the three guides up top. No tensioner? How often do you replace the tensioner?

Also on a slightly related topic a few posts have suggested replacing the oil rails. Are those the plastic rails that run just above the injectors? Should they be replaced also? I thought I was looking for the rail above the cam bearings.
I agree with you on the chain. I think I am looking at less than a degree of stretch. I don't think I need to change that out.

Steve you have been so helpful and put some of my fears to rest.
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  #10  
Old 03-09-2008, 09:12 PM
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Unfortunately the tentioner in these 116/117 motors is not a ratcheting one. I replace a tentioner when it regularly looses its pressure overnight. This is noticed by a rattling sound on start-up that goes away after oil pressure fixes the tentioner. This rattling is what does in the upper rails when they are brittle. It will happen occasionally no matter how good the tentioner. If I heard it twice in a hundred starts I would probably replace the tentioner. Maybe twice in 50. if the rattle didn't cease instantly with oil pressure on the gauge I would also replace it.

The point of doing nothing is that since you will need a valve job when it comes apart anyway the only difference that letting it fail will bring is the need for four exhaust valves, maybe a couple intake but not often. All the labor will be the same and 80% of the parts which is the small part of the job.
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  #11  
Old 03-10-2008, 08:56 AM
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Steve - not to hijack Damon's thread, but I'm about to do the upper guides on my 560SL. It has 40,000 miles on it and I am assuming everything is original. I have a new tensioner on hand that someone gave me and was debating on whether to change it now or not. No issues noticed with rattling on start up. Just because I happen to have one on hand, should I go ahead and change it or save it for the next time?

Thanks.
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  #12  
Old 03-10-2008, 09:03 AM
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If I had one, and it was Genuine I'd go ahead and replace it just for good measure.

Otherwise I'd consider just freshening the seal on a unit w/ only 40K.

Jonathan
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  #13  
Old 03-10-2008, 09:04 AM
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I see no technical reason to change it now. The part is very durable. I believe the only problem that ever occurs with the tentioner is contamination from debris in the oil. The act of loosing pressure over time is a matter of the check valve not holding and I have always thought this was due to oil grit in the check ball. As a result of this concept mileage is not really an issue. The issue is performance.

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