Chain guides: Upper, or all with the timing cover off? - Mercedes-Benz Forum
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post #1 of 12 (permalink) Old 02-06-2016, 09:55 AM Thread Starter
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Chain guides: Upper, or all with the timing cover off?

Timing chain guides are often the topic of our conversations. Recently the thread 'Timing chain service, $9800?!?!?' (https://www.benzworld.org/forums/w126...ce-9800-a.html) captured also important technical information that (I thought) should be captured in a separate thread, with an easier to search title.

The main issues that arose are:

1) Should the chain guides be changed due to mileage, age of the engine, or both?
2) When changing the guides should only the upper ones be changed (accessible after removing the head covers), or all (in this case the timing cover has to be removed to gain access to the lower ones)?
3) If the answer to 2) is 'all of them', can the timing cover be removed with the engine on the car, or does the engine have to be removed first?
--------------------------------------------------

My personal opinion on the above (I also welcome disagreements) are:

1) The mileage is the primary factor (as everybody knows that is 100,000 miles) if they have not been in the engine for more than (approximately) 20 years (the same "age limit" was also brought up by Nitro88). It happens that our cars are all past the 20 year mark, so IMO they are due for a chain guide job if the original guides are still in, regardless of mileage. The first two pictures show what I pulled out of my engine: 1989 car with 90,411 miles. Do I need to say more?

2)The timing chain has straight guides and round guides. The straight guides are only to prevent the chain wobble, so their contact with the chain is intermittent and with low chain pressure. The round guides have permanent contact and with high chain pressure because they "deflect" the trajectory of the chain, forcing it on a particular path. These guides have more wear. For that reason, is my opinion that when doing a chain job, all guides have to be changed: the upper AND the lower. The third and fourth pictures show by comparison the wear amount (from left: chain tensioner guide, lower timing chain guide, the oil pump chain guide). The last picture shows the wear on the lower timing chain (round) guide. The reason (IMO) the lower chain guides do not get much attention is simply because most people avoid changing them due to the fact that getting the timing cover off is not by any means an easy or pleasant job.

3) If all guides will have to be changed, and the lower will require removing the timing cover, the question becomes: can the timing cover be removed and correctly reinstalled with the engine on the car? According to Vaifanatic he has done it, and jackjtr is in the process and is trying to put together a writeup (https://www.benzworld.org/forums/w126...eassembly.html). I cannot offer correct details in this matter (I first took the engine off the car for other reasons), but I don't see why the cover cannot be removed with the engine in the car (some small engine raising may be required) provided that everything is done with attention to details, you document every step (especially at disassembly) so you put things back together the way they were, read the FRM FIRST, think everything thoughtfully, and have plenty of time (this is no "Saturday morning job").
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Last edited by liviu165; 02-06-2016 at 10:11 AM.
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post #2 of 12 (permalink) Old 02-06-2016, 10:14 AM
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Has anyone ever had one of the lower guides fail and grenade the motor?
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post #3 of 12 (permalink) Old 02-06-2016, 10:16 AM
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Just to contribute to your point, I'll post a picture of my lower curved guide:



The black plastic seems to be a lot more sturdy and less susceptible to cracking than the white plastic used on the other guides, probably because the Mercedes engineers knew how hard it would be to get into the timing case. However, pretty much ANY plastic at this age is going to be brittle, and with constant chain tension and movement on them, the risk of them cracking goes up a lot.

I do too believe that this job should be done to any old Benz engine like this that's still in service. It's just not worth it in the end to bend valves and trash an otherwise beautiful engine because of a little $10 piece of plastic. That's my two cents.


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post #4 of 12 (permalink) Old 02-06-2016, 10:57 AM Thread Starter
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Originally Posted by bflobob8 View Post
Has anyone ever had one of the lower guides fail and grenade the motor?
Vaifanatic found one of them broken (post 59, https://www.benzworld.org/forums/w126...-9800-a-6.html). Because the number of people doing a replacement of also the lower ones is extremely small, it is not possible to correctly know their integrity.

I am not aware of any catastrophic engine failure so far due to a lower guide, but their capacity to maintain their integrity will only go down in time. If only a piece of plastic get caught up in between the sprocket and the chain in operation, the chain may jump. Personally, I don't look forward to be that example. Considering the cost of the replacement parts and the outcome if not done, I consider safe practice to replace them. The opportunity can be used to also replace the water pump.
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post #5 of 12 (permalink) Old 02-06-2016, 12:48 PM
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We all understand the implications on the lower guides. Yet they're the stronger link of the group of guides historically-wise.

It's just the labor involved, plus the proper gasket sealing of the timing cover that puts us off comparatively to the benefits.
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post #6 of 12 (permalink) Old 02-06-2016, 01:43 PM
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The original motor in my car had just around 94,000 miles when the guides broke. It was definitely an AGE related issue, and those were almost certainly the original guides which at the time were about 28 years old.

The motor I swapped in had about half of that mileage (out of a euro '84 500SEC) and I pulled the valve covers to inspect the guides and noticed they were also chocolatey, everything else was nice and clean. The motor had surely been taken care of, but age had begun to deteriorate the guides.

It was at this point that I decided to drop the oil pan and take a look at the bottom guides (since those are rarely mentioned, ever) and discovered that one had actually already broken! I should have taken photos of all this, and done a write up for the work I did, but I just wanted to get it done and not have to create yet ANOTHER DIY thread for something that there are plenty of resources out there for already.

All I can say is, that with my experience with two different motors, one just below 100,000 miles at 28 years of age, and one with 46,000 miles and the same age, age will definitely play a factor in deterioration of the guides!

Here are some poorly taken photos I took of the low-mileage motor after I refreshed the guides:





So clean you could eat off of it, and that's how it looked when I first pulled the valve covers and discovered chocolately brown guides! I'm definitely glad I did. I almost foolishly assumed that low-mileage would suggest that the guides would still be good for another 54,000 miles til the 100,000 mile mark for this motor. I was totally wrong, and glad I found out before risking another motor!

TL;DR - Age will brittles these guys just as much as mileage, and inspect the lower guides anyway!

ETA: Despite having been broken, that lower guide didn't cause an issue (but maybe I just got super lucky); so far I'm the only one I know who's experienced that and fortunately didn't have a catastrophic failure.
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post #7 of 12 (permalink) Old 02-06-2016, 02:02 PM
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Totally agreed VaiFanatic! They really shouldn't be neglected as much as they are.

bondavi, I have little automotive experience in general, but I will say that hearing these stories about broken lower guides really sounds like they should be replaced. Mine are worn down pretty bad, and I don't know how long they would have lasted with me driving my car the way I do.

As far as I know, any shrapnel can get sucked into the timing cogs, jump time, and ultimately destroy a beautiful engine. I say it's totally worth it to pull the thing apart. It gives you a chance to clean stuff up and replace seals as well.

One of my water pump o-rings was completely hardened, so I'm glad I got the chance to do this job. Who knows? I could have started to get coolant in the crankcase, chased a head gasket, and found out in the end that it was just that o-ring. Hey, just another reason to pull that cover.

Definitely check out my thread and contribute if you have any helpful info:

https://www.benzworld.org/forums/w126...eassembly.html
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post #8 of 12 (permalink) Old 02-06-2016, 04:25 PM Thread Starter
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One of my water pump o-rings was completely hardened, so I'm glad I got the chance to do this job. Who knows? I could have started to get coolant in the crankcase, chased a head gasket, and found out in the end that it was just that o-ring. Hey, just another reason to pull that cover.
You are right, I completely forgot about the O-rings (total of four). I was probably caught up in the "plastic material debate", and completely forgot about the rubber rings. Very good point! After all, some of our cars have reached the 30 year mark, how much more can we ask for from some parts that are subject to age degradation?

Personally I consider that the amount of labor involved for the removal of the cover is a worth while effort because it results in several other new parts that one day will have to be installed anyway: new timing cover O-rings (4 pcs.), lower guides replacement, a new water pump, and a new front oil seal. If at the same time (because the parts are out anyway) we consider an alternator bearing job and a steering pump seals job, new coolant, and new P/S fluid, cleaning and inspecting the front area, then it looks even better! IMO this is a perfect winter job for an enthusiast to refresh the engine with new parts for many years to come. And if still in doubt, throw in new valve seals (covers are off anyway), and you are set!
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post #9 of 12 (permalink) Old 02-06-2016, 04:38 PM
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Definitely agreed! I also cannot believe how many top end rebuild threads I've read where the timing cover is totally neglected. This is definitely a job that more people have to be doing on these things. It's better to pull the whole thing apart and refresh it than "guess" and wait for things to go wrong. Good insurance too for possible problems ahead.

Does anyone know what that upper part above the water pump does? I believe it's completely sealed off from the water pump area.

Another question, should sealant be used on the mating surfaces of the timing cover? There isn't any from the factory, but it's probably good insurance to apply some during reinstallation right?

PS-This job should be called a "Front End Rebuild".
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post #10 of 12 (permalink) Old 02-06-2016, 07:34 PM Thread Starter
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Yes, for the timing cover a non-setting compound should be used. Permatex makes plenty oil resistant ones, but I think MB used Halomar. Is the most sticker one I've ever seen. I bought mine from EBay. I strongly recommend you read the FRM very well before proceeding to assembly.

No sure which part above the water pump. Picture?

To clean parts of oil go to Home Depot and buy Simple Green. I let parts sit in the cleaning substance for at least 24 hours (depending on how dirty they are and how consumed the cleaning substance is), then I wash them with warm water. Have a look below at the results. Try it with your timing cover.
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Last edited by liviu165; 02-06-2016 at 07:41 PM.
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