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Old 11-08-2008, 07:15 PM
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Cam Sprocket Wear

Here's the story. Changed the timing chain, tensioner, upper guides, oil tube fittings, rail, on a M116 85 380SL (double timing chain factory) at 94K. At approximately 105K, something (NOT the oil tube fittings popping off, more likely a blockage in the oil tube itself from a loose piece of RTV) the LH cam, cam bearings, and rockers had to be replaced, and that side oil tube was cleaned like a gun barrel and new fittings installed. Recently at 120K, I've heard "timing chain slap" occasionally when left overnight. I thought that maybe the tensioner was leaking down but it seems to be OK and may be near the limit of its travel. The cam sprockets seem to be awfully shiny and pointy. There are faint score/wear marks on the chain. I haven't checked stretch yet but what do you think of the following theory: The LH cam, being extremely difficult to turn while failing, put undue strain on the chain and stretched it, also putting a lot of pressure on the gears, wearing them at a fast rate, and also making the "chain stretch/tensioner limit" problem worse. Therefore, I'm going to:

1. Examine the guides and replace as needed (could be a hairline crack)
2. Replace the chain (AGAIN, GRRR)
3. Replace the cam sprockets (only) - I don't want to take the timing cover off.
4. Check the tensioner for proper operation and replace if needed (it couldn't possibly have failed at 25K?). It is a Febi.

Since that 94K, the oil has been changed at 3000 mile intervals. However, this engine and car were clearly not taken care of before I got it (I replaced the RH cam also at 95K due to oil tube fitting failure and lobe wearout).

I'm going to check the stretch during the week but I'm hoping someone will tell me that this theory sounds reasonable. I really don't want to get into replacing the crank gear, although that also probably took a beating (maybe less considering it's probably in an oil bath).

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Last edited by Strife; 11-08-2008 at 07:21 PM.
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  #2  
Old 11-08-2008, 07:53 PM
CWW CWW is offline
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Timing chains are one of those things, much like tires and brakes, where the car just needs it when it needs it. You can complain about little use you got out of it, and I feel your pain (been there before), but there's still no way around it.

I have a 1985 380SE with the same motor, and everybody has always said to do the chain every 50k (or less). Have you ever seen the chain out of the car? If you uncoupled it, it would probably be 8' long. The design was ridiculously complex and lengthy, and the double-row chain was really only a bandaid for this. Even the double-row chains don't last long, they really just help with preventing catastrophic failure, but their lifespan still isn't great compared with any other motor. These really do eat chains.

It's the one real weakness in an otherwise bulletproof motor, so I think we're still doing ok overall with the 380s. One throttle body or a wiring harness or an ADS shock on my M120 R129 would cost as much as two or three timing chain replacements on the 380se. Lol. It can always be worse.
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Old 11-08-2008, 10:27 PM
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Additional Info

Surprise: My LH inner guide is broken. Fortunately, it's broken on the side AWAY from the timing gear. Beacuse the common catastrophe is to have the other side break and ride up under the gear, causing a jump (And often a broken cam cover), I suppose that this was...uhhh..."lucky", if you can call it that. Man, did I dodge a bullet. I couldn't find a store today when I went out and didn't stop. I'm not sure how many successful starts I had left.

Now, I am NEVER going to do this again, and am considering getting a METAL replacement for this. But...

Why did MB go to plastic? Noise? Cost? Bits of metal in the oil from wear?

Wouldn't a very loose timing chain maybe catch a metal guide and instead of breaking, cause an automatic calamity (instead of what happened here)?

Any opinions?
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  #4  
Old 11-09-2008, 07:16 AM
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Quote:
Originally Posted by Strife View Post
Why did MB go to plastic? Noise? Cost? Bits of metal in the oil from wear?

Wouldn't a very loose timing chain maybe catch a metal guide and instead of breaking, cause an automatic calamity (instead of what happened here)?

Any opinions?
Hey Strife,

I'm getting ready to do a valve job on my 380sl with about 200k, and I plan on replacing those junky, plastic guides with the metal ones while I'm at it. In fact I have three of the latter here in front of me.

I wouldn't worry too much about the timing chain catching on them any more than i'd worry about it catching on the plastic guides, as there's really no place for it to catch. And the metal can't wear off because hard rubber is bonded to the upper surface, where the chains rides. What I was a little concerned about was the possibility that the rubber could wear off or separate. But the Meyle rep assured me that they've been making and selling these things in Europe for quite a while, and nothing like that has ever been reported.

Whether that's true or not remains to be seen. But we all now for a fact that the plastic ones are problematic. So i'd say it's worth taking the chance. In any event, the metal ones are really far more substantial and seem to be far more appropriate for these cars than those crappy, little plastic ones,and I assume that MB originally used metal guides, and maybe even those that Meyle now sells? The cost probably led to the switch: retail price for the plastic ones are ap. 3.00 vs app. 20 for the metal ones times 5 per car times how many cars?.

If i'm correct, maybe someone here knows when they switched and in which cars?

Last edited by mbboy; 11-09-2008 at 07:35 AM.
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  #5  
Old 11-09-2008, 07:20 AM
88Black560SL
 
Join Date: Oct 2004
Location: CT
Posts: 3,510
I cant tell from your post if you replaced the guide on the chain tensioner. If not you want to check that one as well. The chain tensioner guide can be easily replaced without removing the timing cover.

Also you reported a Febi tensioner. I have heard recommendations from a reliable source on this sight that only the genuine MB tensioners should be used. I have no experience with this myself. I generally don't replace tensioners with a timing chain unless I feel there is a problem. I replaced my chain at 100,000 and now I'm just starting to hear the morning rap, maybe I will reconsider that.

As far as your theory left cam dragging might be a reason for both the left sprocket to get pointy as it will cause high tangential loads on the left sprocket. The right sprocket would only see higher radial loads and force the chain to ride lower in the groves and not wear to points. Replacing the cam sprockets sounds like a good idea. Mine were also pointy at 100,000K and I felt the I would have liked to replace them if I had the money.

Glad you found your cracked rail on time. I'm starting to feel that only genuine MB guide rails should be considered for this engine. Its to expensive a problem if it screws up and they aren't that expensive. Let us know if you find a source for metal rails.

For my black car I am seriously considering pulling the engine and trans. Pulling the chain cover and replacing everything under it as a preventive maintenance. While I have the engine out I can seal up everything and detail the engine and engine bay.

I have found that the only problem with having 2 107's is that you have to work on 2 107's.
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  #6  
Old 11-09-2008, 07:39 AM
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Let us know if you find a source for metal rails.
thebenzbin.com

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I have found that the only problem with having 2 107's is that you have to work on 2 107's.
"Double your pleasure, double your fun"
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  #7  
Old 11-09-2008, 09:05 AM
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sprockets

My 560SL will reach 100k miles about late Spring and although the engine is smooth and quiet I will do the chain-tensioners-guides-oilers job at that time.

In regard to sprockets, I would inspect those closely for hooking. If any sign they should also be changed. My major sproket experience is with Shovelhead Harleys and they usually required a sprocket change with every other rear chain change. A hooked sprocket will shorten the life of a new chain drastically. The Benz cam chain runs in a sealed environment (as does the dual row primary chain on the Hog) and is well lubricated hence the approximately 100k mile lifetime.

100k miles is pretty good for a complex mechanical assembly that has a hard life like an automotive roller chain. One factor in chain life is sprocket diameter; the smaller the radius, the shorter the chain life. One can assume the Benz engineers designed the optimum configuration.

The summary of all this boushwa is: If your sprocet shows wear by "hooking" or "pointing" change it. If you don't your chain life will be even shorter next time.

If you guys already knew all this, just call me Captain Obvious.
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  #8  
Old 11-09-2008, 06:14 PM
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Originally Posted by mbboy View Post
thebenzbin.com
The bad news is, it seems Meyle makes the metal part for the 116, and not the longer ones you would need in an aluminum 117.

I wonder if the length has something to do with harmonics...

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