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'82 380 SL, '82 300TD, '85 500SEC
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Discussion Starter #1
I've read all the admonishments about updating the timing chain, lest it'll break, destroying the engine; and by golly, I'm a convert. But...

I just read an ad in my area for a 91 560 with a timing chain failure and "bent valves". I assume nobody has taken this engine apart to verify the damage...it's more like, well, if the timing chain breaks throw in the towel.

So, the real question is...what really happens to an engine when the chain goes? What's the worst case? Not hypothetical, what's really happened? If it were "just" bent valves, wouldn't the engine be rebuildable? If someone GAVE you such an engine, wouldn't you open it up, or would even that be too "disturbing"?

Not to bring back suppressed memories, but has anyone had this happen and is still around to talk about it? Thanks, Dave
 

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1973 450SL
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138 Posts
A number of things could happen when the chain breaks. First off, when the break happens the camshaft no longer turns which means the valves are stuck in their present position. However, the crankshaft continues to turn which means the pistons come up and hit the valves, bending them and also may punch holes in the piston crowns. Also, that sudden force on the pistions could bend (or break) the connecting rods, transferring that force down to the crankshaft and possibly causing damage there also. So bent valves may be the least amount of damage. There are other chain reactions going on throughout the system that I may not even have touched on here. All of these things may or may not happen. But you did ask for worst case.
 

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I've wondered about this also; the way I see it, it could go like this, from best case to worst:

1. The valves don't get bent through absolute luck.
2. The valves get bent.
3. 2, and The cam(s) is/are broken.
4. 2,3, and the cam bearings are pulled out of the heads.
5. 2,3,4 and the pistons get smacked (dented)
6. 2,3,4,5 and one or more gets holed.
7. 2,3,4,5,6 and the connecting rod(s) or crank is damaged.

I suppose if such a car turned up in my driveway, I'd HAVE to get into it to find out if it was fixable. Problem is, I'd have to invest a lot of time and probably low four figures in gaskets and valve jobs alone to find out if there was any damage within the block itself.

Scenario #1 is so rare (but it HAS happened...) but take a look at #2. Are the cams still straight? Are they weakened internally so that they will go unexpectedly in the future? Are they bent just a little so that there will be a future cam bearing failure?

#2 is bad, Then there is the matter of crossing your fingers on getting the heads on again without helicoiling - it would take a special tool and probably a few hundred$ of helicoils to do this job right. Even after all that, that engine might go 200,000 miles well maintained, or 200 - just no way of knowing without a complete tear down and measurements with equipment I don't have, in which case you might as well get a different/rebuilt engine.

If it were as easy to remove/install the intake/exhaust manifolds and heads as with a carburated small-block Chevy, and if a "head set" of gaskets could be had for $50, maybe the story would be different and more people would look into this.
 

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1995 Sl320 Roadster
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Believe me when I tell you there is no such thing as a cheap fix when the timing chain breaks.My neighbor had a 1975 SL and his chain broke and bent some valves,connecting rods and pistons.It cost him about $3500 to fix everything.It would have been much cheaper for him to have had the timing chain and guides and tensioner replaced according to recommended maintenence.He didn't do it and it cost him big time.
 

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'82 380 SL, '82 300TD, '85 500SEC
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Discussion Starter #5
SL3204me said:
Believe me when I tell you there is no such thing as a cheap fix when the timing chain breaks.My neighbor had a 1975 SL and his chain broke and bent some valves,connecting rods and pistons.It cost him about $3500 to fix everything.
1. A consensus seems to be building suggestion at least one possibility is bent rods. I can certainly visualize how that 'might' occur. I've been "lucky" enough to see a 380 bent rod and diagnose the cause, so...it's very hard for me to believe this would be an outcome of a chain failure. Obviously the rod is in the direct line of fire, and the neighbor above may have had a rod involved in the repair, but you're sure one actually bent? Thanks.

2. Since you brought up the money question, I'll play devils advocate: Frankly, a $3,500 "rebuild" by a third party doesn't seem that outrageous if one looks at other forum threads and postings discussing the costs of rebuilding. So...an engine with 150,000 miles might be a candidate for a rebuild; and, one with a chain break certainly would...what's the difference in costs?

Thanks, guys. Dave

-engr693: you have a current cylinder index of 37...I think I see, but why an odd #?
-strife2: aha! everyone would EXPECT bent valves but maybe not! Right! Tell me more about the need for helicoils, lots of them...I have a day off today and my brain is obviously elsewhere.
 

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daveward said:
Tell me more about the need for helicoils, lots of them...I have a day off today and my brain is obviously elsewhere.
In theory, the aluminum block engines require helicoils to hold the replacement head bolts. Some folks clain they can make do without 'em.

Perhaps this is another reason the old iron block V8 in the 450SL is considered "bulletproof" in comparison to the later engines. (But it will still suffer the same damage in the event of a timing chain failure.)
 

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1973 450SL
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Dave, odd cylinder index count in one word....



Weedwacker
 

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page62 said:
In theory, the aluminum block engines require helicoils to hold the replacement head bolts. Some folks clain they can make do without 'em.
Occasionally the guide plates for this are available on eBay (really, really expensive); I seem to recall that the guides are deliberately misaligned by a degree or so! I was interested in this because, well, someday, I'll need a valve job and it's a big gamble to see if the head bolts would hold. If it doesn't work, you are out a lot of $$$$ in gaskets and maybe a lot worse than that.

Apparently, if you are lucky and skilled with a torque wrench and "the touch", head replacement can be done without helicoiling.
 

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2008 E 320 Bluetec (ELEGANCE) 2005 CLK500 (avant-garde)
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The timing chain broke on my '84 380SL when the previous owner had it and the damage was so catastrophic (estimate from the MB dealer was $6500) that he decided to bite the bullet and get the entire engine replaced with an MB factory rebuilt engine.

I've never had one break on me and I had a friend that worked on Mercedes-Benz for 40 years and said that he'd never had one break, but I feel like that I just can't take the chance. I have them replaced every 100K. The one on my S500 was $1200 (with guides and tensioner) but a new engine would be upwards of $12,000.

Good insurance for the money I think!
 

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1979, 450 SL 1983, 500SL
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Chain break

The engine in my 79SL came from a SLC (I think) and even it has heli-arc weld marks on the left valve cover. Previous oner had guide break and bent valves. He redid the valves and menchioned chain. I read a few mesages in here about the timing chain and did not hesitate.

And now that I have the odometer working again I'll know when to do it again.
 

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2004 SL55 AMG, 2006 E500
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Here's my 2 experiences:

Mom's POS Mercury Van: Timing belt went out - nothing happened - $135.00 repair

Brother's and my old project: 68 Camero, Just shy of 700 Hp, 9.03 1/4 mile - I was stationed overseas. Came back to 3 screwed valves, broken connecting rod which allowed the Piston to smack straight into the head. He was est. at 165+ when this happened. Never ran the same.

From what my normal mechanic told me, newer cars usually don't have catastrophic outcomes from a belt/chain breaking.
 

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1979, 450 SL 1983, 500SL
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Bent Valves

Prana25 said:
From what my normal mechanic told me, newer cars usually don't have catastrophic outcomes from a belt/chain breaking.
They are called non-conflict engines. Once you get the compression ratio up there then the space between the top of the combustion chamber and the top of the piston gets very small and there is no longer enough room for the valves to be extended and the piston to be top dead center. You get conflict.
 

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Well, it depends on the design of the engine, whether or not it is an "interference" or "non-interference" design. A non-interference design has no possible position of crank/timing that can cause a valve and piston to meet. I think it is safe to say that GENERALLY, low-performance engines are more likely to be non-interference. Case in point; the absolute POS 1.8L Brazillian engines in 1980's era GM cars (I had 3). I got so good at changing the timing belts on these after failure that I once did it on the side of a road under extremely trying weather conditions (a tornado wiped out a good part of a city about 8 miles south of where I was while I replaced the belt).
 

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1979, 450 SL 1983, 500SL
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??

I thought that was what I said?

Ford Ranger $22.00 timing belt. 30 min change. Non- conflict engine, at least the four cylinder.
Ford Pinto $36.00 belt. 30 min change. Non conflict
450SL $130 chain first time 115 min Change. Conflict
Masda truck
Courier
Chevy 454 in an Airstream Clasic motorhome. That was a monster to work on.

These are the vehicles I have put a timing chain / belt on.
 

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Outstanding Contributor , Bob's Your Uncle!
-----'83 280 SL----- 5 speed....The PIG
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Noeljb........Sounds like the story of my life...Never owned a new vehicule...always bought $500-$1500 beaters. I too learned how to change belts/chains......and rads...starters.....waterpumps...etc...etc... Always in the rain or on the coldest day of the year (up here in the great white north).
 

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Hey... I'm with you guys. When I was 16 I was told that I had to change the Freeze Plug on the old Monte Carlo if I wanted to borrow the car.

I had no idea what a Freeze Plug was, but with the hormones of a 16 year old, I found out what it was and changed the damn thing within a couple of hours.

(No score, by the way..... but the car got fixed)
 

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Freeze Plug

Prana25 said:
Hey... I'm with you guys. When I was 16 I was told that I had to change the Freeze Plug on the old Monte Carlo if I wanted to borrow the car.

I had no idea what a Freeze Plug was, but with the hormones of a 16 year old, I found out what it was and changed the damn thing within a couple of hours.

(No score, by the way..... but the car got fixed)
And have you see where they put two of them on a dodge truck? Got to pull the transmission just to see them clearly. I like my truck, I don't like working on it.

You should have seen me at 12 fighting rear wheel skirts on a GranPrix.
Mom couldn't change that tire in the rain.

All that a Degree in Entomology and I learned how to use a periscopic sextant at 7 mile a minute.

Won a war too!

BORAH!

Oh and sorry about you not scoring.:(
 

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1985 380SL
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Thanks for posing this question about timing chain breaks. My 380 SL is my first MB experience and I'm a bit overwhelmed just yet. I'm impressed with the quality and design of these machines but like anything new I worry about the things that I don't understand well. When mine was running I had heard some chain slap during startup and I started worrying (I'm at 110K). This thread has opened my eyes.

Regards
 

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Depends on the day!
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If you are hearing slap I'd change everything out yesterday.

More often than not @ this point I see more timing rail failure causing timing to jump then outright chain failure.
Usually piece of a broken rail makes it's way b/w the timing gear and chain..after that it gets ugly.

As for helicoiling, chances are if your car is pre 88/89 it will need it.. If the car has been overheated it will Definitely require HC's.

You can check if your threads will hold prior to R&R'ing the heads by backing the bolts off a touch and re-torquing. Touch or no touch, if the threads won't hold the requisite touch your up a creek.

The guide plate for drilling/tapping runs about 500$ last I checked and is Necessary to get the angle of the bolt holes/threads correct.

On today's docket is a full chain/guiderail/tensioner swap on a 1990 560SEC.. 85K but I Do not trust nearly 20 year old plastic;-)

Jonathan
 

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Jonathan is right - if it is slapping, it's about to fail. The primary failure mode here is NOT the chain breaking, it is once of the four plastic upper chain guides getting brittle with age and heat, then failing. As Jonathan said, a piece of that guide will follow the chain up to the cam gear and, thanks to the amount of chain stretch allowed by the chain tensioner mechanism, be pulled through the cam gear for 1/2 of a revolution. That normally allows the cam gear to slip timing, which WILL result in V8 engine valves hitting the pistons. Something will break or bend (or both) at that point and it just got expensive.

If you pull your valve cover, take a look at those plastic chain guides. If they're a deep maroon color rather than the new milky white color, it's time.
 
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