420 SEL timing chain?

[BenzBoy]

A friend called this evening seeking help. He started his 420SEL and the timing chain snapped. Compression is down on 3 cylinders and the loca mlechanic told hie to throw away the motor and get a second hand one - that the crankshaft would probably be bent. Hmm - sounds a bit dramatic to me. Wouldn't damage be limited to the heads?
Your thoughts?
Regards,
Brian

 

[Styria]

Hi BenzBoy, if indeed the 420 engine would react similarly to the 380 and 450 engines in the event of timing chain problems, damage ought to be restricted to bent valves on head(s) only - I believe it is usually the one on the left hand (passenger) side. You refer to a broken chain - this seems unlikely, as it is normally the plastic guide(s) that break and broken pieces wedge themselves between chain wheel and chain.

As a rule, head refurbishment and replacement of bent valves is all that's required to put things right. From what I have heard over time, this job should run out to about the $3K. mark. In my opinion, and presuming the engine to be in good condition overall, the purchase and fitting of a replacement engine would not seem to be a favourable solution. The notion of a bent crankshaft holds little merit - in fact, none at all. Regards Styria

 

[WGB]

That is the standard disaster scenario for these engines and is why many great cars end up parted out.

Usually Left hand head, two bent valves or occasionally the camshaft posts will snap and rise and can penetrate the cam cover if high revs at time.

I assume it will need a new chain, tensioner, chain guides and a valve job as Styria says.

I think finding good 420 engines is problematic due to the risk of water having entered the bore in storage and corroding the nicasil coating. On a personal level I would be much happier having a good running engine of this type rebuilt.

It is a salutory lesson in why all M116/7 motors of any vintage need their tensioner and usually their chain replaced at regular intervals.

Bill

 

[SEL_69L]

I have seen a 420 engine in this type distress, with the heads pulled off. Unfortunately, it appeared to be terminal.

In another instance I saw a W126 '560 SE'. I enquired of the owner. He explained that the original 380 engine failed, (can't remember why now), and was replaced with a 560. He had labelled the bootlid appropriately, which was the reason for my questions about the car.

The car was seen at the Eastern Creek Classic about 5 years ago.

 

[B13]

420SELs seem to have the highest value retention of all the W126 series... the 380 and 500 are often very cheap, and the 560, when the show up on eBay, are not that expensive - I would imaging the 560's engine size puts some people off due to perceived expensive running costs (petrol) whereas in most peoples minds the 420 seems a good compromise. The 380/500 are 1st generation W126 and seem to be worth much less than the 2nd generation.

Just looking on eBay while I write this, there are four 420SELs on offer, two are asking about $2.5-3K ea, and two are asking $8-9K.

If the overall condition on your 420SEL is above average, then you might consider shopping around and getting the engine fixed if you can find someone who will do the work for under, say, $2500.

TIP: with the good $A against the greenback, buy all the bits you will need from Autohausaz and then just get the local guys to do the labour.

I've never heard of a chain let go - have heard of it jump teeth on the sprockets or bust up the plastic sliders. After its all fixed, put a note in your service log book to change the chain and sliders again in 160,000km time, no exceptions.

I.

 

[B13]

Actually the more I read this the less it sounds right to me...

The mech says the chain has snapped - has he even removed the cam covers to see if that is so? Then he goes on to say the compression is low, but you can't do a compression test unless the chain is in place... so I'd be after a 2nd opinion.

I had a chain jump on my red 450SE back in the 90s but it was as I was starting the car, not at highway speed. I was lucky enough that I didn't bend the valves and a new chain, sliders and tensioner later I was back up and running. So I'd check what they mean by low on 3 cylinders - if 5 are reading 160PSI and 3 are reading 120-130, you might get away with just a new chain, sliders and tensioner...

Like the others said, post some pictures.

I.

 

[BenzBoy]

Thank you gentlemen but as you can see from my original post the car is not mine and the inquiry comes form a friend who is not terribly mechanically minded.
I can hardly post pictures of something I have not seen.
Regards,
Brian

 

[Styria]

Thank you gentlemen but as you can see from my original post the car is not mine and the inquiry comes form a friend who is not terribly mechanically minded.
I can hardly post pictures of something I have not seen.
Regards,
Brian

So BenzBoy, what is the verdict ? Some of us have raised some quite valid and knowledgeable opinions, and I for one would be very interested in future progress. Don't worry about pics.,, as they are not all that important. I am more interested in what B13 refers to as far as a compression reading is concerned - I also can't see how a compression reading could be obtained. Generally, you get about three valves touching pistons with resultant bent valves. As for the rest - I am highly curious - again, as B13 also says, chains don't break as a rule. Regards Styria

 

[Tony66_au]

On a non running engine you can test for compression or leaks doing a blow down test.

Basically you use a rig that screws into the spark plug hole and this has a pressure gauge on it and an air feed, you then feed compressed air into the rig at a set rate and see what the pressure gauge reads and/or follow the sound of escaping air with a Mechanics stethoscope if it wont pressure up although this is futile if you cant close the intake and exhaust valves.

But if you know your stuff you will know what's supposed to be open and shut.

I also use this to keep valves from dropping into the chamber if im doing stem seals or similar and dont want to pull a head.

But from here it sounds as if the Mech wants a quick fix instead of performing surgery.

 

[BenzBoy]

I think the problem might be the reluctance of the owner to spend money on investigation and an inability to do it himself. He has been offered a replacement motor plus fitting for the cost of a head rework - I would prefer to investigate and repair but I am not the owner.
Regards,
Brian

 

[Tony66_au]

The ever looming spectre of Sticker shock.

I hope he sorts it out

 

[B13]

I don't suppose its worth mentioning that after this replacement engine is fitted and the car back up and running, the owner then needs to do a chain stretch check lest the same thing happens to this replacement engine. If it's done 100,000k since the last chain replacement then its almost certainly on its way to being stetched to the same danger zone which terminated the last engine.

Cheers,

I.

 

[BenzBoy]

I don't suppose its worth mentioning that after this replacement engine is fitted and the car back up and running, the owner then needs to do a chain stretch check lest the same thing happens to this replacement engine. If it's done 100,000k since the last chain replacement then its almost certainly on its way to being stetched to the same danger zone which terminated the last engine.

Cheers,

I.

It is worth mentioning - I did so and am assured it has been changed recently. Personally, I would want to KNOW for sure...
The whole thing sounds a bit like dodgy motors to me.
Regards,
Brian

 

[Oversize]

Has everyone heard of the service update for the 420 / 560 engines? It's been a long time since I was involved, so the memory's a little hazy, but it involved removing the tensioner, checking for chain stretch, checking the length of the spring, cleaning & resetting the tensioner & re-installing the components with a teflon insert. The large nut on the tensioner was then painted with a red, or yellow dot to indicate the update had been undertaken....

 

[WGB]

I have no real knowledge about the 126 tensioner but I assume it was supposed to be an advantage over the version fitted in the 116 M116/7 motors.

It is just a pity that a ratchetting version of the type used in 280 and 6.9 motors wasn't used as it seems to be that initial sloppiness on startup or running at low speed with low oil pressure that allows the chain to start it's "Terminal Death Throe".

The 6.9 chain in my car had 18 degrees of stretch before I changed it and yet was kept tight by the tensioner at all times even though the ratchet was probably in the last notch.

Bill

 

[Oversize]

I believe the 420/560 tensioners would only move in one direction, so in that regard they did have a type of ratchet. Oh & it was probably a nylon insert & not teflon as I mentioned earlier...

 

[mathew]

I change my oil every 5000km in my 420SEL and I simply replace the tensioner every 60,000km at $160 and twenty minutes work why wouldn't you. The chain I do every 150,000km my mate at MB classic in Germany says the reason for chain stretch is poor quality oil changed infrequently, it's rare for the chain itself to break. He reckons with my oil changes the chain would easily go to 200,000km but I like to er on the side of safety. I change the top guide rails when the chain is done.

Importantly these engines give you heaps of warning of pending chain issues the rattle at start up and you would have to be death not to notice it the moment you can here that chain slap it's into the workshop and you should avoid this costly breakdown.

Cheers Mat.

 

[Styria]

Hi Mat, just not too sure that I can agree with you and your friend regarding chain problems and poor quality, contaminated oil. Also, it's my understanding that you do not always get warning of an impending breakage. Some years ago, I came across an absolutely immaculate 380SE with a whisper-quiet engine. In addition, it was a low mileage car and the owner, however, wanted to sell the car at a quoted price given to me. When I rang him a few weeks later the price had gone up on account of a chain problem, causing at least $3000.00 dollars worth of damage to head(s) and valves. The chain itself was fine, it was a plastic guide problem that caused the chain to be derailed.

Mat, in your opinion, can the tensioner be part of an impending chain problem ? I would have thought that problems in the chain/guide/tensioner areawas one of time rather than contamination of oil. Regards Styria