Author Topic: 6.9 engine seized while cranking  (Read 1579 times)

s class

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Re: 6.9 engine seized while cranking
« Reply #45 on: 22 September 2019, 05:02 AM »
Piston dimensions - rember that the piston is slightly tapered, so when comparing thrust and land directions to assess ovality, you must measure at the same distance below the crown.  I agree with Aaron that piston has taken a beating, and it must be cleaned and examined very critically for cracks in the ring lands and gudgeon pin area.  If it passes the crack and dimensionality aspects, it should be usable.  You will need to then test fit it with the new rod to check piston protrusion.  I sometimes do this without rings installed until I'm sure everything is OK.
« Last Edit: 22 September 2019, 07:57 AM by s class »

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

s class

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Re: 6.9 engine seized while cranking
« Reply #46 on: 22 September 2019, 05:07 AM »
Heads... If you have a machine shop fit new guides, you will typically need to reseat the valves due to the possibility of differences in axial precision between the old and new guides.  A capable machine shop can do that with only minimal loss of valve seat material, but yes, as Aaron says, the static valve clearances will need to be assessed. Machine shops will typically want to re face the heads by default, but this should be avoided if possible.  Read the section of the 4.5 shop manual on refacing heads.  The same principles apply.
« Last Edit: 22 September 2019, 07:58 AM by s class »

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

UTn_boy

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Re: 6.9 engine seized while cranking
« Reply #47 on: 22 September 2019, 10:12 AM »
Dave, I meant connecting rod bearings only.  They come in a set.  They were never available individually. 
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raueda1

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Re: 6.9 engine seized while cranking
« Reply #48 on: 22 September 2019, 10:47 AM »
OK fellow enthusiasts!!  This has all been both thought provoking and incredibly useful.  There are so many potential paths forward, each with very legitimate pros and cons.  It was really quite overwhelming sorting through it all.  Finally though, my approach has now snapped into sharp focus.   I'm narrowing the project scope to the "just-fix-what's-broke approach."  There are practical considerations too.  I'm under some logistical pressure to get my car out of the big garage before winter (under its own power) and get my parts car out of my driveway and into the big garage.

The rationale is that, as others have commented, it seems to be an otherwise healthy, low miles engine.  The swap engine idea certainly has merit but introduces all kinds of other uncertainties.  Better to stick with what's better known.  To be sure, this means checking it all very, very carefully.  If another piston/rod or 2 needs to be replaced, then it will be.  I've got 1 new rod in the mail and others are available if need be.  Bearings too, of course.  Will replace cam chain and rails.  Maybe cam sprockets, need to look at them more closely.  I wouldn't even do that except that I've got the chain and I want to ensure cam timing is spot-on.  This is the perfect opportunity.  Also replace all the odd rubber stuff.   Who knows what vacuum leaks there were?  I've now got almost all the parts on hand to do all that. 

However, I'm not going to venture into the "deep" sprockets, crank seals or other bottom end stuff.  Sure, the engine is out and opened up but there's little basis to think that anything is appreciably worn, that would be more in the category of "well, while it's all apart why not also do [fill in the blank]."  The oil pump chain sprocket looks brand new.  Same goes for the valve guides and associated machining work.  Hell, before this the car didn't smoke or even burn oil driving very fast for hours in very hot conditions.  None of the pistons have hit valves (which I'll certainly inspect more closely).  Maybe I'll replace the valve stem seals cause it's so easy (I assume).

After this exercise just pulling a head, or even the whole engine again, doesn't seem so bad.  Rounded bolts, frozen nuts etc shouldn't be a problem.  [sidebar comment: I've always used Never-seez or similar when reassembling pretty much anything needing to be torqued.  Makes torquing easier, more accurate and easier to disassemble later.  Anybody have comments on this practice?]
Now I've got all the oddball tools that I had to order and wait for.  Pulling the engine doesn't seem like such a big deal any more.  Now I'd do it in 20% of the time it took just took me.

So that's my path forward, now written in stone - unless something very unexpected crops up.  This was kind of the direction I was headed anyway but now I'm firmer and more confident that's it's a pragmatic and low risk approach.  Maybe I'll have more questions along the way.  Otherwise, the next you'll hear on this is what happens when I fire it up!   ALL your comments have been incredibly helpful in thinking it through, so thanks again to everybody.   Wish me luck.  Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

rumb

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Re: 6.9 engine seized while cranking
« Reply #49 on: 22 September 2019, 11:08 AM »
If I recall correctly the front crank seal is easy and cheap and you are going to take the front cover off anyway to do the chain and guide. The front pulley came off straightforward on my engine. I also dont recall but many mb engines have a replaceable ring on the crank that meets the front seal and wears quite a bit.
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

raueda1

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Re: 6.9 engine seized while cranking
« Reply #50 on: 22 September 2019, 11:20 AM »
If I recall correctly the front crank seal is easy and cheap and you are going to take the front cover off anyway to do the chain and guide. The front pulley came off straightforward on my engine. I also dont recall but many mb engines have a replaceable ring on the crank that meets the front seal and wears quite a bit.
Hmmmm.  Not sure why changing chain needs front cover to come off, though I hadn't considered the rail back there.   I admit, I detest the idea of pulling the balancer off.  Maybe I need to just need to get over it.  Will investigate further, thanks.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

s class

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Re: 6.9 engine seized while cranking
« Reply #51 on: 22 September 2019, 11:30 AM »
If I recall correctly the front crank seal is easy and cheap and you are going to take the front cover off anyway to do the chain and guide. The front pulley came off straightforward on my engine. I also dont recall but many mb engines have a replaceable ring on the crank that meets the front seal and wears quite a bit.

I'll have to differ there.  The 2.8 engines have a replaceabke ring where the front main seal runs, the v8s don't.  On 3.5, 4.5 and 6.9 it's part of the front flange.  Replaci ng the seal means drawing off this flange, on the smaller v8s its a routine procedure.  On the 6.3 and 6.9 it's an absolutely epic task.  Refitting the flange on the M100 is the stuff that will put hair on your chest

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

rumb

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Re: 6.9 engine seized while cranking
« Reply #52 on: 22 September 2019, 01:27 PM »
If I recall correctly the front crank seal is easy and cheap and you are going to take the front cover off anyway to do the chain and guide. The front pulley came off straightforward on my engine. I also dont recall but many mb engines have a replaceable ring on the crank that meets the front seal and wears quite a bit.

I'll have to differ there.  The 2.8 engines have a replaceabke ring where the front main seal runs, the v8s don't.  On 3.5, 4.5 and 6.9 it's part of the front flange.  Replaci ng the seal means drawing off this flange, on the smaller v8s its a routine procedure.  On the 6.3 and 6.9 it's an absolutely epic task.  Refitting the flange on the M100 is the stuff that will put hair on your chest

Thanks for correcting me,  I was going from my faulty partial memory... ::)  I did remove mine and replace seal and I did not think all that hard, though I do know you have to get the keyway -( actually 2 pins) lined up exactly.  Maybe it was because I have done the same about 3-4 times on other MB engines. I may be wrong again, but it seems accessing the bottom chain guide was enhanced by having the front flange off.  I use a sharpie and mark on the flange and pulley shaft where the keys/pins are so that it is easier to keep everything lined up.

« Last Edit: 22 September 2019, 01:48 PM by rumb »
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daantjie

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Re: 6.9 engine seized while cranking
« Reply #53 on: 22 September 2019, 06:11 PM »
I would absolutely do the valve stem seals with motor out of the car. It's a back breaker of a job with the motor in situ :o
Daniel
1977 450 SEL 6.9 - Astralsilber

Randys01

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Re: 6.9 engine seized while cranking
« Reply #54 on: 23 September 2019, 02:08 AM »
I think you know my approach to this right from the start. Just repair the obvious and any other minnow issues you see.
I think you are leaning this way after canvassing some damn good opinions.
That engine is in good knick. I suspect this engine had no obvious overhead valve gear clatter..did not blow smoke ..did not use  oil.

Make sure the piston is good to be re-used. Get a pros measurements like I said.
Replace the rod...make sure it is the same variant...
Have the piston to the rod set up and weight balanced.
Buy a complete set of big ends and replace the two on the same journal.

If this bent rod was other than hydraulic lock, there would be other signs eg detents in the top of the piston.
As far as the heads go, I just have a feeling your valve guides will be ok.
Without turning this into Ben Hur, remove  two exhaust valves No 1 and No 8.
Measure the diam of the stem in a no of places...compare to spec. I bet you will find negligible wear. Determine the amount of wiggle/slop between the valve and the guide.  ..without the springs etc. If it is badly worn, you will be amazed at the slop. if there is no wear by feel you are good to go. This may not sound scientific but that's how it works. Just change the valve stem seals if everything seems ok. If you read the M100 site, you will see I have done a Masters on this topic!!
Do not disturb the valve seats. A basic test to determine if a valve is leaking is to fill each combust chamber with Metho. Yes, even fully assembled a burnt valve will leak. Even dual valve springs cannot seal a weeping seat. If the metho disappears then you will have to review the way ahead. Yes..I know this is heresey to the purists but we do not need to go looking for trouble.  A few basic tests will pin point if there is an issue and then the $ budget can decide the forward strategy.
Setting up the valve lifters after a valve grind, as highlighted in an earlier post is just a nightmare. I have had to do this from square one and it is best avoided.
What you must do tho is pull every valve lifter out of its threaded home and purge the galleries.
You will be horrified at the grunge.
Now the matter of refacing the heads. I know this is done as a matter of course "just because".
.......I see no reason in this instance coz the engine was not overheated .If you can clean the head faces as well as you have to clean the block face, you should be able to refit without disturbing the head face. If you simply must face the heads, then of course all the valves etc will have to come out and we are back to an escalation.
If you end up having to face ; and can encourage the machine shop to take the absolute minimum [ they never listen ] then the impact on the timing will be miniscule. There will be the issue of raising the CR on a Euro to approx. 9.+ :1. Not sure what your ULP options are in your part of the world.
Cheers


raueda1

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Re: 6.9 engine seized while cranking
« Reply #55 on: 23 September 2019, 01:48 PM »
Great news.  ;D ;D ;D

Got piston evaluated by a very serious machine shop.  They do mostly auto work but also some enormous work on giant mining stuff. 

Anyway, piston checks out.  No cracks and dimensions are fine (my measurements were crap, as I suspected).  He said to reuse it without any reservation.  The new rod checks out too.  It weighs within 1 gram of the old one and all dimensions same.

The piston comes in at least 3 variants and I feared an endless search for the right one, so this is an enormous relief.

Finally, he said that the bearing looked just beautiful, zero apparent wear. Still, replacing it makes sense since its easy at this point. I'll do them all. 

Looks like now it's just a matter of clean up and reassembly per my plan above. Hopefully my next post here will be to dept that it runs like a scalded cat.  Again, thanks to all and 🍻.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

p. rex

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Re: 6.9 engine seized while cranking
« Reply #56 on: 23 September 2019, 02:59 PM »
Congrats!

This is really fascinating reading.  Most folks, myself included, are not brave enough to dig into the M100 engine internals.  Thankfully, few of us need to -- we are lucky to have pretty robust motors.

With any luck, I'll never have to do anything more than valve lifters or a timing chain tensioner, but all the same, kudos to you.  You've got some gumption to tackle something like this.
1979 450SEL 6.9 #7184 “Die Marschallin”

raueda1

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Re: 6.9 engine seized while cranking
« Reply #57 on: 23 September 2019, 06:11 PM »
I did forget one amusing detail.  At the machine shop the old timer expert guy looked at the rod first and shook his head. He then said, "Let's see the piston," and I handed it to him.  He gave it a quick look over, looked at me and said, "Looks like you had hydraulic lock."  He concluded in about 2 seconds what we've been discussing for 6 weeks.  Make of this what you will.   ::)
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Jed

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Re: 6.9 engine seized while cranking
« Reply #58 on: 23 September 2019, 06:32 PM »
"Looks like you had hydraulic lock."

there is no substitute for experience

thanks for sharing...great read

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1979 Mercedes 6.9 #6424 - ongoing restoration
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Randys01

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Re: 6.9 engine seized while cranking
« Reply #59 on: 24 September 2019, 02:12 AM »
OK.    a most unfortunate "misadventure" but you can  wriggle out of this with your backside largely in tact!
The reason the rod bent was because you managed to get the motor partly started. You may recall I asked if it was only ever on cranking...you confirmed she partially fired up.   bingo!
Good luck from here.