Topic: 6.9 engine seized while cranking

[daantjie]

I was also noticing the cleanliness, especially the piston for sure looks new - ish.

[rumb]

I dont know if you could purchase just one bearing so may need the entire set. You can use plastiguage on one of the other rods to see if any wear and mike the journal. Entirely possible that there is minimal wear. It ma be possible to use the bearing that would come with a good used piston and rod. As mentioned need to find out what size pistons are in there currently.  Also picture of other side-bores to see it any lip at top of cylinders. May as well have a shop measure everything for you.  Dont just push out all the pistons without making first sure the lip if any would damage piston ring lands.

[raueda1]

Yep..the dreaded hydraulic lock...and we all know how this came about.
...and so to recovery. Without going back over the history can you confirm that you never got the engine started...that it jammed under starter motor conditions?

The answer is a qualified yes.  Engine was firing weakly but very far from actually running.

Hydro lock occasioned by kjet flooding usually fills all cylinders equally. When the engine is cranked the 1st cylinder to approach Top Dead Centre is the one that is going to lock. In most engines there are usually 2 cylinders at TDC. In this instance No8 rear lh bank] is the offender. Calculate which other cylinder was at TDC and give it extra forensic examination. All other cylinders cannot theoretically be impacted.

I'm not sure how to "calculate," but I'll have a look to see what other cylinder is in the same position.

I am amazed at how clean that engine looks and in particular how little wear there is on the piston skirt. This is either a remarkably low mileage motor that had the oil changed every fortnight or this engine has been apart before. I'll be interested to learn if the piston is std bore.

It did just have a rather strong flush with gasoline.   The car shows 72k miles on the clock.  I doubt the engine has been out just based on grime, condition of the bolts, etc.  Bolts on oil hoses looked virgin, for example.  Admittedly, this isn't a reliable indicator, but it's usually apparent that an engine has seen a wrench.  When I got it the fuel delivery system needed work and somebody had messed around with the fuel injection (other than me).  As for piston, we'll see soon enough. 

Options
I'd be inclined to go with your suggestion...pull the piston.. rod.. and big end...replace. new gudgeon of course!
I'd pull another piston rod set so that the replacement assembly can be at least weight matched. The original is also a valid balance weight reference.

By this you mean to ensure that the replacement is same weight as the original?  I assume that all the rods+pistons are balanced at the beginning. Hmmmm.  It's easy enough to balance by grinding weight off.  What if I end up with a rod or piston that weighs less than the old parts?  Is there a balancing tolerance spec someplace?

You will want to be dead certain you get the right piston /rod combo as there were a couple of production variations over the course of the motor.

Oh my.  How do I do that?  Are there different part numbers for those variations?

As usual, questions always beget more questions.   Here are a few more:

> What's the size of the external torx (??) socket for the rod bolts?  My searching hasn't helped for some reason....
> Also replace big end bearing, I assume?
> Any reason to worry about the other rod (or its bearing) directly adjacent to the bent one?
> Any recommended sources for 6.9 rods and pistons?  I'll search by part number when I get it apart, but still...  I've got an inquiry active with the Classic Center.

Regardless, the next step is getting the head off and the parts out of the block.  Thanks again to all for the help and support on this.  And always remember:  when reinstalling your fuel distributor remember to put the metering piston back in.     Cheers,

[oversize]

Bite the bullet and do a proper rebuild.  I'd be looking at much lighter, custom high compression pistons if it were me.  I'm sure you'll find the valve guides are shot and you'll be kicking yourself that you didn't take the opportunity when it was available.  In the end you'll have a much more powerful engine that will outlast you!

[Randys01]

No doubt the endless ball of $$$tring will unravel as you pull the beastie apart......72KM.........if it's been looked after, you'll barely find anything actually worn out.  It will be an example of "it's worn well but it's well worn".

Always a dilemma as to whether to just repair the immediate problem...or repair that and anything easy and obvious..or go the whole Monte.
The answer always comes down to money and the intention of "am I keeping this forever.?"
I would be trying to localise the scale of the repair.

The lh head has to come off so as mentioned, is the issue of the dear old valve guides. My favourite topic. You might be lucky and they are in good condition.  Even so. care for a valve grind Your Honour?...what's the rocker wear like? Valve lifters?...and so it goes.
I'd be confident that cyl bore wear will be minimal..but take double note of the lip on no 8.

Going back on a few points. As the motor is locked it will be obvious which other cyl is at TDC. give it special attention to the extent that you can. without calculating this, I reckon it will be on the rh bank..and you don't want to take that head off if you can avoid it.
You are going to have to buy a full set of big end bearings so I'd fit the whole set: but if that's too hard, just buy a set and fit up no8. They are not a King's ransom.

Getting a correct conrod and piston will be a challenge. I'd be looking for a Unicorn horn with more optimism.
Frankly I would be preparing to reuse the piston........whaat?  .but not before it is tested by a reputable engine shop. It will need to be crack tested. measured up with laser precision. Special attention will need to be paid to the gudgeon boss area as this is what took the brunt of the load.
Now before the purists go into meltdown, remember this damage was sustained at cranking speed.  this did not fritz itself at 4000rpm boring down Route 66.
Are you ready for this?  Explore options for having the rod straightened. Yeah I know .impossible..you're kidding? right? No my friends it's done all the time in racing circles. Worth a shot.
On the matter of balance. whatever combo you end up with, .new piston/new rod...old piston new rod, the piston will have to be aligned on the conrod and at least the weight of the new combo compared to at least one other piston rod assy.
Any decent engine shop can do all of this including permissable variations plus and minus.
I would definitely pursue this option whilst still trying to find an obscure piston...and rod.

So I'm advocating the "minimalist approach"..I'm cost sensitive...the end result of course cannot be anything less than "highly satisfactory...we are not talking a bodged up job. That is, it has to accord with accepted engineering standards and practices.

[daantjie]

Boom let me drop the mic and make Dave's day for a new con rod:

http://oldbenz.co/mercedes-405sel-69-connecting-rod-1000302620

Check part# of course, but hey for $100 it's the king of no brainers

[Randys01]

Well done Daniel. I can't believe it. 
 That hopefully deals with the King kong problem of the 2 unicorn horns!

[rumb]

I have offered up a rod and piston from my spare engine if needed. Need to find out what size pistons are in both engines.

[raueda1]

Thanks so much to all forthe info and support.  My partner just can't believe there's a whole global community of nutty old-benz people at all.   But there is, and that community makes it possible to keep these wonderful cars alive. 

Much as I'd like to do a full rebuild, I just can't justify spending the $$$.   I'd love a super-6.9 but what I really need is just a nice driver, which is what the car was in the first place. That leads back to the fix-what's-broke approach while fixing leaks, old rubber, etc.

Any way, amazingly I discovered that same rod.  They seem to have lots of other 6.9 stuff as well.  There are also some rod/piston sets in Germany.  But are they the right ones?  TBD.

Now I'm still disassembling.  2 of the intake manifold hex bolts got trashed (stripped) and will need to be drilled out.  That will happen on Monday and the head will hopefully be off soon thereafter.  Then I can start figuring out the full extent of damage and exact parts needed.  Robert, thanks so much again for the offer, will be back at you when I know more. 

Finally, as for valve guides, who knows?  The car didn't use any oil and no smoke going down long mountain grades in 2nd gear, so maybe ok.  Again, TBD.

Stay tuned for the next installment of this exciting problem and thanks again for the help.  Cheers,

[raueda1]

Today was the day that the mystery was solved.  The heads came off and the bent rod and its piston were finally removed.  The crank is now freed and moves freely in both directions.  I'll start with the cause.  It clearly was hydraulic lock.  There was still a half cup or more of fuel sloshing around in the plenum under the intake manifold.  There was also a fan shaped area on the bottom of the air sensor that was extremely clean - clean, like, where it had been washed by raw fuel pouring down from the FD. 

Now, on to getting this thing fixed.  I've got a NOS rod in the mail.  Open issues and options, as I see them, are:

Dealing with the piston
The cylinder had no lip whatsoever and it was easy enough to push the piston out.  It didn't seem to be totally trashed but didn't completely escape either.  See pix.  There doesn't seem to be any catastrophic damage or cracks.  It was easy enough to tap out the pin.  I did have to tap it out though, it was too tight to just push by hand.  However, on one side there is some obvious scuffing.  It's pretty superficial, although in some places it does obscure the tiny circumferential grooves around the piston.  Still, it's very difficult to feel the scuffs with a fingernail.  On the opposite side there's some mild scoring.  Maybe this is something terrible or maybe not at all.  I don't know.  The piston itself isn't oversize.  Disturbing however, is that it is definitely a little out-of-round.  Measured across the skirt it's ~107.65mm while across the pin it's ~ 106.85mm, so it's a bit oval by 0.8mm.   That seems like a lot to me, but I don't know.  There's no obvious wear beyond this though.  The cylinder looks fine, though I'll examine more closely.  Crosshatching is clearly visible and it didn't seem to suffer collateral damage. 

My tentative plan is to take the piston and rod to a local machinist to balance the rod and evaluate the piston.  Depending on feedback from W116 and the machinist I may either reuse the piston or look for another one.  Obviously your comments would be very helpful.

How much to rebuild? 
I'll be replacing the chain and guides.  As others have said, it's a no-brainer.  I'll also be replacing all the other no brainer stuff discussed elsewhere (everything rubber, etc etc.).  I've also got a cracked exhaust manifold that will be replaced along with a nice stainless steel exhaust system from our friends at TimeValve.   All the little bits and pieces are at the plater to make them pretty again.  Everything else is being cleaned up like new. 

I'm on the fence with rebuilding the cylinder heads.  Valve guides and seals aren't too much if I recall right, so it probably makes sense to just do that.  What about refacing the valves and seats?  I have no reason to suspect that they're a problem, but why stop at just the guides?

I don't see any reason to mess with the crank or main bearings.  New bearings for the new rod seems sensible, along with the adjacent rod (cause why not?).  Other thoughts or suggestions?

-------------------
So there you have it.  Again, and as always, thanks to everybody for all the help with this.  Stay tuned as the adventure continues.  Cheers,

[UTn_boy]

A few things.  First, the connecting rod bearing aren't available separately....at least from Mercedes.  They're only available as a whole kit.  Part number 100-030-00-60 at a retail cost of $350.  (standard size, 55mm).  Then again, a reputable machine shop can also have bearings made to the dimensions needed.  But if you want Mercedes parts (highly recommended for engine and transmission internals) you can simply fit all new connecting rods since you'd have the whole kit anyway. 

The 0.8mm far surpasses the maximum eccentricity allowed, so the piston will have to be dealt with.  Naturally, Euro pistons are no longer available.  Though, I find it both odd and extremely disconcerting that 6.3 pistons are still available, but not 6.9 pistons.  I guess it's all part of Mercedes's mission to get all of the old models off of the road that they can.  I digress.  If the piston can be made round again and has no structural damage, then all you'll need to do is measure the height of the ring landings and the gaps between the piston rings and the ring landings themselves.  Superficial scoring on the outer circumference of the piston is insignificant if the scores are very shallow. 

Since you'll be down so far in the engine and renewing the timing chain and rails, don't forget to renew all of the sprockets, camshaft gears, idler gears, distributor drive gear, and most importantly the one on the crankshaft. 

I'm very skeptical about the connecting rod having bent at starter speed from hydraulic resistance.  At starter speeds the excess fuel usually stops the engine from turning, but not cause any mechanical damage.  I've seen this particular situation happen a handful of times over the years, and the worst thing that ever happened was a piston ring cracked.  Hence my skepticism.  As a precautionary measure remove all of the valves and check to see if any of them are bent. 

On the subject of heads and valves....be very careful about re-seating valve faces and seats.  If you want to do this then do so ever so lightly. Often when this is done the valves will seat deeper within the head....which in turn requires resetting of the valve lash.  That job is pretty difficult to do because the ball stud assemblies have to have oil pressure pushing up on them to check the lash.  In addition, try not to have any material taken off of the bottoms of the heads.  There is a minimum allowable deck height, but again, taking material off of the heads will also require resetting of the valve lash.  This also requires checking of valve timing and the possible use of an offset woodruf key to compensate for the change in geometry from having material taking off of the bottoms of the heads. 

[raueda1]

Thanks for taking the time with the comments, very helpful.  Gives me some stuff to chew on.  More below . . . .

A few things.  First, the connecting rod bearing aren't available separately....at least from Mercedes.  They're only available as a whole kit.  Part number 100-030-00-60 at a retail cost of $350.  (standard size, 55mm).  Then again, a reputable machine shop can also have bearings made to the dimensions needed.  But if you want Mercedes parts (highly recommended for engine and transmission internals) you can simply fit all new connecting rods since you'd have the whole kit anyway. 

You mean new rods AND bearings or just the bearings?  As far as I know the rods are NLA from MB, though there NOS rods popping up here and there.   But finding 8 new rods would be pretty hard.  As it is I feel lucky to have found just one.  I'm hoping further inspection doesn't reveal more damage.

The 0.8mm far surpasses the maximum eccentricity allowed, so the piston will have to be dealt with.  Naturally, Euro pistons are no longer available.  Though, I find it both odd and extremely disconcerting that 6.3 pistons are still available, but not 6.9 pistons.  I guess it's all part of Mercedes's mission to get all of the old models off of the road that they can.  I digress.  If the piston can be made round again and has no structural damage, then all you'll need to do is measure the height of the ring landings and the gaps between the piston rings and the ring landings themselves.  Superficial scoring on the outer circumference of the piston is insignificant if the scores are very shallow.

Scoring is very shallow.  After looking at pix from people asking "is this piston OK?" on other forums it's obvious that my piston looks pretty good!
       re: eccentricity - Yeah, seems kind of crazy.  If numbers are right I'd figure the piston would be hard to get out, maybe.  But I also know I measured wrong, using a digital caliper not a proper micrometer (the "C" shaped kind).  The calipers can only nmeasure at the bottom edges, not the piston body.  Still, those were the numbers and they were highly repeatable.  Bottom line is the racing shop will need to confirm.   

Since you'll be down so far in the engine and renewing the timing chain and rails, don't forget to renew all of the sprockets, camshaft gears, idler gears, distributor drive gear, and most importantly the one on the crankshaft. 

OMG.  That's the one I dread the most.  Let's take a step back.  This motor seems in pretty good shape and was running fine until I decided to mess with it.  It just wasn't quite as peppy as I think a Euro car ought to be.  My rationale for replacing the timing chain was to get valve timing perfect without chain stretch entering the equation.  Then it's a very small step to replace the rails.  Of course if all those sprockets are worn it's a problem, so now I'm struggling again with the "if it ain't broke" conundrum.  My goal here was really just to fix it, not do a total rebuild.  But it's a slippery slope and I'm sliding down....   Crankshaft sprockets are easy enough and available.  But the other stuff - I don't know.   Is there some way to evaluate sprocket wear? 

I'm very skeptical about the connecting rod having bent at starter speed from hydraulic resistance.  At starter speeds the excess fuel usually stops the engine from turning, but not cause any mechanical damage.  I've seen this particular situation happen a handful of times over the years, and the worst thing that ever happened was a piston ring cracked.  Hence my skepticism.  As a precautionary measure remove all of the valves and check to see if any of them are bent.

I'll certainly be checking the valves.  The hydraulic lock theory can't be proved, we'll never really know.  But something happened immediately after goofing up the FD and I know it was pouring fuel straight in.  And it wasn't just cranking from the starter, it was also firing weakly and chugging.  So I guess it's not a good theory, I just can't think of a better one.  Lesson:  when you install FD don't forget to put the metering plunger back in.   

On the subject of heads and valves....be very careful about re-seating valve faces and seats.  If you want to do this then do so ever so lightly. Often when this is done the valves will seat deeper within the head....which in turn requires resetting of the valve lash.  That job is pretty difficult to do because the ball stud assemblies have to have oil pressure pushing up on them to check the lash.  In addition, try not to have any material taken off of the bottoms of the heads.  There is a minimum allowable deck height, but again, taking material off of the heads will also require resetting of the valve lash.  This also requires checking of valve timing and the possible use of an offset woodruff key to compensate for the change in geometry from having material taking off of the bottoms of the heads.

This convinces me not to do a valve job, just replace guides.  And I may not even do that - "if it ain't broke...." 
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Thanks again.  I'll certainly be posting more on developments, progress and setbacks.  Cheers,

[daantjie]

Dave, some food for thought, use it don't use it of course

Why not source a good used Euro motor?  They pop up occasionally for little money.  I recall there was one on ebay forever (might even have been a member here selling it) for under $1K USD.

As you say, the rebuild slope is a slippery one, and especially on the 6.9 motor if you piece out the individual parts, you will rapidly stack up the Benjamins

[s class]

Dave, some food for thought, use it don't use it of course

Why not source a good used Euro motor?  They pop up occasionally for little money.  I recall there was one on ebay forever (might even have been a member here selling it) for under $1K USD.

As you say, the rebuild slope is a slippery one, and especially on the 6.9 motor if you piece out the individual parts, you will rapidly stack up the Benjamins

I can't agree with that.  A used engi e will have hundreds of thousands of miles on it, and will be an unknown quantity with other potential problems.  This original engine looks to be in excellent low mileage condition apart from the issue at hand.  I would repair the damage at hand without going to a full rebuild.

[s class]

Dave, now that the engine turns and the heads are off, you must measure piston protrusion of the other 7 cylinders.  This is the distance between the piston face and block deck with each piston at its TDC.  You will need to clean the block faces properly to get accurate measurements.  I would expect a variation of 0.05mm between pistons, or perhaps a max of 0.1mm.  Any more than that and you have issues such as another rod bent just slightly, not apparent to the eye.