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Garage => Mechanicals => Topic started by: raueda1 on 06 August 2019, 04:46 PM

Title: 6.9 engine seized while cranking
Post by: raueda1 on 06 August 2019, 04:46 PM
Preamble:  this is a fresh reboot of this https://forum.w116.org/mechanicals/6-9-ring-gear-destroyed/ (https://forum.w116.org/mechanicals/6-9-ring-gear-destroyed/).  The old thread got too long and the problem seems to be something else anyway.
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Background:  After doing Fuel Distributor work I tried to start the engine.  I messed up the FD a little and flooded the engine.  I then rectified the FD and tried to start again.  The engine was firing and almost started.  It stopped abruptly however, with a hard clunk.  In the process I apparently cranked too hard for too long and burned out the starter.   I replaced the starter with a rebuilt unit from a reputable source.  On the next try the engine cranked about 1/2 a revolution and stopped abruptly with the same hard clunk.  Thereafter it wouldn't crank at all.

What I tried and looked at

In all cases I tried rotating CW (the right way) but also CCW just to see if it would go backwards even a tiny amount. It didn't - jammed solid in both directions.  I'm using a 24" breaker bar and lots of muscle to turn it over.  There is absolutely zero crankshaft movement.  Clearly something is jamming the crankshaft.  I think it's significant that after the first clunk the new starter DID rotate it a little but was followed by the same clunk. 

Now I've now exhausted all my logic and am at my wits end.  Clearly something is jamming the crankshaft.  I'll try any ideas, however far fetched.  Thanks for any and all help.  Cheers,
 

Title: Re: 6.9 engine seized while cranking
Post by: Pete49 on 06 August 2019, 08:35 PM
Drop the sump and see if a conrod has dropped and jammed it though turning it in reverse should allow movement.
Title: Re: 6.9 engine seized while cranking
Post by: oversize on 06 August 2019, 09:59 PM
I'd pull BOTH rocker covers and thoroughly inspect the chain & guides. Check the bellhousing that you didn't drop a bolt or something in there.  If no joy you'll have to pull the motor and drop the whole sump. If not joy there then it's front covers off. If nothing is revealed it's then heads off....
Title: Re: 6.9 engine seized while cranking
Post by: s class on 06 August 2019, 10:52 PM
Dave I would agree with the others that you should remove both valve covers and inspect thoroughly.  If everything looks good and the chain etc is all in order then I would be looking at external issues eg in the bellhousing.

Get yourself one of these cheap USB I section cameras from ebay and look down each plug hole
Title: Re: 6.9 engine seized while cranking
Post by: s class on 06 August 2019, 10:54 PM
Removing the sump in situ is possible but not easy, need to lift the engine quite high off the mounts.  Before doi g that try looking into the sump drain hole with the USB camera.
Title: Re: 6.9 engine seized while cranking
Post by: rumb on 07 August 2019, 07:15 AM
This is a tough one!  As others have said I would start with the other valve cover and look for broken chain guide pieces that may have fallen and jammed somewhere. One of those usb cameras could help look down into engine. You could a look at both cam alignment marks to see if they are somewhere near the same on both sides. Something broken in valve train? Chain jump I would think most likely at start up whereas rod I would think fail at high rpm.
Overall I would rate valve train damage as most likely, something on the backside of engine 2nd and rods last.
I cant remember doesnt the 6.9 have an inspection plate on the front bottom of bell housing?
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 07 August 2019, 09:10 AM
This is a tough one!  As others have said I would start with the other valve cover and look for broken chain guide pieces that may have fallen and jammed somewhere. One of those usb cameras could help look down into engine. You could a look at both cam alignment marks to see if they are somewhere near the same on both sides. Something broken in valve train? Chain jump I would think most likely at start up whereas rod I would think fail at high rpm.
Overall I would rate valve train damage as most likely, something on the backside of engine 2nd and rods last.
I cant remember doesnt the 6.9 have an inspection plate on the front bottom of bell housing?
Thanks to all for the comments and ideas.  I'll start with the other valve cover.  My new borescope has been ordered and will arrive today.  The real fun will begin tomorrow.  Robert, yes there is such a cover plate.  Unfortunately what can be seen looks good. 

Seems like an engine manual would be helpful.  The manuals here just seem to stop with the 4.5l engines. I'll start scouring the interwebs, but if anybody has a lead on an 6.9/M100 engine manual please do let me know.  German is fine.

I'm amazed that after all the triple digit speeds, high temp desert driving, etc etc, that just starting the car in a garage has so utterly screwed it up.  I'm being punished.  Good grief!  Regardless, I'll post findings as they're found.  Thanks again,
Title: Re: 6.9 engine seized while cranking
Post by: daantjie on 07 August 2019, 09:47 AM
Dave there is a 100.985 service manual in the Library under "Technical".  It also has a Combustion and I think Emissions sections too.  It is transposed from micro fische so the pics are useless but info is good.
Title: Re: 6.9 engine seized while cranking
Post by: rumb on 07 August 2019, 03:33 PM
At some point you are probably going to need these valve cover gaskets, NLA and relative hard to find.

https://www.ebay.de/itm/Ventildeckeldichtung-Ventilschaftd-Mercedes-M-100-985-450SEL-6-9-1975-80-W116/264391592115?fits=Platform%3AW116%7CMake%3AMercedes-Benz&hash=item3d8ef788b3:g:rLsAAOSwdVpdJz3O
Title: Re: 6.9 engine seized while cranking
Post by: Randys01 on 07 August 2019, 08:29 PM
As soon as I read the 1st sentence I thought "hydraulic lock". it is easy to do with K jetronic. However, removing the spark plugs should have allowed the motor to turn.
...mm

OK......b4 the borescope arrives take a piece of wire and carefully wrap some tissue around it tightly.  lower into a the plug holes and see if it comes out wet. keep going until all 8 are done. If they are dry then drain  the engine sump oil only from the sump. Ordinarily you only get about 3 litres.
Examine the nature of the oil..does it seem super thin and watery?  did you get significantly more than 2..5 l to 3 litres?
With the plugs out and the sump drained........disconnect the fuel pump relay.  and crank the motor.
If it's no go, then you have deeper problems but I would eliminate any possibility of hydro lock b4 hitting the panic button.
Do not force anything with crowbars/ whatever if she won't turn on the starter.



Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 07 August 2019, 09:42 PM
As soon as I read the 1st sentence I thought "hydraulic lock". it is easy to do with K jetronic. However, removing the spark plugs should have allowed the motor to turn.
...mm

OK......b4 the borescope arrives take a piece of wire and carefully wrap some tissue around it tightly.  lower into a the plug holes and see if it comes out wet. keep going until all 8 are done. If they are dry then drain  the engine sump oil only from the sump. Ordinarily you only get about 3 litres.
Examine the nature of the oil..does it seem super thin and watery?  did you get significantly more than 2..5 l to 3 litres?
With the plugs out and the sump drained........disconnect the fuel pump relay.  and crank the motor.
If it's no go, then you have deeper problems but I would eliminate any possibility of hydro lock b4 hitting the panic button.
Do not force anything with crowbars/ whatever if she won't turn on the starter.
Interesting idea.  What exactly is hydraulic lock?  Fuel filling one of more cylinders so it can't crank?  And then fuel gets into oil and thins it out?  Or the same but with coolant?
Title: Re: 6.9 engine seized while cranking
Post by: Randys01 on 08 August 2019, 12:33 AM
Hydraulic lock occurs when one cylinder or more fill with petrol. This condition can occur when  mucking around with the FD on K jetronic resulting in a continuous stream of fuel squirting into the cylinders. I have encountered this problem myself...as have quite a few other folk ...who have delved into the wonders of K jet.
The condition can also occur if we get water into a cylinder..eg blown head gasket. This happened to me 2 days ago when a Nissan  popped the head gasket..water was drawn into the cylinder and damn near locked the motor.

But having now read your earlier threads on starter motor -to ring gear- to fuel distrib, something tells me this is a problem that is all related and has been some time in the making. Seemingly, the case for conventional "cylinder hydraulic lock "seems to weaken partic as you have removed the plugs. ie the lock up is not in the cylinders. You have removed all the plugs ?...yes?

 It is drawing a long bow, but it is possible to jam a motor from turning if there is too much oil/fluid in the sump.
Hence my suggestion to drain it. As u are aware the 6.9 is dry sumped. This is a bit misleading coz of course it has a sump but only to collect and scavenge. Oil supply management is drawn from the reservoir tank.
It is theoretically plausible and possible for the sump/crankcase to overfill and lock the piston downstroke. There are a couple of ways this can happen...all of them bizarre.
Impossible I hear the experts chortle!

Ordinarily in this motor, the scavenge pump would keep the sump at the right level even if it had filled with surplus petrol draining from the cylinders past the rings and into the sump..
But what if the scavenge side of the pump is not returning the oil to the reservoir.? The reservoir would be empty and the sump would be full.
So you need to determine that the oil level in the reservoir is in the right range [yes I know..that level is determined when the engine is running but we can't run the engine ]HOWEVER...a static dip of the tank will tell you if it's emptyish.
If heaps comes out of the sump  then you have the answer.

Resolve "crankcase hydraulic lock" before moving on to the next scenario.
...and yes...it is a long shot.
 
Title: Re: 6.9 engine seized while cranking
Post by: TJ 450 on 08 August 2019, 12:37 AM
I hate to say it, but with the sheer volume of fuel K-Jet is capable of delivering, hydro-lock is likely.

It can and will bend the conrods, a car Nathan now owns had this occur and the engine was rebuilt as a result.

Tim
Title: Re: 6.9 engine seized while cranking
Post by: daantjie on 08 August 2019, 07:57 AM
Man, the suspense is killing me here :o
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 08 August 2019, 09:53 AM
Thanks for all this, extremely illuminating.  I've seen several cars with hydraulic lock from water - driving through floods, at the beach.  It once happened to my wife when her car was submerged.  But it never occurred to me that it could happen from fuel.  Anyway, see more below....

Hydraulic lock occurs when one cylinder or more fill with petrol. This condition can occur when mucking around with the FD on K jetronic resulting in a continuous stream of fuel squirting into the cylinders. I have encountered this problem myself...as have quite a few other folk ...who have delved into the wonders of K jet.
This most definitely DID occur, at least briefly, and the engine was cranked until it was clear that it wasn't going to fire.

But having now read your earlier threads on starter motor -to ring gear- to fuel distrib, something tells me this is a problem that is all related and has been some time in the making. Seemingly, the case for conventional "cylinder hydraulic lock "seems to weaken partic as you have removed the plugs. ie the lock up is not in the cylinders. You have removed all the plugs ?...yes?
Yes, removed plugs.  But the borescope arrived.  Same day delivery from Amazon!  Sso I'll be having a look inside anyway shortly.

It is drawing a long bow, but it is possible to jam a motor from turning if there is too much oil/fluid in the sump.
Hence my suggestion to drain it.  As u are aware the 6.9 is dry sumped.  This is a bit misleading coz of course it has a sump but only to collect and scavenge. Oil supply management is drawn from the reservoir tank.  It is theoretically plausible and possible for the sump/crankcase to overfill and lock the piston downstroke. There are a couple of ways this can happen...all of them bizarre.  Impossible I hear the experts chortle!

Ordinarily in this motor, the scavenge pump would keep the sump at the right level even if it had filled with surplus petrol draining from the cylinders past the rings and into the sump...
But what if the scavenge side of the pump is not returning the oil to the reservoir? The reservoir would be empty and the sump would be full.
Interesting.  What scenario(s) would cause this to happen apart from pump failure?

So you need to determine that the oil level in the reservoir is in the right range [yes I know..that level is determined when the engine is running but we can't run the engine ]HOWEVER...a static dip of the tank will tell you if it's emptyish.  If heaps comes out of the sump  then you have the answer.
We'll know the answers in a few hours!  Keeping my fingers crossed that this IS the case, and nothing else is broken.  Thanks again.  This mystery deserves a youtube channel with millions of followers.  Cheers,
Title: Re: 6.9 engine seized while cranking
Post by: s class on 08 August 2019, 01:11 PM
I recently assisted a local main dealer with a 450SL kjet.  Someone had mad a mess of trying to fix the fuel distributor.  They had tried to install the metering barrel without any lube on the 8 orings that seal the metering slots.  This resulted in 7 of the 8 splitting.  This resulted in max unregulated delivery to those cylinders.  I advised a compression test as I was confident hydraulic lock had occurred... Three workshops had tried cranking the engine ad nauseum.  Any how, I rebuilt the fd, but we still really struggled to start the engine.  Short version of a long story
Title: Re: 6.9 engine seized while cranking
Post by: s class on 08 August 2019, 01:14 PM
Short version of a long story is we then found that the left cam was timed 180 degrees out.  We corrected that, and I then got the engine to run fairly well, despite probable bent valves and probable bent rods.  The workshop manager and I were both pretty surprised to get it running that well.  But this is not the recommended way to treat an engine.
Title: Re: 6.9 engine seized while cranking
Post by: s class on 08 August 2019, 01:17 PM
I've Sen 450 motors survive unregulated fuel delivery a few times, but your situation is a bit different.  Running without the meter g piston would dump fuel directly I to the intake, not via the injectors.  Remember the pump can deliver approx 2 litres a minute.  The possibility of crank case lock did occur to me, but it would take quite some time to get enough fuel to full the crank case to pass through the rings. 
Title: Re: 6.9 engine seized while cranking
Post by: s class on 08 August 2019, 01:19 PM
Have you tried depressing the air mass meter flap, and opening the throttle to see if there is fuel pooled up in the intake manifold?
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 08 August 2019, 03:17 PM
For some findings to report:

> drained the oil.  There was a LOT in the sump but I'm not sure how much would be normal.  Oil stinks of gasoline.  It seemed to be thinned, to be sure, but not to the extent of "watery."  No metal chunks or anything came out.  I still couldn't move the crank after draining the oil.

> looked at valve timing.  The timing notches don't line up with the index lines on the post of course.  I made a little gauge to see how far offset they were from the index mark. Both sides were offset by the same amount, so it didn't look like their relationship with each other got changed.

> borescope: 
   *  looked in all the cylinders and saw nothing obvious amiss.  Stupidly I left the 90deg angle mirror at home so couldn't see everything or look at valves. I'll do that tomorrow.
   *  looked down the chain path as far as I could.  Nothing obvious - couldn't see broken teeth, metal chunks, anything like that.
   * ring gear:  poked as far as I could (which was pretty far) and couldn't see any damage.

So, it does look like there is some kind of mechanical blockage.  Or maybe hydrolock lead to something else?  The mystery continues. 

Next step, I guess, is to start >>really<< taking stuff apart.  Seems like 3 choices - from the rear (transmission), the bottom (oil sump) or top (heads).  Any further theories or suggestions are certainly welcome.  Seems like pulling the sump is probably less involved than the heads. 

Oddly, a sense of calm has overtaken me.  It's so baffling that I'm now philosophical about it.  Cheers,
Title: Re: 6.9 engine seized while cranking
Post by: Randys01 on 08 August 2019, 05:39 PM
So you only got a couple  of litres out of the sump. How full is the reservoir?
However, I think the journey now takes us to timing chain/tensioner/guide issues.
In order to narrow this down I would de-tension the main chain tensioner to relieve the tension on the banana tensioner rh  bank. With the chain slack I would try to turn the cam shafts [spanner on the cam sprocket] We don't need to wiggle it far to determine the cams will turn a bit. any rotation either way tells us the top of the motor is not the culprit inc the myriad of cogs/wheels/guides n the top timing chain entourage.
Now try turning the crank pulley a smidgeon..to date it has been locked solid but if you can now get a bit of action it would indicate something is binding in the timing chain department.
Report in!
Title: Re: 6.9 engine seized while cranking
Post by: rumb on 08 August 2019, 06:03 PM
Transmission in neutral? Though even in park I think you should still be able to turn crank.

Total oil in pan I would think would be about 10 quarts with probably 2 left in the filter and cooler.

Random ideas:
can you wiggle each cam a few degrees? Grab with vice grips to see if they each move at least a little bit.  If it moves and all your upper valve train looks normal -no loose rockers -  then that would reduce odds in that area.  You may need to remove chain tensioner to allow a little chain slack.

Have you run the scope in the oil pan?
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 09 August 2019, 07:20 AM
So you only got a couple  of litres out of the sump. How full is the reservoir?
Reservoir was full.  The gasoline thinning was very notable on the dipstick.

However, I think the journey now takes us to timing chain/tensioner/guide issues.
In order to narrow this down I would de-tension the main chain tensioner to relieve the tension on the banana tensioner rh  bank. With the chain slack I would try to turn the cam shafts [spanner on the cam sprocket] We don't need to wiggle it far to determine the cams will turn a bit. any rotation either way tells us the top of the motor is not the culprit inc the myriad of cogs/wheels/guides n the top timing chain entourage.
Now try turning the crank pulley a smidgeon..to date it has been locked solid but if you can now get a bit of action it would indicate something is binding in the timing chain department.
Report in!
This will be today's project, along with more thorough borescope exam. 
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 09 August 2019, 08:07 AM
Transmission in neutral? Though even in park I think you should still be able to turn crank.
Yes, in neutral.

Total oil in pan I would think would be about 10 quarts with probably 2 left in the filter and cooler.
Seemed like more than 10 quarts came out, more like 12.  Hard to measure exactly, but my 5 gallon oil drain tank was easily 2/3 full.

Random ideas:
can you wiggle each cam a few degrees? Grab with vice grips to see if they each move at least a little bit.  If it moves and all your upper valve train looks normal -no loose rockers -  then that would reduce odds in that area.  You may need to remove chain tensioner to allow a little chain slack.
Yes, Randys01 suggested similar.  Good suggestion, gentlemen.

Have you run the scope in the oil pan?
Since this is going in from the bottom I guess that's an automotive colonoscopy.  Will perform procedure today. 

All this now has me wondering about a different path.  I'm picking up the beater 6.9 early next week.  Then I could conceivably do an engine/tranny swap. Is a USA engine a simple bolt-in on a Euro chassis?  I think there are significant differences in how the oil and transmission cooling is set up.  AC might also be an issue.  I'd like to preserve the euro AC which is nicely charged with Freon 11.   USA engine would also need to be desmogged too I guess.  Anyway, I'd then have a running car to play with while the weather's still nice and sort out the original Euro engine in the meantime.  Maybe even rebuild to it's former Euro greatness.   It would also be an opportunity to redo some other stuff on the car that would otherwise be harder, like engine mounts.  This might sound kind of nuts but at some point the cumulative time & effort messing with the seized engine might exceed the effort of swapping.  And I'd eventually be pulling the donor car's engine anyway.  I haven't yet spent any time exploring this option.   Hmmmmm.
Title: Re: 6.9 engine seized while cranking
Post by: rumb on 09 August 2019, 11:37 AM
I think you are best with keeping/building a Euro engine. More horsepower from different pistons and no smog.  Additionally with your euro engine you can get if you dont already have an permanent EPA waiver. Putting smog crap on will only anger your emissions department and they might demand cats which have a different floor pan to make room for them. AFAIK the heads are the same. You could build a US engine to look like a euro from the outside.

I dont remember if the parts car is euro or not, but I have a spare euro engine that parts can be obtained from if needed.
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 09 August 2019, 01:56 PM
More info!  Loosening the chain tensioner gave enough slack to move the cams a bit - they aren't frozen.  However, the chain slack did NOT translate to the crankshaft.  It"s still stuck.

I tried to do the sump colonoscopy but failed.  The 90deg mirror on the scope is crap, crappy reflective surface, so the LED illumination is diffused and totally clouds the image.  Terrible design.

I'm not sure where this leaves me, but I doubt it's good.  I guess it means either drop the sump or pull the engine.  I'm somewhat inclined to pull engine, especially if I can do it but still leave AC compressor etc in place.  Seems like there are various possibilities that would require pulling the engine anyway, so might as well bite the bullet.  If the concensus is "yes, pull engine," then I'll start another thread on that.  I expect I'll need so guidance.  :-\

Further thoughts welcome!  Thx,
Title: Re: 6.9 engine seized while cranking
Post by: Randys01 on 10 August 2019, 05:30 PM
We gave it our best shot and whilst we didn't determine a positive diagnosis we eliminated all the soft options thus justifying a major project pulldown.  Good luck.   I have read the new thread.......
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 10 September 2019, 07:47 PM
Mystery solved!  The problem was exactly as some of you surmised.  See picture.  The crank counterweight is jammed against piston #8.   I'm amazed at how little clearance there is so, it didn't take much of a bend to jam it up.  The rest of the rods look fine, at least visually.  No metal flakes, chunks, or any other obvious problems.  The sump was surprisingly clean.  It doesn't look like the failure event took anything else with it, like broken valves.  Still, any thoughts would be welcome.  For example - new vs used rod/piston?  Special tricks or things to be aware of?

It would also be great to get any comments or views on how far to take this now that the engine is out.  Barring any surprises my inclination is to replace the rod, piston and bearing and call it a day.  I'll do the timing chain too as I know it has a little stretch.  I also discovered an exhaust manifold crack, though it doesn't seem to actually leak.  Nevertheless, I'll replace manifold too.  What else?  Others might say redo the whole bottom end etc etc, so discussion of pros/cons would be welcome.  By way of context this engine didn't ever smoke, rattle or have any other fundamental problems that I know of.  My work on the car has mostly been along the lines of making sure it's working 100% and not compromised by WUR problems and other small stuff like that.  Of course that's what got me into this whole mess. :-[ 

At this point I'm actually pretty excited.  The job seems do-able and created an opportunity to do lots of other stuff that I'd probably have ignored, like those oil hoses, resealing AC, etc.  Thanks to all for the ongoing support and help on this thing. Cheers,
Title: Re: 6.9 engine seized while cranking
Post by: daantjie on 10 September 2019, 10:50 PM
So can we conclusively say this is as a result of hydro lock?
Title: Re: 6.9 engine seized while cranking
Post by: Randys01 on 11 September 2019, 01:22 AM
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?
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 am amazed at how clean that engine looks and in partic 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.

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.
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.
Title: Re: 6.9 engine seized while cranking
Post by: daantjie on 11 September 2019, 02:53 AM
I was also noticing the cleanliness, especially the piston for sure looks new - ish.
Title: Re: 6.9 engine seized while cranking
Post by: rumb on 11 September 2019, 08:14 AM
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.
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 11 September 2019, 09:54 AM
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. :o  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,
Title: Re: 6.9 engine seized while cranking
Post by: oversize on 13 September 2019, 02:26 AM
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!
Title: Re: 6.9 engine seized while cranking
Post by: Randys01 on 13 September 2019, 05:35 PM
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.
Title: Re: 6.9 engine seized while cranking
Post by: daantjie on 13 September 2019, 06:03 PM
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 ;)
Title: Re: 6.9 engine seized while cranking
Post by: Randys01 on 14 September 2019, 01:11 AM
Well done Daniel. I can't believe it.  ;D
 That hopefully deals with the King kong problem of the 2 unicorn horns!
Title: Re: 6.9 engine seized while cranking
Post by: rumb on 14 September 2019, 07:18 AM
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.
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 14 September 2019, 10:41 AM
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,
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 20 September 2019, 04:35 PM
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,
Title: Re: 6.9 engine seized while cranking
Post by: UTn_boy on 21 September 2019, 03:07 AM
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. 
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 21 September 2019, 09:24 AM
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.   :P

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...." 
---------------
Thanks again.  I'll certainly be posting more on developments, progress and setbacks.  Cheers,
Title: Re: 6.9 engine seized while cranking
Post by: daantjie on 21 September 2019, 12:59 PM
Dave, some food for thought, use it don't use it of course ;D

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 :-\
Title: Re: 6.9 engine seized while cranking
Post by: s class on 22 September 2019, 04:53 AM
Dave, some food for thought, use it don't use it of course ;D

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.
Title: Re: 6.9 engine seized while cranking
Post by: s class on 22 September 2019, 04:58 AM
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.
Title: Re: 6.9 engine seized while cranking
Post by: s class 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.
Title: Re: 6.9 engine seized while cranking
Post by: s class 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.
Title: Re: 6.9 engine seized while cranking
Post by: UTn_boy on 22 September 2019, 10:12 AM
Dave, I meant connecting rod bearings only.  They come in a set.  They were never available individually. 
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 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,
Title: Re: 6.9 engine seized while cranking
Post by: rumb 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.
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 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.
Title: Re: 6.9 engine seized while cranking
Post by: s class 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
Title: Re: 6.9 engine seized while cranking
Post by: rumb 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.

Title: Re: 6.9 engine seized while cranking
Post by: daantjie 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
Title: Re: 6.9 engine seized while cranking
Post by: Randys01 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

Title: Re: 6.9 engine seized while cranking
Post by: raueda1 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 🍻.
Title: Re: 6.9 engine seized while cranking
Post by: p. rex 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.
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 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.   ::)
Title: Re: 6.9 engine seized while cranking
Post by: Jed on 23 September 2019, 06:32 PM
"Looks like you had hydraulic lock."

there is no substitute for experience

thanks for sharing...great read

Title: Re: 6.9 engine seized while cranking
Post by: Randys01 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.
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 27 September 2019, 06:36 PM
Update:

The engine is now pretty well stripped apart from removing the crank.  Toady I made an interesting discovery after pulling the pistons for carbon removal and inspection.  The engine has indeed been rebuilt.  Maybe that's why it was so clean, though clearly it wasn't all that recent given the carbon on the pistons. 

The pistons are +0.5 oversize, code 75.  This corresponds to 8.4:1 compression instead of 8.8:1.  That makes it a demi-Euro engine in my mind, so I'm annoyed.  The main difference between the versions is mostly the depth of the cavity on top.  From what I make of the specs it looks like MB progressively reduced the euro engine's compression over time.  I wonder why.  Too many knocking issues?  Declining fuel quality at the time?  Who knows.  Maybe somebody can cheer me up with news that 8.4 is actually pretty close to 8.8, so no big deal.  ::) 

Haven't checked bearing journals to see if they messed with that (need a suitable micrometer).  There was an old mouse nest in the V between the heads and under the intake plenum.  Slowly progressing.......              Cheers,
Title: Re: 6.9 engine seized while cranking
Post by: Randys01 on 28 September 2019, 01:22 AM
...not surprised....I thought the minimal scuff on the piston skirts suggested low mileage= rebuild.
When you pop the big end shells out you will see on the back if they are oversized or stds.
The 6.9 variant of the M100 saw dramatic modifications compared with the 6.3....As I recall there are 4 compress ratios and some of these also saw a change in gudgeon offset.
In Aust we got the so called 8 :1.  Introduct Down-under was towards the end of production. A mate of mine has a lively Euro import aka 9:1.no smog etc. Seat of the pants, there's nothing between them but I reckon I reckon mine goes very well for its spec. [ I have spent hours finessing it ]
So 8.4 or 8.8...if there ever was any discernible difference, the gap narrows as the old dears start to wear.
Title: Re: 6.9 engine seized while cranking
Post by: raueda1 on 28 September 2019, 09:01 AM
...not surprised....I thought the minimal scuff on the piston skirts suggested low mileage= rebuild.
When you pop the big end shells out you will see on the back if they are oversized or stds.
The 6.9 variant of the M100 saw dramatic modifications compared with the 6.3....As I recall there are 4 compress ratios and some of these also saw a change in gudgeon offset.
In Aust we got the so called 8 :1.  Introduct Down-under was towards the end of production. A mate of mine has a lively Euro import aka 9:1.no smog etc. Seat of the pants, there's nothing between them but I reckon I reckon mine goes very well for its spec. [ I have spent hours finessing it ]
So 8.4 or 8.8...if there ever was any discernible difference, the gap narrows as the old dears start to wear.
Hmmm.  I can't make heads nor tails of the bearing designation "100 3110 KZ".  However, they're also stamped 11-75 which, per Mahle's system, suggests that they were manufactured in Nov 1975.  If so then perhaps they're original.  The car is #923 and was built in April 1976 but first licensed in the USA in 1977.  Engine is also original, matches the build card.  So, this chronology seems to make sense.   And if so, that suggests that somebody did a piston job at some point but left the big ends alone. 

Or maybe not?  I guess there's the possibility that they are oversized and were on the shelf for some time before the piston job.  The bearings are all in good shape.  The shop guy actually commented on how good they looked.  The other pistons all look good as well.  Obviously the piston job was done when pistons were still available - when was that?  Given low wear I'm guessing that the car got little use after that job.  Now I wonder what necessitated piston replacement.  I can't wait to see what surprises are lurking with the heads!  ::)

Well, all this stuff is fun I guess, but doesn't really impact the task at hand.  I'll measure the crank soon enough.  In the meantime there's a lot to inspect, clean, carbon to remove, stuff to repaint, plate, reassemble etc. 
Cheers,