Bore and stroke are both slightly larger, and longer, respectively

AR03.10-P-6111AW repairing connecting rod.

AR01.30-P-5800HX Remove/install cylinder head.

AR01.40-P-9291AW Boring and honing cylinder bores.

AR01.40-P-9273AW Installing cylinder liner.

AR01.40-P-9202AW Measure cylinder bores.

AR01.40-P-9272AW Remove cylinder liner

AR01.40-P-9271AW Widening cylinder bore in crankcase.

RA05.00-6-0103-41X Removal and installation of the guiding rails.

I really appreciate the input from all of you very much. It is unfortunate that Mercedes-Benz has taken the dishonorable decision not to admit to an obvious design and/or fabrication error. Designing the connecting rod would be a challenge because you want it to be light and tough and it is fatigue loaded. But to pretend there is no problem seems wrong.

Now one thing I forgot to mention is that Jack's wife is rather hefty and he takes her everywhere. She weighs about the difference between a W126 SDL and W140 SD. Some people think the extra weight is a factor. I hate to be the one to suggest it but...........could Jack's wife be the rod bender here?

I am a hair's breadth from buying the car myself just to see what is up with it? But please, if you have any more insight, I will aprpeciate the benefit of your experience. I promise I will report back when Jack's car is properly sorted out.

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JMF

His wifes additional weight percentagewise overall is of little consequence in my opinion. What may have aggrivated this problem could be pushing the car harder than the last owner did or just luck of the draw.
Good old coincidence in action. The 3.5 can really accelerate if you come onto it. Something to see in a good example. Just get it moving a little first. The head cracking with car not showing any signs of overheating is a possibility. If no overheating observed the head gasket gains in favour. These are just my opinions though. Others will and should vary. Please post whatever it is you find out. Again most the 3.5s give a clear signal the rod is bending or has bent over time.

I had one with a broken head gasket, and cracked head because of overheating. (broken hose) rods were fine.
I also saw one that had the 10mm bolts that hold some plate under the crank come loose. the bolts swirled around in the pan until one managed to get through the screen. it went into the oil pump and busted a hole in it. engine lost oil pressure, rod bearings didn't get proper lubrication, engine failed. rods were still fine...

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1964 datsun 1500
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this is where I quote some dude, so I look smart because of quoting that dude that said something smart once

Indicates that some of the rod bender engines were caused by the EGR valve. Little balls of tar like substance are formed with an active EGR valve and any leakage of oil from the turbo. Also mentioned is any pump back into the intake of unburned diesel fuel. Anyway these small tar balls apparently go/went into the cylinder to be lodged between head and piston resulting in bent rods. Sometimes allowing antifreeze/coolent into the cylinder also due to head lifting/cracking. IF so, how is Ma Benz gonna piss and ***** about a mandated "green" device mandated by the government?

This is all conjecture at this point, but a good precaution would be to inactivate the EGR valve via a plug in the line to that device.

Regards

Run-Em

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1983 300SD - SPARKY THE DIESEL
2004 Audi TT 3.2 Quattro Roadster
1999 SLK - aka - LIL RED
1956 Chevy Nomad - aka - REAL MAD
1940 Ford dbl dr. Deluxe - aka - ORANGE U GLAD
1998 Kawasaki Nomad bike --

I honestly feel like the W123's were the last cars produced by Mercedes Benz that were built to a standard and not a price. It seems like even though these cars retained their reputation, they became more stylish than engineered. I can't imagine the old Mercedes Benz company not doing their research and homework and letting an obvious flaw get mass produced like this!

Nah the W140 is built better, they spent $1B and 8 years of R&D on that one. Lost money on every one they sold until the mid 90's. MB expanded the 603 and it didn't work well, I guess it was a stop gap until the 606 came out. However all the other engines the W140 was fitted with are supurb in all respects.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

The 606 was out before the midpoint of the 3.5's life. The turbo 606 powered the W140's outside the USA.

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The rods are Fine. Its not the rods fault. This problem is caused by hydrolock. Any rod will bend under that condition. Fix the reason its hydrolocking, and you dont have to build a new rod.

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Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?

As long as they would add one additional commandment for you to keep thy religion to thyself.
George Carlin (Wonder where he is now..)

1981 240d (engine donor 1983 240d) recently rebuilt engine hurray! - No more.. fought a tree and the tree won.

pearl black 1983 240d 4speed (Converted!@$$%) atleast the tranny was rebuilt.

The older 300SD/SDL had heavier connecting rods and no such problem. The post '95 350 had no problem. Why? Stronger connecting rods.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

I am a proud owner of a 1991 350SD. In my opinion it is the last car that looks like a real Mercedes. It is rare with only 2888 made in 1991. The fact that some of the original engines had problems makes them even more rare.

My car is in very good condition and has a lot more power and better fuel economy than the earlier 300SD. My mechanic drives a 300SD that is cherry. After working on my car and taking it for several test drives he said "This is the best 126 I have ever seen. I wish it was my car."

Some 350s may had problems. I have done what I can to eliminate them. It is a 17 year old car and people still ask me if Mercedes has come out with a new retro model.

So, next time you pull up next to me with your 300 I will let you smell my 100% biodiesel exhaust as I pull away from you effortlessly.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

When you compare the pressure at full-throttle/boost on the piston to what it would take to crush a small ball of tar, doesn't make much sense.

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Gone to the dark side

- Jeff

Mercedes has upgraded rods for these engines, I priced them for $200.00 @ rod.

The stress/strain on a rod during operation of a diesel engine are pretty well known. By the '90s, good modal finite-element packages had become available in P-Method analysis, very reliable for determining the design, material, hardness/temper etc. of the rods. I find it hard to believe that the design of the rod was inadequate for use in this engine. It is possible that something such as a resonant frequency that created repeated osclilations in the rod during operation lead to their fatigue and was not predicted, but again IMO unlikely.

I'm not trying to punch holes in anyone's theory here, but being in on the process of designing and engineering parts for automobiles (including Mercedes-Benz) for over 20years there are some things that we go through each time iinvolving analyzing the design, including likely process variations, and creating a design and process that can operate in the worst-case tolerance stack-up. This includes everything from the clips holding vacuum lines to the brake calipers. Critical parts (determined by good sense and FMEAs) are closely controlled and often real-time X-rayed in the production.

It seems that, in theory, if the rods yeild due to fatigue, and being that it is nowhere near 100% failure rate, it cannot be a design failure or it would affect 100% (all being of the same design). The attrition being more random and a smaller percent indicates to me that it was a process variation, possibly in the material, or in the heat-treatment process, or even in hand-finishing of the part, or outside influence (hydrolock and rock theories for example).

If it were a hydrolock, IMO again, it would have other indicators. If it were a hydrolock due to a head gasket etc., are we to believe that it occurred during the cranking/starting phase? If it were, is the starter capable of creating enough force to yield the rod(s) on top of the infinite compression in that cylinder that would precede impact with the liquid or would the high-compression slow and stall the starter? Would the rod be the failure mode of a hydrolocked cylinder? I've had this problem in a gas engine, cracked pistons, didn't hurt the rods, high-torque starter in a high-performance engine (jet boat).Different engine design, but the question remains and we haven't done any analysis, anyone want to donate a bad 3.5 so we can hydrolock a cylinder and crank it to see what fails?

If the hydrolock were to occur during running, it seems it would require a lot of liquid to enter the cylinder in a very short period of time. Again, possible? If such a substantial breach were in the gasket wouldn't the car be belching white (steam) smoke from the exhaust?

Are there any other possibilities? If we had a couple of "bent" rods, a couple of good ones from the same engine, could do X-ray and metallurgical analysis and compare it to the original Engineering specifications, could run a couple of 3.5s with bad head gaskets etc. we might be able to solve it. I've spent many hours trying to fail parts in a quality lab until the lab techs were tired of me, I know how hard it can be to reproduce a failure in a lab envronment, even harder on the internet.

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Gone to the dark side

- Jeff