I'd like to know if the design of the engine with additional stroke, increasing the angle of the rod to the bore at mid-stroke, required a compromise between the width of the rod and the lower placement of the wrist-pin simply because of rod-to-block clearance at that midpoint. If so, the wrist pin being lower causing a shorter than ideal rod and increasing further the angle at midpoint, increasing the mechanical advantage of the compression & combustion pressure on the rod, increased cylinder side-loads, not enough space to thicken the rods, ... I've been in this Engineering corner before, ... maybe M-B Engineers just reduced the normal safety factor in the design to make it fit and goofed?

__________________

Gone to the dark side

- Jeff

It seems unlikely that the best engineers in the world goofed on something so important and fundamental as a connecting rod.

A non designed event in the combustion chamber or a foreign object remains the only persuasive answer to me.

Tom W

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

...........so they redesigned it with, presumably, more strength and fatigue resistance just as an exercise because they had nothing better to do with their time.............??

Look at the facts............nothing "unlikely" about it..............

But, not to be a pain, I went and researched the metallurgical events leading to a "fatigue failure" in the classical sense of the terminology. Without a notch or other stress riser in a part free of defects, before there is a significant crack formed the grain structure in the area that is going to fail, meaning break in two, there are changes that weaken the area and eventually form a crack. My postulate is that this phenomena allows the rod to bend before a crack forms, which then lowers the stress and takes the assembly out of the operating realm likely to cause the rod to break into two parts. There is also, depending on the steel alloy, the chance that bending acts to cold work the part in the area of the stress, and that also prevents further damage at the lower stress levels.

I have had direct experience with nickel based alloys that make them near impossible to work with using procedures for pipe bending, machining or forging developed for "other" materials. They work harden and you cannot imagine the changes. This is an alloy dependent characteristic though and I don't know what MB uses for conrods.

I don't believe the only result of fatigue that is possible is two parts where there once was only one. It isn't like there isn't a per cycle cumulative "remembered" adverse effect on the part, and to my knowledge none of these parts includes any inherent way to count and track the effects of each cycle, then add them up so when they reach a specific number they just suddenly go from perfect part to two parts. Along the way in our engines some of these rods bend.

Remember, your definition of fatigue failure is a broken in half part. That is a statistically predicted event. In reality there is more going on than just waiting for the part to break in half. I also consider it possible the initial bending takes place before the endurance limit is reached and the effect of this bending manifests at the mileage where other operating and maintenance evolutions make them inevitable. Meaning people who were not really concerned with regular oil changes, used improper oil grades, ran hot, etc. saw earlier failures, with failures defined as the onset of serious oil consumption and the characteristic loping idle. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

So far nobody has posted a picture of the "weak" rods and the "strong" rods nor a bent rod. Nobody has posted any direct evidence that supports fatigue either.

I remain unconvinced in the face of no evidence to support fatigue other than "strong rods" that none of us has any facts about.

The theory about a detonation event causing more pressure than designed for could, for that matter, result in a stronger rod as well.

Tom W

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

So, without a photo..........you fail to believe that they were redesigned by the factory.........??

... yet it's likely that they goofed on the early 602 head, the early 603 head, the early 602 & 603 head gaskets and subsequently redesigned them to remedy the flaw, ... puleeeze! It happens, and connecting rod designs can fail, so can the specified production process, and sometimes the design doesn't adequately consider real-world production limitations and tolerance stack-up.


BTW, one of the things I've been responsible for is "cost of quality" analyses. This is where you identify a possible failure, determine the probability, quantity, distribution, and effect of that failure, and decide if it is more costly to design out the failure or to accept the numbers. We usually looked for zero defects, and about 4 parts per million failures can get you tossed out of an automotive assembly plant, but there are times that a failure is a statistical probability and the failure mode is acceptable. Not all failures are Engineering oversights, some are expected. I don't feel that the connecting rod failure is expected, but understand that in the real world with all of the variables in alloys, forging, tooling, assembly, machining, heat treating, etc., etc. there just might be an anomaly that created weak connecting rods. A re-design could be the reaction to the stackup of these variables and a more robust design to keep this stackup from creating failing rods. Or to put it another way, the rods might have been designed to be strong enough in a perfect world, but with production variations in the real world they were inadequate, thus the redesign. Just a thought.

__________________

Gone to the dark side

- Jeff

>>But, not to be a pain,

Thanks Jim, until you posted that, I didn't really understand your position.

The local strain effects which are going on when a crack is being intitiated are a part of the fatigue process that I have never been too involved with - there's nothing much to see or measure without extremely elaborate equipment which I've never had access to.

I'm not sure that he bending will actually work to change the stress regime and protect an area of the rod. I think there are two reasons for this - 1) the fluctuating stress would remain unchanged 2) as bending is an anti-symettrical deflection, any area that could claim a benefit would have a counterpart on the opposite side of the rod where there would be a detrimental change.

I've had a few dealings with materials like Inconel - I was involved in a project with Rolls-Royce to develop tooling and processes to enable the material to be ground at high material removal rates. Awful stuff!

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

__________________
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.

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

Seeing may be believing.

Not seeing does not help.

Tom W

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

The rods on my 1991 300SD engine that bent were found by removing the head and measuring the difference in height at TDC for each piston, relative to the block head gasket seating surface. In my experience, I was so startled by the diagnosis I asked to see the engine. Still in the car (they only took the head off to do the diagnosis) the difference in height was visually apparent. I don't believe the shop actually measured the height differences, because the condition was so obvious. Which I will guess was more than 0.015" short on the two pistons with bent rods.

That kind of difference will reduce compression loading significantly. Which should reduce the loading on the rod and possibly push the load point away from the fatigue limit, putting an end to the conditions that caused the rod to bend. The work hardening aspect may or may not contribute to pushing the actual operating loads further from the material's endurance limit.

The other consideration is we don't really know when the actual bending of the rod takes place. I think it is possible it happens earlier in life than we suspect and the symptom we detect is the oval shaped cylinder, the resulting high oil consumption and performance degradation (noisy, loping idle, and smoking as it burns oil, for the most part). A bent rod by itself won't smoke or cause the loping idle problems. How long between the bending of the rod, and the onset of noticed smoking and idle changes may well be dependent on other regular maintenance intervals and quality of oil.

And Tom, I have seen a side by side photo, on this forum I believe, of the two rod designs. Unfortunately I think they went through more than one rod design change and have never seen all the iterations side by side. The more than one changed rod design may just be a rumor, though. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

>>Which should reduce the loading on the rod and possibly push the load point away from the fatigue limit,

I'm not so sure about this bit of your logic Jim.

I would say that reduced compressions will reduce the temperature in the cylinder when the fuel is injected. This will increase the delay period and make the sudden uncontrolled stage of burning more violent. So, although you have reduced the compression loading, you've probably significantly increased the peak load you get from irregular combustion (re nailling).

Seems to me the conditions you describe can occur in a relatively precise set of circumstances only. The reduction in compression would have to fall in some limit preclude the uncontrolled stage of burning to be destructive, as you describe. If I understand you correctly, you are still calling for the delay to be less than the time it takes to get to TDC after injection. Otherwise the temperatures will be further lowered and there will be no real ignition.

This does seem to be condition that could be brought on by poor fuel quality, poor injector nozzle spray patterns and faulty injection timing due to chain stretch as the engine ages. All in an engine without bent rods.

It also seems like a condition that would occur in the life of every Diesel, eventually, as the engine wears out, or has other failures. And it would be accompanied by nailing.

The only times I have experienced nailing I cured it with RedLine Diesel fuel additive. From my 1975 240D up to my present 1998 E300D, I use RedLine's Diesel fuel catalyst regularly. My 1975 was picked up at the factory and began nailing on my second tank of fuel. MB changed injectors, made no difference. Nailed again. Told me to get used to it, it was not a problem for the engine. Got it home and had the injectors changed again. Got better but not for long. MB at the time blamed it on fuel quality. Was introduced to RedLine in the early 1980's. No more nailing. Not even on the bent rod 350SD.

The point of all this is (a) nailing is common, damage from nailing is not
(b) the conditions to create a more hazardous environment inside the engine from what used to be normal operating conditions due to nailing is going to be sensitive to fuel quality, injection timing (chain stretch), other consequences of wear in the engine (valves, valve seats, rings, cylinder, head gasket), and how much the rod bent. All pretty independent of each other, and not likely to conspire to hit the exact conditions necessary to cause uncontrolled burning stages to be more violent than is typical for these indirect injected engines before the day of the fancy pintel design used on my 1998.

I am not an expert in Diesel engine design though, just an experienced user and reflecting on my experience as a user. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=280214920845#description
1991 MB 350 SDL 128,150 miles

".....Q: are u original owner ? has heads or engine been replaced ? thanks!
A: Hi, yes the heads were replaced and I have receipts for all the work done, the 2nd owner owns a MB repair shop in fla. He did everything on the car that was needed to be done, using MB parts only. He had the car a long time but only put about 500 miles on it, he has about 20 cars and decided to sell some of them. It drives like a dream, I can assure you that this car is just fantastic in every way, everything works! .... "

__________________
'83 SD, 2x '85 SD
You are entitled to your own opinions, you are not entitled to your own facts.