Howdy I have ben lurking for a while and want to thank everybody for the great help and info. Hope to be able to return the favor. I own and operate a heavy duty truck shop for 27 years and have rebuilt hundreds of diesel engines but new to mb. my 3.5 is running great.but? Are there any replacement rods that I can install now and head off the problem? Thanks again.

Did you use the search feature for "bent rods"? There seems to be no consensus on why this happens but regardless, the factory is supposed to have stronger rods but they never officially admitted the problem and I don't believe there is a distinct p/n for an "improved" rod.
Someone said that they thought the pistons get a layer of carbon on them from leaky turbo oil seals (or worn turbo bearings?) and that reduced the volume raising the compression ratio or something?
cyl #1 has intake that is positioned to allow more crud or water to pass if the headgasket leaks, etc.
Lots of theories on this.
I would use the car and listen carefully to any noises, etc. that might indicate something is about to "happen" and catch it before it does any serious damage, I figure most people that have had this problem were not carefully attuned to their engine, like who would expect the engine in a $50K car to just blow up suddenly for no reason?

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

there is not a consensus for why the rods get bent. but they do.

i am partial to the idea that the 3.5 liter motor is bored out so much that the block flexes too much causing head gasket sealing problems. this in turn causes partial hydro lock and bends the rod slightly.

another angle that i find persuasive is the idea that the carbon buid up on the pistons can finally bend a rod. though this would seem to hold true for the 3.0 liter engines too and they hardly ever seem to bend rods.

the replacement benz factory engines supposidly have stronger rods, but several folks here have experienced bent rods with them, so.....

the third theory expressed here is metal fatigue. for me it would seem that they would fail in tension rather than bend if fatigue were the cause.

the consensus is to run appropriately weighted synthetic oil, clean the crossover from time to time and keep your oil clean.

for my 3.5, i am keeping my fingers crossed and looking around for a 3.0 liter block to have on hand if my rod bends. if it does i would put the (rebuilt)block under my fresh head.

tom w

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[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.

Thanks for the info, will do the search. Makes me think about the the service truck at the shop.1991 f 350 7.3 . Treated allful( change oil when we think about it -that type of thing. 478,000 miles .The valve covers have never ben off. Would like the same life with the MB,but doubt it.

The rod available now are the 'improved' ones. I don't have the part numbers but the 'old' ones are not available. I remember seeing a side by side picture of old rod next to 'improved' rod and there was a visual difference. I did some searching before I bought my 3.5 and like the above posts, there is no definitive answer as to why they bend.

I have seen a 91 3.5 sdl w/ 400+K miles on original rods...still running strong.

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82 300D....went to MB heaven
90 350 SDL....excercising con rods

Thank, Makes Me Feel Better

Dealer will be able to tell you the updated part numbers, they also will have how many times the part was updated on their screen. With some parts like the engine vac pump for example their are like 3-4 part numbers meaning the part was changed several times.


Personaly if it is running fine I'd just drive it. Wait for it to blow and swap in a short block. It may never blow why waste the time and money now?

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

Good Idea. I Like Your Way Of Thinking--if It Ain't Broke,don't Fix It.

I beleive in preventive repairs and maintaince, but to a point. Yes it has been proven that a certian percentage of these engines will bend a rod. But no one has proven what percentage or even what causes a rod to bend. So why fix a problem that may never be a problem?

I have an original "14" head on my 603, these are known to be prone to cracking if overheated. I am not going to change the head because it may crack someday. If it does crack at that point I will deal with it. Until then I will just drive it and forget about it.

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