I had a 300 SDL that caused me endless grief with head problems. I am looking to get another and was looking at a 350SDL. Then I started picking up info that this motor was BAD. What do you guys think. A 300 or a 350??

Welcome to the forum.

There is quite a bit of info in the archives on the 603 engine in the 350 SDL.

In summary, some members have the vehicle and they love it.

Others have owned it and have suffered bent connecting rods. This then requires a replacement of the block to the tune of about $8,000.

So, it's a bit of a roll of the dice. If the rods stay together, it's a fine vehicle. If they don't, and you will not know they they don't until the damage is done, then you are looking at a serious expense.

The smaller 3.0 engine in the '86-'87 SDL does have issues with cracking of cylinder heads in the early versions of the casting. Again, some crack, some don't. The cost to replace one, however, is much less than replacing the entire 3.5 engine in the 350 SDL.

You can read all about the 603 engine in the 350 SDL. Do a search with the title "rod bender". You will need a couple of hours to enjoy it all. It isn't pretty.

Is there a certain mileage on the 350 when you can safely assume it probably won't bend rods? I mean, if the car has 100 or 150K on it, would it have already happened if it was going to happen?

I love my OM617, but the increased performance of the 603 intrigues me.

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1982 300SD

It's a statistical thing with high cycle fatigue in the rods. If you happen to get a full set of six rods that are all above the bottom 20% (guessing) for fatigue life, then they will, effectively, last quite a long time.

But, if one of the rods, statistically, is in the bottom 20% for fatigue life, then it will probably fail during the life of the body (say 300K).

There certainly is no mileage whereby they will fail and then, above this level, you are home free. In fact, the opposite is the case. If you were to drive the original rods for 1M miles, the failure rate would be much higher than it currently is.

Fatigue happens due to a specific load occurring over a certain number of cycles. A properly designed rod should have, effectively, an infinite fatigue life, because the load is kept low enough that fatigue is not an issue.
But, in the case of these rods, no other explanation makes any sense other than random failures due to high cycle fatigue.

Most rotating components of jet engines must be removed and scrapped after 20,000 cycles of use to prevent any possibility of failure due to high cycle fatigue. The calculated fatigue life is 60,000 cycles, but, fatigue is a tricky business, and, it's very possible that one part out of 1,000 may drop down close to the 20,000 limit. Of course, with a jet engine, there can be no possibility of a fatigue failure under any circumstances, so, the life rating is very conservative. Probably 1/2 the components removed and scrapped would go another 40,000 cycles, but, nobody can say for sure.