I currently own a 1990 420sel but I've always loved the turbo diesel Mercedes(the sound is what got me). I would like to either buy a 300sdl or 350sdl. I've heard of the head warpage on on 350s and connecting rods bending. But I think I've heard certain 350sdl engines were ok depending on the number on the head or engine or something like that. Could someone just inform me a little more on what the pros and cons are of each or what to look out for and possibly which years were the best. Just stuff like that. Thank you all.

Personally I would go with a nice, well maintained '86-'87 300SDL.

Neither. I've been very fortunate to have dodged the bullet of ever having owned either. What history has taught us, is that the cost of ownership to keep one in repair and on the road is so great on average, that the risk is outsized to any benefit.

As usual you have no idea what you are talking about. Why don't you tell us the story about how you bought an E300 new, or how you used to own a motor-coach. You've had good luck with the what, three benzes you've had? Good for you, recommend those cars. You answered none of the OPs questions and brought nothing to the conversation. Solid work.

To the OP.

You've got the problems mixed up. The 1986-1987 300SDL has the #14 head which was prone to cracking. The 1990-1991 350SDL was known for rod defects but always had the later head castings which addressed much of the cracking issues. However with ANY aluminum head, if you overheat the car, you need to have it checked for flatness.

I have a 1987 300D W124 with the same drivetrain as an 86-87 300SDL. I have records for the car since new. At 255k miles it has the original #14 head and the original headgasket under it. I try to change the anti-freeze every other year with genuine MB coolant and distilled water.

I used to own a 1991 350SDL with the original shortblock. It did not consume oil and ran great, it had 135k miles on it when I sold it. Not all 3.5s bent, the opinion of most is, if it hasn't bent and its over 100k, it's more than likely not going to.

Things to look for:
A decent number of 350SD/Ls got factory replacement shortblocks with revised thicker rods. These do not bend, and they will have the later casting heads the 3.5 was built with, making for a very durable motor. So if you find a 3.5 where the owner has service records indicating the motor was replaced and you can find the tag on the block, you are solid.

The 3.0L shortblock from an 86-87 300SDL or 300D is a fairly direct swap into a 90-91 350SL/L. You can take the later revised head off of your bent 3.5, put it on your 3.0L and have a great motor. A friend has a 1990 350SDL and we did this for him. He bought a 350 consuming lots of oil and we put in a 1987 300SDL shortblock. Car runs and drives fantastic. I think DIY the bill was around $1,000 for the motor, machine shop checking the heads, gaskets, bolts etc. Selling the spare head, spare IP, and turbo on ebay made it a new $0.00 cost, other than our labor.

The 3.5 later revised heads will fit onto the 3.0L block, so if you bought an 86-87 300SDL with a cracked head you could get a later head, put it on, and have a great motor.

The #14 head issue on the 3.0ls is really overblown in my opinion. As someones sig used to say (Babymog?) turning your nose up at the 603 over the #14 is like turning Cindy Crawford down because of her mole.

If the temp gauge goes up, turn the car off. Pretty simple. Lots of people used to the "good 'ole days" of all iron motors will try to make it the last few miles home with the temp gauge in the red, then get on the forum the next morning and post about how Mercedes motors are "junk" because the head cracked.

I've autocrossed and tracked my 87 300D on a #14 and it hasn't cracked yet.

Between the 300SDL and the 350SDL, I am torn. I prefer the refinements on the later 2nd gen W126. I like the interior in the 89+ cars. However they are both great cars and you will find many members with hundreds of thousands of miles on 603 powered W126s. I really liked my 1991 350SDL.


Lots of resources on this forum if you move forward. There are threads on how to find the factory build plate on the revised 3.5 blocks, how to read the casting numbers on the heads, and how to deal with inclined vs. non-inclined injectors if you are going to do a head swap.

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68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i

I think you've proven my point rather well. In several ways.

Actually, my family has owned many diesels over the past 50 years. Most all bought new, BTW. So, yes, I'm a die hard diesel owner.

I provided the information the OP was looking for, you just came here to attempt to reassure yourself over the validity of your purchases. As usual, offering nothing.

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68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i

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Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

Thank you very much DieselPaul for all the information. I was very unsure about either model but you seem very reputable. Now I know I can go with either model. I will probably try to find a 300sdl

The SDL is the better choice if push comes to shove. As long as you know the pitfalls that both have encumbered, (as I later learned,) you'll be an informed shopper or buyer.

I skimmed past all the childish bull****, so excuse me if it was covered.



It would depend if you would like the updated interior as the main choice. I do but it is not a 'MUST HAVE' for me. The powerband is a bit narrower on the 3.5 from what I recall, but not much.

I have had my SDL for ten years now. 3L bottom ends are all over the place if the 3.5 craps out (just saw a car in the JY that was Astral silver on blue with a failed HG, owner junked it... crosshatching still in the bores. Sad). But the question is... are you able to do the swap yourself? If not, you are better off in a 3L car.

The early head can crack if overheated, gee... what head does not have that propensity if overheated? The aluminum head to iron block will require a head gasket or two during its life. Mine needs another, PO did first...poorly, and I am going with a No 17 cast off a 3.5 car that I have here. Is it a requirement? No, my No 14 is fine. Just will wrap that up as a spare.

The 603 drives sooooo different than the 617, it is really an amazing difference considering both are IDI.



FWIW, the 420 is the best deal on second gen gassers to me. A bit better on fuel economy than the 560, no SLS to deal with as on the 560, and a bunch more torque at a more useable range over the 300SEL (which is a dog, silly that motor was offered in the LWB).

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

At this point just buy whichever is in the best shape, any engine that makes the 20 year mark is a good motor.

IMHO having owned a 300SD and a 300SDL if I were to go diesel W126 shopping again I'd hunt for a 350SD. I think the updated W126 is a better car, so I'd want a post 86 example, or better yet a post 89. I also like how the SWB cars drive around town.


The head problems on the 3L 603 are greatly overblown on this forum, and have been for years. IMHO the 602/603/606 are some of the best diesel motors anyone has made, they really are very good you will enjoy a properly tuned one.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200