Originally Posted by DieselPaul

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.