Background and History

I wanted to do an interesting project involving swapping a diesel into gas vehicle. In the research I determined that there were a limited number of potential diesel engines that could be used, and rather quickly I settled on a Mercedes for its quality, reliability and reputation. I thought about some early Jags, or some early US cars like a Firebird or Camaro, and a bunch of other possibilities that were quickly discarded. I began to think that a 70’s or 80’s Mercedes SL would be an interesting target vehicle They are definitely cool looking and it seemed somewhat logical that putting a Mercedes diesel engine into a Mercedes chassis might be a good combination.

So I began doing some research. The research indicated that it had been done, but there was scarce information at best about how, and how difficult the swap was. The consensus was that the OM617 just wouldn't fit without serious surgery and adding a hood scoop.

There were a series of posts that indicated that while there were significant problems involving any number of serious mods, required for clearance that it was theoretically doable and allegedly had been done.

OM603 swap into a W107

OM617 swap into a 107

OM603 into a 107

380SL and an OM617

SL Diesel

380SL with 240D Engine

Pictures of a 617 in a 380SL

I began to do some assessment and believed that with some combination of modified subframe, different oil pan, subframe spacers, motor mount mods, remote oil filter setup, hood scoop and a variety of other mods like driveshaft, transmission and so forth, the project could be done.

On the legal front I determined that a legal swap in California needed to meet three criteria.

    • The engine had to be of the same year or newer as the vehicle (unless it was a 75 or earlier)
    • The engine had to have all of the smog equipment that came with the original vehicle
    • The engine had to be from the same class of vehicle (car from car, light truck from light truck, etc)  

Once certified by a BAR referee (Bureau of Automotive Repair), the vehicle could be re-titled as a diesel and thereafter be exempt from any and all smog requirements.

Research showed that the rear end ratio was very close to that of the donor car, so no change in the rear end was required and the performance would be extremely similar to a 300D.

With that working assumption I began researching target and donor vehicles. I was very patient and after lots of research and waiting, found a 1985 380SL with a blown engine.  And later I found a donor vehicle, a 300D which was also a 1985 model. 

This was specifically a budget project, so expenses had to be kept to a minimum at all cost.

Preliminary Research

The first thing I did was to pull the engine on the 380SL. I also pulled the subframe, assuming that I would need to do some cutting and welding to provide clearance. Next I pulled the engine on the 300D which provided the ability to do some trial fits of the engine to the subframe.

This turned out to be rather futile, because I had no way to “level” the subframe relative to the engine. So I really couldn’t judge accurately clearances. It was painfully obvious that the big old GM R4 A/C compressor would never clear the subframe, so I removed that right away.

So I put the subframe back in the car and did a trial fit and determined that I had two major clearance issues - the oil filter housing and the steering gear and the oil pan and the subframe.

Project Phases

There are three phases to completing this kind of project.

Engine/Transmission Fitment

The secret to swapping an OM617 engine into a 107 chassis and making it fit is solving the three clearance issues - oil filter-steering box, subframe-oil pan and valve cover-hood.  And the secret to making it a “bolt-in” proposition, is to use engine support arms from an early 300D. Eliminating the need to fabricate engine supports and motor mounts was a huge savings in time, expense and most importantly complexity.

First to get clearance between the oil filter housing and the steering box, you need to get a filter housing from a 300SD that provides the proper clearance.

Second, to get clearance between the subframe and the oil pan, you need to get an upper oil pan from an early 75-78 300D, that doesn’t have the ribs.

Third, to get clearance between the valve cover and the hood, you need to drop the subframe .5” using custom spacers and longer bolts and raise the engine mounts .5” using spacers under the motor mounts.

It’s still just a bit tight so to prevent interference as subframe spacers and engine mounts begin to collapse, the secret is to fill both with urethane. This prevents the rubber from collapsing by supporting it from underneath with a material that is both soft and substantially impervious to deterioration.

Hook Up

Once the engine and transmission are bolted into proper position, it then becomes a series of tasks to hook everything up in a proper manner. I’ll explain system by system how I solved the myriad little problems and issues that come up.  It’s literally just a matter of taking one system at a time and figuring out how to make it work. I’ll explain cooling, fuel, steering, transmission, gauges, vacuum, oil, intake, exhaust, electrical, glow, shut off, etc., in order. In each case there are some little tricks that can make things really simple.

Clean Up/Phase II

The final phase involves getting the car titled and legal, as well as fixing all the things on the car that would have needed attention whether a swap was accomplished or not, detailing the engine compartment and engine, and most important fabricating a bracket for an aftermarket air conditioner compressor to provide cooling.

Summary

Using all the things I learned, this swap is within the capabilities of most do-it-yourself mechanics. It is possible to do the entire project without welding (provided you have the exhaust system modified by a shop and just use rivets on the radiator support), everything is hand tools and bolting things together. The driveshaft does not need to be changed; the transmission linkage does not need to be changed; no hood scoop is needed, and the only fabrication required is the subframe spacers, the engine mount spacers, the radiator support and the oil lines.

Specialty tools needed are a 2 ton engine hoist, engine leveler, cutoff saw, a couple special sockets, and the usual collection of jack stands, cribs, jacks and hand tools.  Specialty materials include urethane resin.