When I started the project, I fully expected that I would have to fabricate engine support arms to go between the engine and the subframe. One of the things that I learned was that the 1975-76 300D W115 with the 617.910 engine, has support arms that would work because those years used the same identical front subfrme that the 107 chassis used.  That one breakthrough saved a ton of time and fabrication and more importantly, automatically positioned the engine properly relative to the chassis.

A preliminary assessment of the fit of the OM617 engine in the 380SL, showed that there were two major problems - The oil filter housing hit the steering box, as shown in the image (orange arrow). This is with the right motor mount in place, and the left motor mount is still about a half an inch from rotating into position.

The second clearance issue was with the oil pan to the subframe.

The subframe is a bit too close, as shown, but the lip can be cut, and I elected to box the rest of the subframe at that point just to be safe. 

While there's ample clearance, that tab just has to go. It's unneeded anyway.

A second breakthrough came when I learned that an upper oil pan from a W115 1978-9 300D  had a smooth lower surface and not the ribs that the '85 (and many other 617 engines) had. This alone gave me at least a half inch of clearance between the sump and the subframe.

When the engine was again installed for a test fit, there was clearance, but it was literally the thickness of a piece of cardboard. It's hard to see in the below image, but there is just a bit of clearance. So the engine would need to be raised a half-inch to get the target half inch of clearance all around I set as the minimum.

Raising the engine is easy, just need to create 1/2" spacers to fit under the motor mounts. I used steel, but aluminum would be a far better material for this part

So now we have the clearance we need between the engine and subframe, but we now create two problems. First, the engine sits too high relative to the chassis, and second, we create an issue with valve cover to hood clearance. The the solution here is to drop the subframe by the same half inch we raised the engine. This restores the same relative position of the engine to chassis and provides the hood clearance we need.

To drop the subframe we need to fabricate spacers. Here are the spacers I turned on the lathe. There is nothing special except to make sure that the spacers fit snugly in the subframe mounts and that they also fit snugly in the chassis. You don't want any slop that would allow lateral movement of the subframe relative to the chassis. You will need half inch minimum longer bolts to reinstall the subframe.

I should say a word about getting the subframe in and out of the car. The easiest way to do this is to get the car up on jack stands and then use the engine lift with a sling. I used a cheap Harbor Freight movers dolly to wheel the subframe under the car and then lifted it into place with the sling.

Engine Mounts, Transmission Mount and Subframe Mounts

Although we've got the clearance we need for the engine and transmission, we don't have a lot of extra clearance to keep us safe as the mounts age and begin to collapse. A quick search of the forums about engine and subframe mounts shows that collapsed mounts is a big problem.

To provide some insurance against collapsed mounts, the solution is to fill the mounts with urethane resin. Notice the extra support provided by the urethane filling compared to a standard motor mount. It's important to fill the transmission mount and the subframe mounts as well.

It might seem that the urethane would be too hard and would transmit a lot of vibration, but in practice it's just fine.

To pour the subframe mounts, install the new mounts in the subframe (make sure they are grounded fully) then seal the bottom of the subframe with tape and make sure no urethane can get into the mounting hole. Pour to the top and let set fully before moving.

Note:  Check to make sure your subframe has the recall supports installed for the lower control arm. The additional stress of the diesel will make this a requirement. Here's a link with complete information Subframe recall information

Now you are almost ready to install the subframe and the engine.


While not technically part of this step, it is a ton easier to get the radiator mounted before the engine goes in for the last time. So before you drop the engine in, hop over to the cooling hookup section and look at the radiator fabrication and installation section.

Oil Filter Housing

As we mentioned above, there is a clearance problem with the oil filter housing and the steering box. This is solved by using an oil filter housing from a 300SD. As shown in the image below, the different housings provide the clearance we need.

And when we get the engine in the vehicle, we'll be just fine.  It looks closer in this image that what it actually is.

Tip: Get a new "plastic waffle" gasket for the oil filter housing from the dealer and install it while the engine is out of the car. It's a ton easier.

You are now ready to install the engine and bolt it in and start getting things hooked up.

Clearance Checks

Wnen the engine is bolted up, double check all clearances. If you need to make an adjustment it's easier when everything else isn't already hooked up.

You can do a visual check of the subframe to sump clearance, and the oil filter to steering box clearance, but checking clearance between the valve cover and the hood is tricky. Here's the secret. Get a piece of a zip tie and a piece of tape. You will tape this to the front of the valve cover. Use a little piece of paper on the tape to allow the zip tie to be firm, but slip easily.  Set it about an inch and a half above the valve cover, then carefully close the hood and take a reading of your clearance (I've got 3/4")

 It's now time to start hooking everything up and getting ready for a first start!

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