Anyone know if it's possible to put a 300SD 3.0L diesel engine into the body of a 420SEL (4.2L gas)? I've read they're both W126's, but what sort of size difference is there with the Long wheelbase (ie SD/SE vs SDL/SEL)?

Are the transmissions compatible (ie diesel engine work on the transmission that went with the gas engine)?

If so, is there anything else obvious to worry about? Anything with the fuel tank/lines to change?

Basically a buddy of mine is giving me his rundown 420SEL (engine problems) and I have a working 300SD engine on another body.

The driveshaft is different due to the long wheelbase.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

the transmission might bolt up however issues that may come up are starter being on opposite side and most certainly the gear ratios will not be good for a diesel. The fuel lines will have to be flushed since gas and diesel dont mix well. Also consider swapping over the differentials for performance reasons.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

How can I learn more about the challenges/difficulties of putting a random engine and transmission together? I'm really not familiar with transmissions in general - is there something I can read/learn to become more familiar/understanding of these types of issues? ie books, etc

I have a diesel transmission that died... I'm not sure if maybe some parts are interchangeable with the gas transmission to make the diesel engine happier? This is more of a pet project, definitely a challenge.

Here are some general thoughts based on the research I've done relative to my diesel swap project which is underway (OM617 into a 380SL)

The consensus seems to be that it is best to keep the diesel transmission that came with the diesel engine - the reasons seem to be mostly around gear ratios, torque converter and other internals. If you keep the diesel transmission that originally came with the engine, you will probably find that you'll need to swap the yoke from the gasser to the diesel, which could make the connection to the existing drive shaft a bolt up job. You will most likely actually use the smaller flex disk because the diesels put out less power (You might need to make a minor adjustment using the splines on the driveshaft)

If you use a 617.951 you'll have the best luck.

As far as fuel is concerned, you'll need to flush the gas but you've got return lines to the tank already in place, but be aware that the connectors may be different. You'll need to do something with the fuel pump. Either take it out, replace it with a low pressure pump, put a fuel pressure regulator on the existing pump or something like that. There is too much pressure in the stock gas fuel pump which is designed to drive fuel injection.

I don't think you'll have any clearance issues.

As Oracle12345 said you should check the differential gear ratios, it could be a problem.

You'll probably need to do something about your exhaust too. Generally the gassers have smaller diameter tubing which will cause backpressure issues with the turbo. In my case I've got two parallel exhausts running about 75% of the way to the back, so all I've got to do is fab a larger segment to transition from the diesel down pipe to the dual section (essentially where the cat used to be)

You might have to do something about the radiator if your gas engine does not have an oil cooler. Easiest solution is to just use the diesel radiator if this is the case - might just bolt up if you're lucky.

I don't have specific answers for you, but these should be the issues worth researching before starting your project.

Oh, and where to learn? These forums of course! A wealth of knowledge, experience and helpfulness.

Good luck!

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Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

Transmission is the same. 420 V8 also has the starter on the passenger side just like the 617 does. Gear ratios are the same, only difference is the shift points....it could probably be made to work with proper adjustment.

Its a very doable swap. I'd go for it.

The 420 has a huge exhaust system, cut out the cat and it's equal to the normal diesel system.

Fuel system is easy, clear out gas from the tank, saw off lines in engine compartment and clamp on hoses for the diesel like normal in the diesel versions.

Overall very easy swap IMHO.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

That's the answer I was waiting for! I read there's a model/serial number on the side of the transmission for me to compare.

The mechanical bits aren't the whole story.

1. You're going to have to either graft in the glow system out of an SD, or install a manual glow controller relay with a switch in the cabin. The 420's wiring harnesses aren't set up for that.

2. You'll have to work out how to connect up the vacuum shutoff, as the gas cars don't have that.

(I would think that addressing #1 and #2 will mean getting an entire steering column out of an SD, or preferably an SDL if the 420 has an airbag.)

3. I don't know if the gassers use the same rad, and/or if the SD oil cooler lines will bolt up as-is.

4. Front springs: someone needs to chime in here on what the weight difference is (if any) between the V8 and the 3.0.

You need to work all of this out carefully. If you are paying shop time for some or all of this, it will add up.

Engine swaps like this are MUCH easier when you have two complete cars that you can sit side by side.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

look it up in wis they are different

and zacharis made some good points to consider

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

You mentioned that the turbodiesel engine you have came out of an SD. If that means a w126 SD, should be okay.

But if it was a w116 you may have issues with the oil pan contacting the cross member and/or you will have to do some adapting under the hood. For example, I don't think the main vacuum line from the brake booster will bolt up.

Everything is do-able. However, it's all relative to how much time and expertise, and what sort of working facililties and tools you have yourself, versus how much you'll be paying someone else to supply.

Also bear in mind that what you will end up with won't necessarily be easily saleable. You can probably sell it within this community, but the mainstream market won't be overly enthusiastic. It will always be a frankencar. Of course, if you plan to keep it forever, that won't be an issue.

I'm not against motor swaps. In fact I still regret, 9 years on, that I sold the funkiest car I ever owned, an '82 Volvo 245 with a complete Mustang 5.0 + Getrag drivetrain.

But I AM into risk management and getting into things with your eyes wide open. (Been there, done that, syndrome.)

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

What is different specifically.....I own and work on both, and see very few obstacles in a conversion. I'd probably do one myself if my 420 motor ever gives up and I can't find a decent v8 replacement. Might be a while though, only 47k on my engine since a full rebuild at a MB dealer.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

5G-Tronic - Wikipedia, the free encyclopedia
http://www.w-124.info/w124/mercedes%20tranny_ratios_722.3_722.4_722.5.pdf

as you can see the later 722.3's have different gear ratios

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Different scenario. I looked at the body of the 420SEL and what if I just take the tranny out and put it in my 300SD body? As it's been said, the transmission itself is the same (supposedly), and I have a manual on swapping (removing/installing) transmissions. Would that be easier than putting the diesel in the gas body?

This is a 1987 420SEL, and from the wikipedia page, there are no W126's listed as applications of the 5G-Tronic. So, is it safe to assume it's swappable?

1987 420SEL transmission into a 1984 300SD body, both W126.