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Discussion Starter #1
I purchased a 76 SLC that has been in storage since 2001. What drew me to the car was first I like the SLC coupe as they were not as common and secondly it was Icon gold with mahogany leather. The previous owner tried to get the vehicle running. He spent a lot of time and money and ended up removing the engine and took it apart to free a stuck piston. Knowing this when I bought the car I planned to replace the engine since being a tech for most of my life I figured it was not refurbished correctly. Cranking the engine and hearing the connecting rod bearing noise confirmed this.
I finally decided to update the powertrain with the M117 5.5 and started looking at prices. I also figured the only way would be to buy a whole car to make the parts search easier. I found a 1987 W126 for the price of a used engine. I love driving it so I’m torn on parting the car but on with the project. I sourced used SL exhaust manifolds and the oil filter housing But I have a couple questions that I’m sure the forum can help me with.
1) I have read where some people have fitted suspension from a gen 1 W126 to improve braking. Is it possible to do this with the gen 2 cars? I would consider both front and rear as I would also like to use the larger rear limited slip differential. I know I need the subframe carrier to do this.
2) I figure I will need to fabricate a custom driveshaft but maybe someone has a recommendation since the SLC is longer than the 560SL. I’m going to also use the 4-speed 722.33 out of the W126.
3) Is there anyone in the forum who has done this type of changes and besides the fuel and electrical changes is there anything else I’m missing?
 

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@dgosh28 has a 380slc with a 5.6 in it in Houston. I believe he actually wants to sell the car. He did not do the conversion but bought it that way. @alabbasi hooked him up with the seller I think.

The SLC never came with the 1986+ style offset mount differential. So I believe you’d have to do some welding to make a 1986+ diff fit an SLC. The w126 subframe is larger than the SLC. The SL r107 is shorter too.

I think swapping the hubs, rotors and calipers from a w126 works on the front of a c107, but haven’t done it myself.


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Bloody hell, you've got a long memory.
The 5.6 should bolt up behind a 722.3 transmission from a 380SLC and use a 380SLC drive shaft or you can have one made. You'll need to swap the ignition and injection systems over from the original engine. I'm pretty sure that a Gen 1 500SEC diff is the same casing as an SL diff or a W123 diff and you could probably swap in front brakes from a 560SL if you want bigger brakes. US 560 made about as much power as a Euro 500 which to me have adequate brakes. You might want to try it before you up size.
 

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Bloody hell, you've got a long memory.
The 5.6 should bolt up behind a 722.3 transmission from a 380SLC and use a 380SLC drive shaft or you can have one made. You'll need to swap the ignition and injection systems over from the original engine. I'm pretty sure that a Gen 1 500SEC diff is the same casing as an SL diff or a W123 diff and you could probably swap in front brakes from a 560SL if you want bigger brakes. US 560 made about as much power as a Euro 500 which to me have adequate brakes. You might want to try it before you up size.
I agree on all points, and think that’s what Dan says about the 380slc trans too. I think it sounds like it is essentially the same 3.8 systems now all on the 5.6 block and heads.

The gen2 (1986+) LSD are much more common than any gen1 (1985-) LSD though. So I understand the desire to use a gen2 Diff if possible. I’d recently thought about maybe trying to use a 560sl subframe to modify a c107 subframe, or just swapping an entire 560sl rear subframe into another r107.
 

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The gen2 (1986+) LSD are much more common than any gen1 (1985-) LSD though. So I understand the desire to use a gen2 Diff if possible. I’d recently thought about maybe trying to use a 560sl subframe to modify a c107 subframe, or just swapping an entire 560sl rear subframe into another r107.
I believe that Gen1 SL and SLC sub frames are different so I would assume that there is more to this than just a sub frame swap.
 

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Discussion Starter #6
I would need to do more investigating into the rear subframe compatibility but from pictures I have seen the mounting points are retained for both body to subframe and subframe to lower control arms. I also have a 73 W114 and I might swap the rear diff from the SLC since the final drive on the 280 is a screamer at 3.94 (I might have that last digit wrong).
since I have the luxury of 2 cars side by side. Welcome any inputs from other owners who ventured into the unknown but I will post updates of both success and failure. I started with VW and Audi cars back in the late 80’s and with that manufacturer many components were shared. I was surprised Mercedes was more diverse among models. I love the feel of the old German cars and I feel the newer models have lost much of that. This goes for many other makes as they try to complete with a larger audience.
 

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I believe that Gen1 SL and SLC sub frames are different so I would assume that there is more to this than just a sub frame swap.
Yes. That’s correct. I believe you could swap a gen2 (r107 only) rear subframe into a gen1 r107 without too much trouble. I was thinking that it might be possible to cut up a gen2 r107 rear subframe, cut up a (gen1) c107 rear subframe, and weld the necessary gen2 parts to the C107 subframe, effectively making the nonexistent gen2 c107 rear subframe.


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I would need to do more investigating into the rear subframe compatibility but from pictures I have seen the mounting points are retained for both body to subframe and subframe to lower control arms. I also have a 73 W114 and I might swap the rear diff from the SLC since the final drive on the 280 is a screamer at 3.94 (I might have that last digit wrong).
since I have the luxury of 2 cars side by side. Welcome any inputs from other owners who ventured into the unknown but I will post updates of both success and failure. I started with VW and Audi cars back in the late 80’s and with that manufacturer many components were shared. I was surprised Mercedes was more diverse among models. I love the feel of the old German cars and I feel the newer models have lost much of that. This goes for many other makes as they try to complete with a larger audience.
6 cylinder diffs are smaller than 8 cylinder. You might find that the axles need to go with the diff. I think the w126 axles are longer than 107. You might find yourself seeking some 560sl axles if you end up pulling this off or going through with it.

What are you trying to achieve again?

For your w114, I think a w123 diesel 3.46 would be easier.

For the c107, any v8 1972-1985 differential is going to be easier.
 

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Discussion Starter #9
Fonzi, I think the C107 has a 3:46 final drive So I would swap that for the one in the W114. That car has plans for a M104 engine to increase performance and have better drivability with the fuel injection. If it’s too much of a hassle I will retain the gen 1 final drive. The biggest reason was for the limited slip as I live in an area where some roads are gravel and my house is on a mountain top.
 

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Fonzi, I think the C107 has a 3:46 final drive So I would swap that for the one in the W114. That car has plans for a M104 engine to increase performance and have better drivability with the fuel injection. If it’s too much of a hassle I will retain the gen 1 final drive. The biggest reason was for the limited slip as I live in an area where some roads are gravel and my house is on a mountain top.
I agree with the LSD desire. Gen1 LSD were pretty uncommon, yet so many gen2 have it.


I think you’ve got the 3.46 mixed up with what’s most likely a 3.06 or 3.07 (crazy that MB made both, and not sure how those numbers work out in the exact same size diff). I think a 3.07 is the highest ratio the 4.5 ever came with in a 107 without a special order. The w108 got a 3.23 with a 4.5. The 3.5 came with a 3.46 big diff.

I think we laid it out above what you’d need to do to bolt that gen2 w126 Diff into the 107, not small work. If you want to pony up mega bucks for a gen1 LSD, I know someone who has them in all these ratios, one each: 2.24 from a 1983 500sl, 2.72 (from a 450slc 5.0), 3.06 from a euro 450slc, 3.23 from a euro 380slc, 3.46 from a 350se (w116). He also has two gen2 2.47 from 560sl cars. It might be an action if clawing them out of his cold dead hands or thousands for any one of them. I doubt that’s the type of money you’d want to pay, but if you do, I know where they could be obtained. Otherwise, the hunt is time consuming. Maybe someone knows how to take the gears and/or clutches out of a gen2 LSD and install them in a gen1 housing. I don’t. Furthermore, I think the clutches might be NLA.
 

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Discussion Starter #11
Thanks for the parts lead. If I can’t use what I have then I will put it on the back burner. There is plenty other items I would rather have like euro bumpers. Right now with as much customer work I have this project won’t commence until the fall. Just trying to round up parts to speed the conversion.
 

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I just recently completed the swap and there is a lot more to it than what was already mentioned. Other than manifolds and oil filter housing, you will need oil pan and radiator from an SL as well as swapping engine mount arms and engine shock mounts. Also, numerous small modifications that I had to do along the way to make the swap work. For now I am running it on D Jetronic and hope to change to Megasquirt by the end of the year. Don't forget to do all of the freshening up of the "new" engine before putting it in. I did timing chain, guides, sprockets, valve guide seals, front/rear crank seals as well as intake manifold seals and gaskets. I also took time to change front subframe and engine mounts as well as most of the hoses/belts. You can check my post to give you an idea of what it takes.
 

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Discussion Starter #13
Ivan, so the M116 mount arms are different than the M117? Also is the radiator different between the 4.5 and the 5.5? Yes, I plan to replace headgaskets, timing chain and guides, seals before installing. Fortunately the inside is like new so previous owner took car of the car. Drivers side head gasket is leaking at the back corner. I understand I will have to install heli-coils
 

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Ivan, so the M116 mount arms are different than the M117? Also is the radiator different between the 4.5 and the 5.5? Yes, I plan to replace headgaskets, timing chain and guides, seals before installing. Fortunately the inside is like new so previous owner took car of the car. Drivers side head gasket is leaking at the back corner. I understand I will have to install heli-coils
Head gasket leaking? Or the Valve cover gasket leaking? That’s a very common place for the valve cover gasket to leak.


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Ivan, so the M116 mount arms are different than the M117? Also is the radiator different between the 4.5 and the 5.5? Yes, I plan to replace headgaskets, timing chain and guides, seals before installing. Fortunately the inside is like new so previous owner took car of the car. Drivers side head gasket is leaking at the back corner. I understand I will have to install heli-coils
The arms on w126 are different than for r107 and yes, the radiator is different because the top hose connects on the passenger side on 5.5 vs. on the driver's side on 4.5. I hope none of these things will discourage you. In my case, the swap was worth it and I am enjoying the car now although I am not finished yet. However, I believe it would be beneficial for you to have most, if not all, parts needed ready before you start. On several occasions I had to stop working on the swap for a week or two because I had to order something I didn't plan on.
 

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This won’t discourage me at all. Just helps me source the parts before I start the project. You also mentioned the oil pan is different. I have seen pictures of both and it seems like there is no difference between the SL and the SEL. I even compared part numbers in the pictures. maybe you can shed some light on this one. Fortunately I am keeping the automatic. I mulled it over and I decided I wouldn’t attempt unless I found a donor car Which is why I found the engine I wanted in a running car. I want to keep the fuel injection and this way I will have what I need. I’m not afraid of the wiring as I’ve done it before.
 

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Head gasket leaking? Or the Valve cover gasket leaking? That’s a very common place for the valve cover gasket to leak.


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Unfortunately its a very common place for the head gasket to leak as well.
 

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This won’t discourage me at all. Just helps me source the parts before I start the project. You also mentioned the oil pan is different. I have seen pictures of both and it seems like there is no difference between the SL and the SEL. I even compared part numbers in the pictures. maybe you can shed some light on this one. Fortunately I am keeping the automatic. I mulled it over and I decided I wouldn’t attempt unless I found a donor car Which is why I found the engine I wanted in a running car. I want to keep the fuel injection and this way I will have what I need. I’m not afraid of the wiring as I’ve done it before.
The oil pan on W126 is definitely different. It won't clear the subframe and allow mounting of the downward pointing oil filter housing. I wish it did as it would have saved me $250.
 

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This won’t discourage me at all. Just helps me source the parts before I start the project. You also mentioned the oil pan is different. I have seen pictures of both and it seems like there is no difference between the SL and the SEL. I even compared part numbers in the pictures. maybe you can shed some light on this one. Fortunately I am keeping the automatic. I mulled it over and I decided I wouldn’t attempt unless I found a donor car Which is why I found the engine I wanted in a running car. I want to keep the fuel injection and this way I will have what I need. I’m not afraid of the wiring as I’ve done it before.
Oil pan for the w126 is definitely not the same as the oil pan for the 560sl. 560sl oil pan is not normally cheap either. The 380sl pan might work, but it might not. They are definitely different.
 
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