Installing W124 seats into my W116. Technically, it is possible.

[3l33ter]

Let me just start with some facts, so a) you're not left in suspense while you read the thread, and b) anyone searching in the future will find the answer quickly.

Rails:
W123, W124, or W126 seat rails will not bolt to a W116 floor.
If you want a bolt-in affair, you must reuse your original W116 rails.

Seat frames:
W124 seats do not bolt to W116 rails, but they can be modified to work.
W123 and W126 seats might bolt to W116 rails - I've read that this works, but I didn't try it myself, so I can't say for sure!




OK, now on to the story. My 1975 280S had very nice looking seats, but I didn't find them to be comfortable. Also, one of the driver side rails was broken and wouldn't lock in position, so I kept sliding forward and backward by and inch on one side.

I was on the lookout for nice seats. I did my homework, and what I learned* was that w123 and w126 would bolt in, and w124 would not because it had a different rail shape, but the seat bottom was close in design. I wanted a late w126 seat, but blue interiors are hard to come by, and when I found these very nice w124 seats in blue mb-tex, I just bit the bullet and bought them.
*(When I say "learned" what I mean is what I understood from what I read online, half of which is flat wrong, and the other half is vague [or I didn't read carefully])

I'm not squeamish and I actually like fabricating, so I considered it a challenge. I took my w116's passenger seat out first.

To start with, I took the rails off of the w124 seats and put one on the floor of the w116 and confirmed that it wouldn't work.

I then started attaching the w116 rails to the w124 seat. At this point, I had already done all the thinking and decision making on powered vs manual, weight, convenience, etc. I also considered that a powered seat on manual rails would still need electricity and a switch for the backrest and headrest, and that was OK with me. In this case I decided that manual seats would keep the project simple, and that my passengers can make do without power! Wiring the backrest will come later.

So I'm attaching the w116 rails to the w124 seat and I hit my first hurdle: the w116 rails attach with 6 screws on the bottom. The 124 has 2 screws on the bottom and 4 on the sides, 2 on each side. The 2 screws on the bottom are *almost* in the same place as the w116 ones. Other posts on the internet say they're in the same exact spot, but in fact they are 1-2mm closer together on the 124. I simply spread the rail assembly apart a little bit by hand, and these 2 screws went in. Now with the w116 rails partially mounted to the 124 seat, I marked the location of the other 4 holes I needed and took the rails off again. I drilled holes (thankfully all 4 holes avoided the spring, but the 2 middle holes do come very close to springs).

I mounted the rails again with the 2 front screws, and with my 4 new holes I added a nut from behind (it was a little tricky counter holding these nuts with a wrench) and put the screws in.

Second hurdle: With the w116 rails mounted directly to the w124 seat, the slider handle didn't have enough room to lift up. This is when I noticed that the sheet metal of the w116 seat was shaped in a way that spaces the rails out from the rest of the seat by a little bit (maybe 1/4"). So I made some spacer plates out of some scrap steel (aluminum would've been nice but all I had handy was steel). I drilled 3 holes in each one, put them in between the rails and the seat, and the slide lever worked - yay! I had to use longer screws to reach through the spacer.

I installed the seat and tested it, and it works wonderfully. I still need to run wires for power and ground to this seat, as well as wire in 2 switches for the backrest and headrest. These will be window switches put into the little spill shields that stick out of the seat on the inboard side.

So in summary, to get a w124 seat into a w116, it's really not too hard. You drill 4 holes, use 6 longer screws and 4 additional nuts, I recommend blue loctite, and do the wiring later. 

And how do I like it? I think the styling of these early w124 seats are a close enough match for this DD. But in all of this excitement and frenzy of work, I never even tried sitting in the seats! So I finally sat in it... It's a lot better than the stock seats, but not nearly as good as the late W126 seats in my 1982 300TD (which went in effortlessly.... ahem....). So I might lift up the cover on this seat and add some foam later.

Next I'll talk about the driver seat, ugh, what an ordeal that is being.

First 3 pics: w124 rails in my w116, showing that they don't work.
Last 3 pics: making the spacer, the spacer installed, and the seat mounted into the car.

[3l33ter]

After conquering the passenger side, I decided that the driver side should have no compromises. I wanted:
-full power, because I need the seat bottom tilting feature to find the most comfortable position
-memory function
-w126 gen-II level of firmness

So this means that I won't be using the W116 rails for the driver side. My research led me to believe that W123 and W126 seats would bolt directly into a w116... So I went to the junkyard and bought a late w126 driver seat with memory. This way, I could take the rails, and also open this w126 seat to see what makes it firm, and maybe add that firmness to my w124 seat.

Mounting the w126 rails was same procedure as with the w116 rails. This time, I slotted the front holes of the new rails instead of spreading the rails. Also, I decided to weld in 4 nuts for the new holes, since I had the seat all opened up anyway. And lastly, I needed a 1/4" spacer to lift the seat away from the rails, just as before. You can see the spacer painted blue below.

firmness:
You can see in the pics the differences between w124 and 126 springs. After pushing and prodding, I determined that the w124 really only needs help at the front and back, but the sides were as stiff as the w126.

I swapped over the 2 foam rods from the w126 seat, and made 2 more out of some regular foam that I had. They're not as firm as the w126 foam rods, but better than nothing. Then I went for broke and added more foam, and materials for filler/support. I can take these out pretty easily later on if needed.

Time to put the seat in!

[Harv]

Great work. If you need more support, try using pool noddles. Much firmer than that soft foam and can stand up to the test of time.

[3l33ter]

Do you put the noodles vertically? Or do they lay horizontally (and where do you stuff them)?

[3l33ter]

I noticed these cracks around the front mounts! Yikes! And the car is parked in front of my house with no driver seat. I didn't want to put it together, drive it to my shop, take it apart, weld it, and put it back together again. I just busted out my really old harbor freight mig welder. It doesn't use gas, but hey, it's 220V! The wire feed isn't smooth, so please forgive my ugly welds! What's important is that I got good penetration and it's roadworthy again.

And, oh yeah, my auto-darkening welding helmet stopped working, so I used sunglasses + brazing goggles 
Now the rest of my face is sunburned...

Grind the welds down a bit, a little primer and paint just to keep it from rusting, and done.

[3l33ter]

I put the seat into the car and the M%#*% F$(%*(# rails don't match the floor.

Anyone who says W123 or W126 rails will bolt to a W116 floor is lying!

OK, double or nothing.

The w126 rails are the correct width, but too long. Note, that the front area has 2 extra holes a little further forward (about 1.5 inches). If I used those two holes instead, the rear holes still wouldn't line up (and the seat computers would hit the body). So if you have manual w123 rails or something, maybe this option would be better for you.

I trimmed some excess material, welded in a washer to locate the rear bolt, and then fabricated and attached 2 small spacers. It's funny how an entire day of work gets boiled down to one unassuming sentence.

[3l33ter]

Got the seat bolted in. What a great feeling, because this project has taken about 10x as long as I'd planned.

The new rear bolts are M8 x 40, and they're socket-head because that's all I could find in grade 12.9. They work fine, anyway.

Tomorrow I'll take pics of what the new seats look like. And after I finish the wiring and adjust the seat, I'll let you know how it feels!

[nathan]

I like the crazy lengths you are going to, good pics!