Manual transmission for 300SDL - PeachParts Mercedes-Benz Forum

cmac2012 · #1 08-01-2023, 07:57 PM

If I were a dieselhead with time and money on my hands in excess, I might be into attempting the holy Grail of installing a manual tranny in a W126, specifically my ‘86 SDL.

The engine is outstanding, installed about two years ago, but the tranny has developed a problem. It’s a long story, I don’t want to go there.

I have read of guys using a Mercedes 6 spd from a four-cylinder Kompressor variety. Bolts right in almost I have read. I have also read that transmission is sketchy.

What really looks good is getting a later model BMW five or six speed from a car with as much or more horsepower and about the same torque as my SDL. I’ve read of mating flanges available. I’ve gathered here and there that putting in a clutch pedal is not nearly as tough as you might think. Take the front seats out for the duration, massive padding to lie on your back.

This engine is so fine. It really deserves to be hooked to at least a five speed. I’m pretty sure good specimens are to be had at bone yards - locally Pick n Pull wants $250 for a trans.

The scuttlebutt is, of course, that manual trannies are a lot more trouble free than an autotragic. I need to learn if there are any techniques to identify sound versus malady in a transmission sitting in a carcass. Finding low miles + major body damage is the holy Grail of course.

Air&Road · #2 08-25-2023, 09:27 PM

Cmac,

I an a major leavue manny tranny lover. I have no idea if the six speedcoukd be made to bolt up but if it did, i really dont think that it would hold up to the weight of a big bodied car. If you make it work though i would love to hear about it.

t walgamuth · #3 08-26-2023, 09:12 AM

Carl, which engine is it?

cmac2012 · #4 08-26-2023, 01:30 PM

It’s the OM603. An inline 6, higher performance than the 617, but generally thought to be a little bit less durable. It was a 3.0 L from about ‘86 until ‘90 when it was jumped up to 3.5 L. I’ve never driven one of those - one member who suddenly vanished several years back, Diseasel300 had previously owned a 300SDL, it was totaled by some bozo running a stop sign, he later acquired a 350SD, which was pretty much identical to a 300SD (w126) but with the OM603, 3.5. He said the performance was outstanding. My mechanic in Redwood City, who is practically an expert on these motors owns two of the 350SD. That motor had some difficulty from having made the pistons bigger, there wasn’t enough steel separating the combustion chambers, one of the Pistons would oval out. Numerous recalls. I’ve also heard that it can do pretty well if maintain properly. Who knows.

My SDL‘s engine is way fast. Any sneering about slow diesels does not apply to this car.

I think you’re probably correct Larry about the tranny in question being too small. My impression is that many of the Frankencar adventures turn out badly. Would have to be an enormous amount of tooling and homegrown engineering to put some of that stuff together. And then to have problems of this or that aspect of it being weak, or failing, would have to be discouraging.

I think perhaps a five speed manual from a BMW that weighed about as much as the SDL, just under 4000 pounds, might work. But oh man, you would need Jay Leno garage money and facilities to pull that off. Probably easier and cheaper in the long run to stick with the stock four-speed auto. I recently found an SDL in a boneyard with 259K on the odometer. Cherry body, everything giving the appearance of first rate maintenance forever. The tranny could’ve been mine for $300 and a lot of work. Would have been nice to be able to have called the previous owner. I suspect they would’ve been happy to have a brief chat - did the tranny work just fine? if so, might be worth a gamble. But it would be tough to discover the hard way that a blown tranny was why it landed in the junkyard.

That tranny can be had rebuilt for about $2700. I may go that route eventually.

t walgamuth · #5 08-26-2023, 01:36 PM

I had a 91 350sd. It drove very well. loads of torque. I eventually sold it to BC and bought something newer.

cmac2012 · #6 08-26-2023, 01:46 PM

My mechanic friend has one with an excellent body, about 189k, he wants something like $9000 for it. It’s halfway tempting, I won’t be doing it any time soon. I just remembered that I have a brief YouTube video I made of the car.

https://youtube.com/shorts/PyKfTc8fnF0?si=NSqwFmlowpcIYoKg

ccrelan · #7 08-27-2023, 07:51 AM

Nothing sketchy about the 6 speed that came behind the M111 W203 hatchback. I have it behind my C36 which makes quite a bit more power than any OM603 without issue.

P.C. · #8 08-27-2023, 02:14 PM

Would a manual from a w123/w124 application be a better candidate? What was Mercedes using as a manual transmission for this engine family in Europe? Can this engine/chassis combination offer acceptable performance with a modern manual which may have a pretty deep overdrive top gear?

cmac2012 · #9 08-28-2023, 11:39 AM

I researched that point, and found a discussion on this forum from way back featuring boneheaddoctor, Brian Carlton, and R. Leo:

OM 603 question..............

Opinions are mixed, there was talk about too much torque for the clutches - others said “nonsense.” More and more for me it comes down to accepting the likely fact that Frankencar projects are fraught with peril. Some of them do seem tempting, such as putting a Subaru motor in a Porsche Spyder kit car such as Hoovie did.

My mechanic friend in Redwood City put an LS into a Porsche 928 which had a blown motor. It looks like good work, but the thing has been parked in the same place for a long time. I’ve never asked him if there’s some semi-fatal problem going on.

P.C. · #10 08-28-2023, 07:54 PM

I'm curious if heavyweight versions of the w123/w124, such as those converted to emergency vehicles, and/or vehicles destined for severe conditions, were outfitted with stouter clutches.

P.C. · #11 08-28-2023, 07:57 PM

Found this: https://dieselpumpuk.com/products/mercedes-performance-clutch-kit

savas · #12 08-29-2023, 01:41 AM

This guy managed to put in a Getrag 717 5-speed

https://bringatrailer.com/2018/03/13/5-speed-swapped-w-18-psi-boost-1987-mercedes-benz-300sdl/

cmac2012 · #13 08-29-2023, 08:10 PM

Interesting stuff. But I can’t find a good answer on where the Getrag 717 comes from. Apparently it’s a Mercedes transmission.

I have to wonder why the guy was letting it go for $6500. If all those improvements worked flawlessly seems like it would be worth 10K at least.

The stuff is fraught with peril. Inheriting someone else’s work like that, you might regret it, not knowing the history of how this or that piece was adapted.

Good food for thought however.

savas · #14 08-29-2023, 09:49 PM

This place sells kits to bolt on GM or ford manuals to om603 !

cmac2012 · #15 08-30-2023, 07:23 PM

That’s interesting, it was a little discouraging, however, to hear him talk about the supply of used trannies drying up. It’s not something I’ve ever looked for, I’ll keep an eye out.

I found this link on the web really touting the BMW Getrag G420. They say it will handle up to 600 hp, it was on
E46 M3. That model had a pretty powerful engine, 320 hp naturally aspirated. No doubt you would need a bell housing adapter flange. It all gets pretty involved. Almost sounds like I’m trying to talk myself out of it.

I wonder whether the Getrag would somehow feel more authentically German than a Ford or Chevy tranny. God only knows if there would be a discernible difference that mattered.

https://garagistic.zendesk.com/hc/en-us/articles/360051869394-BMW-Manual-Transmission-and-Clutch-Guide#:~:text=The%20G420%20is%20one%20of,reliable%20in%20high%20horsepower%20applications.