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450 to 5.0 M117 swap

Started by Frankthemoneyman, 19 February 2023, 01:27 AM


Hello, I did search before posting and thought I would share my findings about the 5.0 swap
My recently inherited 450SE with 61000 original miles and a locked-up engine. Three or four shops had a crack at getting it running, before I got my hands on it, and probably did more damage than solve the mystery.
Rather than diving knee deep into the 450 motor, and probably looking at rebuilding the heads, timing chain,  I decided to swap in a good used running 1984 5.0 M117.
Both are Euro's, the 450 and the 5.0 M117, at first glance, dead ringers, no smog stuff, similar exhaust.
I was able to test drive the donor, before the motor was pulled, and it included a recently rebuild fuel distributor. Both cars have the K-Jetronic and things looked like a fairly straightforward swap.
I did some research but exercised caution, with so many keyboard hero's that have never had dirt under their fingernails or bench racers repeating information that they heard somewhere else, posting away on the world wide web.
I had researched that the transmission bolt pattern has a 10mm difference, on the two top bolts, between the cast iron and aluminum blocks, rumor has it, that Mercedes Benz did have 722 transmissions, that had a dual bolt pattern.
Well, I did not get that lucky.
My donor 5.0 came with a transmission, having owned many other Mercedes Benz of the same vintage, I am not a huge fan of the 4 speed transmission, not really an upgrade from the 3 speed 722 transmission.
Significant differences between a few years of production, overall length being a huge deal, and the possibility of having to deal with the driveshaft length. A lack of decent priced driveshaft shops in my area and also the donor 722 transmission looks like it has seen better days, it shifted fine but also leaks out of just about every possible spot a 722 can leak from. Also the output shaft does not match the sedan, probably from being mounted in a G-wagen before.
The original 722 from the 450 had zero issues before the car was parked.
Separating the two is going to be a challenge, since the 450 motor can not broken loose, so I am left with leaving the torque converter on the motor.  I build a frame out of 2X6 and plan on supporting both units, and pulling the transmission back as straight as possible, hopefully without damaging the pump or torque converter. 
Could not find a lot of information on this subject in particular.


If you are really brave why not put in a 5 speed manual transmission  ;D?  5 speed Getrag from older BMW M5 if you can find one.
1977 450 SEL 6.9 - Astralsilber


I put a 5.0 together with the rear diff and transmission in to my 280se , i think that you should be able to use the original driveshaft i used the rear from a 4.5 and the front from a 3.5 because i wasn't able to get a complete 4.5 driveshaft that time.
I prefer the 4 speed over the 3 speed it's much firmer and with the 2.24 rear axle i wouldn't want to miss the 1. gear , it's also nice that you can cruise at 200 kmh with ~ 3500 rpm everytime i drive my 6.9 at 100 kmh i am wondering why its not shifting  ;D  .

The only problem is that the motor is very close to the subframe and often hits it because the rear of the motor is 1cm closer to the front as far as i know.
I was also able to use the w126 radiator with a little bit of massaging , and even without a visco fan i never get above 80C while driving doesn't matter how hard i drive.


Hello Ramiro, if memory serves correctly Mercedes Benz transmissions in the 70's and 80's are direct in top gear, 1.00 final, so the four speed 722 is not an overdrive, by modern standards. Also all of my 722 four speed cars start in 2nd. gear and not first, unless you manually lock the gear shift selector (W126), or press the kick down switch.
There was a first gear start gadget marketed a long time ago, which would make the car start in first gear, every time you start driving from a standstill. 

@ Daantjie, if I would reside out in the country, away from the city, it would be tempting, living in Southern California, before I get to the I5 on-ramp, I am already over a manual transmission in my convertible. So the slush-box if preferred.

Do anyone have any experience in servicing their own transmission, replacement of front pump seal ?

I just was quoted $ 2750 for gasket and o-ring replacement by a local transmission shop, this is the bench drop price, meaning I drop the transmission on their bench, no labor for R&R.
I located a complete gasket and seal kit online for $ 130 and think of replacing the gasket's and seal myself, while the transmission is out of the car.


Sun Valley is the go to shop for transmission swaps.  They are local to you in CA.  I think they only do exchanges but they do have a stellar reputation in the Benz community.
1977 450 SEL 6.9 - Astralsilber


I know that the 4. gear is the same , but with a 2.24 diff the 1. gear on a 3 speed would go to ~140 kmh so in reality the 4 speed transmission is like 1 more gear at the top with the 2.24 rear diff.
Also the car still feels faster from a stop then my 450se that i had , it actually uses 1. without kickdown if i realy step on the throttle from a stop.

Also the 450se always took much more fuel the my 280se 5.0 most likely because of the shorter rear diff and the 3 speed slushbox.

Just saying that i wouldn't waste any time to adapt the 3 speed to the 5.0 engine.


$2750 for seals?  SunValley you get an entire transmission for @$1800.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'21 E450 Cabrio


Yesterday was a partial success and also a partial fail.
I was able to separate the transmission from the engine in a controlled fashion, using my fixture, without damaging the torque converter or pump, I believe.
The fixture allowed me to carefully control the movement and keep things as straight as possible.

The 722.1 style transmission with its separate aluminum plate, bolted to the cast iron block, behind the torque converter, prevents me from unbolting the torque converter on my locked up 450 engine.
If this was the 5.0 scenario, newer style engine / transmission, I would have enough room to unbolt the torque converter.
So now I need to do, what I dreaded to in the first place, tear apart the cast iron 450 enough to get it to spin, and unbolt the torque converter.

@ rumb, thank you, I will check out Sun Valley.
@ ramiro, I get it, long final drive and using the four speed, with starting in first makes for a more useable and enjoyable driving experience.

My 450SE will mostly be used for the occasional weekend drive, cruise down Pacific Coast Highway.
Not looking at fuel economy, when the car will spend 95 percent of its time in the garage.
This car was very happy to cruise at 75mph or 120 kmh, which is perfect for our speed limits.

Next weekend the cast iron 450 is coming apart, more pics to come


Update - The cast iron 450 finally came apart and I figured out what caused the locked up engine.
I took the heads off and no apparent damage and it was still locked up.
Took the oil pan and timing cover / chain off.
Found a bent rod on number 4, it appears to have hydro locked.
Had to loosen the main caps to get it to turn and the crank.


If anybody is planning of swapping a 5.0 into your 350SE or 450SEL you will need the transmission that came with the engine.
There is stories out there of crossover or dual pattern 722 transmissions, but I have never seen one, nor could I find any pictures of one on the internet.

I could only match 4 bolts, out of 12 in total, in addition too, you have have to machine a lot of the aluminum plate, to fit over the rear of the engine. Also the the guide pins do not line up either.

So swapping an aluminum 5.0 M117 in place of a cast iron 4.5 would involve also swapping in the entire engine and transmission.


Quote from: Frankthemoneyman on 18 March 2023, 11:11 PMUpdate - The cast iron 450 finally came apart and I figured out what caused the locked up engine.
I took the heads off and no apparent damage and it was still locked up.
Took the oil pan and timing cover / chain off.
Found a bent rod on number 4, it appears to have hydro locked.
Had to loosen the main caps to get it to turn and the crank.

That happened to me once....   ::)
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0