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  #1  
Old 07-08-2018, 11:59 PM
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Custom Drive shaft, Converting 450sel into 300SD (L) w116

Hello,

Perhaps it would be wise to consult wiser minds. The story is I have a 450sel and a 300SD both w116. I want to convert the 450SEL over into a diesel using the 300SD. The only problem I have is the driveshaft. The 450sel is 1/2" wider and therefore doesn't line up with the small 300SD transmission and differential driveshaft connection ends. The 300SD drive shaft is 2" short (the 450SEL is overall 2" longer than the standard w116 chassis, more back seat leg room).

Question: Is a custom drive shaft possible? Under 1000$, in all honesty under 500$. What am I looking for? All I want is to make the drive shaft 2" longer, but I need it to be the size of the 300SD driveshaft ends (the part that butts up to the flex disc), I assume I would have to send it out and they have to make an entirely new shaft.

Or is this project dead in the water? The car body is 98% rust free @ 150k miles.

Thoughts? Thank you!
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Old 07-09-2018, 12:09 AM
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That's pretty darn easy for a legit driveshaft shop. Depending on where you live you might be pushing that budget you want, but it's nowhere near a dealbreaker. Likely they would use whatever existing driveshaft(s) you have and modify that.

-Rog
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Old 07-09-2018, 12:19 AM
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IIRC the 450SEL through ‘79 has the same 3.07 diff ratio as the SD. You only have to worry about the transmission end. I suppose an alternative is to replace the SD transmission and differential flanges with the SEL counterparts. Might require setting pinion preload on the differential though.

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Old 07-09-2018, 01:05 AM
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I've seen a 450SEL that was converted to a 300SDL before. I often find the rear seat legroom of my 300SD to be lacking. A once-active member of the forum bought a W116 6.9 with a reclining rear seat, and his plans were to use an engine from a 300TD so he could use the pump in the head to run the hydropneumatic suspension. That would have been a fantastic car.
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  #5  
Old 07-12-2018, 12:59 AM
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Quote:
Originally Posted by sixto View Post
IIRC the 450SEL through 79 has the same 3.07 diff ratio as the SD. You only have to worry about the transmission end. ...
....
I think the 450SEL had an option for 210 mm 3.06 ratio It would be close enough in my book.

.
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Old 07-12-2018, 08:56 AM
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Quote:
Originally Posted by Rogviler View Post
That's pretty darn easy for a legit driveshaft shop. Depending on where you live you might be pushing that budget you want, but it's nowhere near a dealbreaker. Likely they would use whatever existing driveshaft(s) you have and modify that.

-Rog

At some point ( 70's ? ) MB moved to a friction welded drive shaft making it more work to change length. These can be identified by a flared ridge about 2" from the flange. To shorten or lengthen these, the tube needs to be cut about 4" from the flange ( 2" or so away from the friction weld. ) a sleeve needs to be made and everything put back together.

Traditional drive shafts used flanges that fit in a tube then are conventionally welded. These are easy to shorten by cutting through the weld , cutting the tube then welding everything back together.
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Old 07-12-2018, 09:33 AM
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Since you have two drive shafts to work with it should be a simple thing to have a driveline shop (ask at trucking repair places whom they use) cut it in the center and lengthen using the other shaft .

Measure thrice of course .

You neglect to mention where you're located, in So. Cal. drive line shops about and usually will do this for less than $500 .

Sounds like a great project .
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Old 07-12-2018, 09:51 AM
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This may be a dumb question (seriously, just throwing out a thought and haven't had coffee yet this morning!), but can you swap the flanges from the V8 transmission and differential to the SD's transmission and differential and then just use the V8 driveshaft? If it's just the alignment of the flex disks that's the issue, I don't see why it wouldn't work?
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  #9  
Old 07-12-2018, 10:23 AM
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Quote:
Originally Posted by Assault View Post
Hello,

Perhaps it would be wise to consult wiser minds. The story is I have a 450sel and a 300SD both w116. I want to convert the 450SEL over into a diesel using the 300SD. The only problem I have is the driveshaft. The 450sel is 1/2" wider and therefore doesn't line up with the small 300SD transmission and differential driveshaft connection ends. The 300SD drive shaft is 2" short (the 450SEL is overall 2" longer than the standard w116 chassis, more back seat leg room).

Question: Is a custom drive shaft possible? Under 1000$, in all honesty under 500$. What am I looking for? All I want is to make the drive shaft 2" longer, but I need it to be the size of the 300SD driveshaft ends (the part that butts up to the flex disc), I assume I would have to send it out and they have to make an entirely new shaft.

Or is this project dead in the water? The car body is 98% rust free @ 150k miles.

Thoughts? Thank you!


Your issue is EZ Peezy


A competent DS shop can do the front DS section for under $200.00. The only issue is getting your measurements spot on before you make one.


As for the differential issue, simply swap out the yoke and get a new funky 12 pt nut. The issues are seals and a delicate hand with the torque wrench. That is, doing your maintenance. I have a 2.65 LSD in my W123 which came from W116 6.9 (M100). Your swap would keep your axles and differential and just swap the yoke. If you want different gearing, you can just swap that out too finding a different W116 differential. If you can find a 2:65 LSD, then that's the ideal rear IMO


You're not the first to make a 450SEL a diesel .... You got some wiring harness issues and vacuum inclusions ...
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  #10  
Old 07-14-2018, 03:42 AM
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A 2:65 "might" in that 450SEL. But in either case I would just keep the existing rear differential, swap to the larger yoke on trans and keep the existing driveshaft.

.
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