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  #1  
Old 01-13-2019, 12:40 AM
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W123 + 3.06 diff swap from r107

Hey all, well i have a w123

2 door coupe' 1985 with turbo diesel engine, however the car used to have a m110 engine in it, so the diff is 3.58 or something.

I got myself a 3.06 ( closest i could find for the 3.07 straight swap ) In Australia we didn't get w123 turbo 300D's, only NA, so the ratios are 3.46 on them all here.

Anyways so 3.06 ratio diff is downstairs. ( from a v8 engine 450slc )
https://mbturbo.com/gear-ratios/

I've read the guides here
https://www.peachparts.com/wikka/W123Differential
and here
https://mbturbo.com/how-to-change-final-drive-w123/

but i read that i may have to swap some spacers and a yoke ?

can anyone help with a guide for them with pictures/tips or something please ?

it would be greatly appreciated. The engine makes too much unsettling noise down the highway with this diff.

3.58 m110 ratio to a 3.06 is a 17% reduction in engine rpm

so 4000rpm down the highway/motorway is now 3320rpm, should ease up on fuel consumption and keep it closer to peak torque for everyday driving.

Also, it is a 4 speed manual, so each gear should last longer before needing to change and i wont be crying for a 5th gear as much
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  #2  
Old 01-13-2019, 02:50 AM
okyoureabeast's Avatar
Rogue T Tolerant
 
Join Date: Jul 2009
Location: North America
Posts: 1,399
Why not go even lower and get a 2.88 diff or one from a 380sl with a 2.07 diff?
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  #3  
Old 01-13-2019, 06:34 AM
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Quote:
Originally Posted by okyoureabeast View Post
Why not go even lower and get a 2.88 diff or one from a 380sl with a 2.07 diff?

there are still hills around here. Also, it's all what i could find. i looked at a dozen or so diffs and this was the only one under 3.46 ratio. Also didn't want to kill all accelleration
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Old 01-13-2019, 11:30 AM
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Join Date: Feb 2011
Location: Modesto CA
Posts: 2,599
Quote:
Originally Posted by math View Post
Anyways so 3.06 ratio diff is downstairs. ( from a v8 engine 450slc ) https://mbturbo.com/gear-ratios/

but i read that i may have to swap some spacers and a yoke ?

From the ratio guide that you linked, you can see that the oil fill volume for the 3.06 from the C107 is 1.3L, and the volume for your 3.58 is 1.0/1.1L.
The ring gear diameter of the 107 diff is 210mm, and that of the 123 is 185mm. Both diffs share a common mounting pattern, but the 210mm diff is wider than the 185mm. If the half-shafts from the 107 are used, it will all go together. Yes, the pinion yoke will have to changed to the one from a 185mm diff.
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Old 01-13-2019, 07:47 PM
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Quote:
Originally Posted by Frank Reiner View Post
From the ratio guide that you linked, you can see that the oil fill volume for the 3.06 from the C107 is 1.3L, and the volume for your 3.58 is 1.0/1.1L.
The ring gear diameter of the 107 diff is 210mm, and that of the 123 is 185mm. Both diffs share a common mounting pattern, but the 210mm diff is wider than the 185mm. If the half-shafts from the 107 are used, it will all go together. Yes, the pinion yoke will have to changed to the one from a 185mm diff.
Thank you.

I saw this.

https://www.pelicanparts.com/More_Info/1233500410.htm?pn=123-350-04-10-M1192&SVSVSI=R107&KeepHeader=&ByPassCat=Y&null=

the same part for both the r107 and my w123 300D, says it fits both.

I guess the only way to get a difinitive answer is to try it and see how it goes and post back.
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  #6  
Old 01-14-2019, 05:32 PM
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Join Date: Jun 2004
Location: West of Ft. Worth. TX
Posts: 4,110
Your "definitive" answer will be to measure the width of the differential. As previously mentioned, one is 185mm and the other is 210mm. I swapped the entire axle/differential combo to eliminate sourcing spacers. You will likely need to replace the pinion yoke. Here is a good video from YouTube "university" that explains how:
Diff rebuild - how to set pinion bearing preload Ep.1 #1023
Andy the Mechanic
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