W124 differential swap and compatibility
 

I have a 1985 300D (W124) imported from Europe. After an engine and transmission swap, I'm finding the gearing much too short and I'd like to swap out the factory differential for something with taller gearing. After some searching, it appears there is some variety of gearing that came with the various 124 models, and all are not necessarily interchangeable (ie, size differences).

I know that mine has a final drive ratio of 3.46:1 because it is etched on the differential housing where you would expect to find it. The cover shows a part number of R124 351 25 08, if that helps. Photos attached.

Goals:

1. ~30-50% taller gearing
2. Minimal modification to car. Ideally keep the same diff housing and just swap out gears. Failing that, a different unit that will bolt up to the same prop shaft and axles

Questions:

1. How do I verify compatibility of a potential target diff with my existing car?
2. Is there a master list somewhere that shows my options?

Diffs
 

Have a look at this; 2nd page.

What is the current power train?

Apologies for the long silence. I tried replying earlier and my replies were all "held for moderation". As it seems I'm now able to reply, I will update this thread shortly.

The car VIN is WDB1241301A033375 (1985 124.130; 300D; EDM).

The factory power train is:

engine: om603.912 (3L I6 N/A diesel)
trans: 717.430 (5 MT)
diff/carrier: PN A124-350-24-74 (3.46:1; 185 mm)

Swapped in:

engine: om606.962 (3L I6 turbodiesel; ~300 BHP)
trans: S6-37DZ (BMW 6 MT)

I see my exact car and diff don't appear on the PDF file you provided, probably because it's imported. It appears to most closely correspond to carrier number 124-350-59-23 on page 5 of the PDF.

Edit: On closer inspection, it looks like the 185 mm and 210 mm carriers use different flex discs and input flanges. A post in this tread states that the internals of the two carriers are the same.

1. Does this mean I could grab the internals from any model using one of these two carriers and have them installed into my carrier?
2. I guess the advantage of the larger carrier is better power handling. Should I expect trouble with my current power train if I upgrade to an LSD with the smaller carrier?

The internals are not the same; the ring gear diameters differ by 25mm (210-185).