Differential question

[gazzol]

Time once again to tap into the fountain of knowledge that is E9 coupe.com
Apologies if this is a repeated question
I would like to increase the ratio on my diff from the current 3.09? (e12 m535). I am well aware of the 3 common ratios fitted to the e9 that end at 3.64, what I would like to know is:- are the crownwheel and pinion sets from other vehicles fitted with the side loader diff interchangeable? For obvious reasons I would like to keep my lsd and get it rebuilt with a higher ratio. I've had a look on real oem and it would appear that the very early e12 520 was fitted with either a 4.1 ratio or a 3.9 and I have a source but I don't know if the crownwheels are the same bolt pattern and size.
Any ideas? Thanks

 

[Wladek]

You need to open side wall of your diff & see on how many bolts crown gear is mounted on.
In general size of the gears/crown gear in side-loaders diffs are the same, just difference in how many bolts; if it 3.09 it's later side-loader, so it will probably have 10 bolts, but as it is LSD it also might be early 8 bolts LSD case, so you need confirm this.

 

[gazzol]

You need to open side wall of your diff & see on how many bolts crown gear is mounted on.
In general size of the gears/crown gear in side-loaders diffs are the same, just difference in how many bolts; if it 3.09 it's later side-loader, so it will probably have 10 bolts, but as it is LSD it also might be early 8 bolts LSD case, so you need confirm this.

Thanks for your advice

 

[Markos]

As long as you stick with e12’s or e12’s based e24 side loaders you should be good.
The e23/e24/e28 diffs were much different. As @Wladek pointed out, the side loader design matters. Since you are souring from “late model” side loaders you should be good.