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I'd like an LSD for my 318i and I have access to a 3.45:1 ratio LSD for very cheap. I figure that's way too long a ratio for my little 120hp M10, but it seems a shame not to get it. Is there any way to combine parts from a standard open 3.9:1 diff with the 3.45:1 LSD to make a 3.9:1 LSD? Or any other way to get what I want using the 3.45 LSD?

cheers.

Sure. Never change the gearing, but put the lsd-core in your 3.91 differential.

This involves getting the core out, unbolting the crownwheel and bolt it on to the lsd-core which you got out of the 3.45. Then put the lsd-core with 3.91 crown-wheel back in your diff and pray the backlash will be fine. In theory this should'nt be affected, but if it's 0.001 inch offset you might end up with a wining and tooth-eating differential.

Thanks! Good to know it's possible.
Forgive my ignorance, but what do you mean by 'backlash' and a 0.001 inch offset?
What are the chances of ending up with a 'whining and tooth-eating' diff?
I don't really want to start with two working diffs and end up with none.
Do you know if anyone has made a 'how to..' thread about this?

backlash is the amount of play in the gearset. Its basically the space between teeth on the gears. Changing components changes this, if its out of spec you either have too much in contact with each tooth or not enough. One can cause premature wear because there is supposed to be a small amount of backlash to allow for cooling and lubrication, the other can see you wear the gears in a different place, quite often even damage them. End result is a noisy differential with a shortened lifespan. Because you havent touched the pinion gear you dont have to worry so much about preload too, but if you havent done this sort of thing before and you want to make sure everything is ok, you are probably better off taking it to someone. If you need to change backlash you need to alter shims and that sort of thing, then check it again. There are instructions in the e21 service manual (big blue folder ones).

I did a diff this way and had no issues.
I did check the backlash before it was used.

Before putting the backplate back on, apply some engineers blue to the crownwheel. A few teeth together.
Rotate the differential using the prop input in one direction at least one full turn of the crown wheel.
Inspect the blue impressions and compare. Pictures of what these should look like can be found from many sources.
Repeat for the other direction.

If your not comfortable with this get a transmission company to check the diff for you after its built.

Ben

That's awesome, guys. Been a great help, thanks!