BMW differentials Help

Hello
I have 2007 e87 120i 1 series petrol. It needs the rear differential replaced,
I’ve looked around to find a reasonably priced differential but can’t seem to find one. Does it need to be a specific spec? Or can I use one from another e87 1 series even if it’s a diesel?

Quarry motors in Sheffield are an excellent specialist BMW breaker, even if they don't have one they will be able to tell you compatibility.

Yes, 123d is also different again...!

I think the small 4 cyl petrol engines have something like a 3.7 or 3.9 ratio, but my E8* knowledge is fading away in the memory cells now.
If you go on RealOEM, and punch your VIN number in it should tell you the diff part number you are looking for (should be on casing) and what the ratio is.
You need a diff from a petrol 1er, NOT a diesel 1er.

All 4 cylinder petrol E8* 1 Series as well as E90 318i and 320i use a small case 168 diff. They have the four bolt prop flange. Diesel ones don't fit. They are bigger diffs with three bolt prop flanges and different driveshafts etc.
They fail because the pinion bearings aren't really up to it. Well, some are and some aren't and it's down to the bearing supplier at the time.

A 2007 120i manual with the N46 engine will have a 3.45 diff. They are hard to locate but a 3.38 will do. Automatics have a 3.91 diff and good luck finding one of those. You can get away with a 3.73 or a 4.1.

If your 120i has an N43 engine then you'll have a 3.73 but a more common 3.64 from a 116i will do.

BUT....

From late 2007 or early 2008, these and E90's have auto stop start and you must fit the correct ratio. This is because the gearbox has a speed sensor (to tell the engine ECU the car has stopped) and if it senses that the rear wheels are not turning at the correct speed, the car will just go into safety mode - reduced power, dash lit up like a Christmas tree etc.

I've got a few diffs kicking around and definitely have a 3.73. I had it earmarked for a job which probably won't happen.