Diff casing sizes explain please

[manse73]

How do you tell what size diff is what
I have an 1.8 diff and want to fit a 2.5 diff
What would I need to also change
The car is running a 2.5 engine and manual gearbox

[Brianmoooore]

Easiest way to tell is to count the bolts that hold the rear cover in place. Six on a small case and eight on a medium case.
Nothing has to be changed to change from one to the other. The narrower width of the small case is made up to the width of the medium case by spacers built into the output drive flanges.

[Grrrmachine]

Everything you need is here:

http://www.e30zone.net/e30zonewiki/inde ... fferential

[daimlerman]

Easiest way to tell is to count the bolts that hold the rear cover in place. Six on a small case and eight on a medium case.
Nothing has to be changed to change from one to the other. The narrower width of the small case is made up to the width of the medium case by spacers built into the output drive flanges.

Almost correct!
The propshaft length needs altering slightly at it's centre join,slacken the collar nut and slide the rear half forward about 13mm,re-tighten the collar nut.
Reverse the above if swapping from a medium case diff to a small case diff...

[manse73]

Thank you both. What would be the best ratio to use so it is not revving its nuts off on motorway usage

[daimlerman]

4:1 gives 65mph at 3000rpm
3:64 gves 73mph at 3000rpm

Above assumes manual 'box.

So 3:73 and 3:91 will fall between the above quotes.

[Brianmoooore]

Easiest way to tell is to count the bolts that hold the rear cover in place. Six on a small case and eight on a medium case.
Nothing has to be changed to change from one to the other. The narrower width of the small case is made up to the width of the medium case by spacers built into the output drive flanges.

Almost correct!
The propshaft length needs altering slightly at it's centre join,slacken the collar nut and slide the rear half forward about 13mm,re-tighten the collar nut.
Reverse the above if swapping from a medium case diff to a small case diff...

Propshaft sleeve should be slackened off at EVERY diff. change, regardless of whether a change of type is involved.

[manse73]

Will I need to change the speedo or sender

[daimlerman]

Will I need to change the speedo or sender

No.
The speedo's are all the same,although I think the M3 may differ,and the sensor reads from the output side of the diff and is the same for all E30's.

[manse73]

Again thanks for your help I am now looking for a 3.64 open diff

[manse73]

Will a diff from an e36 fit and if so what would be the best ratio to look for

[capnmchl]

Nope. Different mounting design.

However, I'm led to believe E36 internals can fit inside a medium case diff, although it seems a lot of hassle.