325 more door diff on a 320 coupe.

[Epyon85]

Today has had me rather confused, a friend and I are swapping diffs, he has a 325 more door, I have a 320 coupe (m20b25 on 320 box)my diff has gone onto his car without issue however when attempting to put his diff onto my car I have run into a problem.
His diff is the same size as mine, looks identical, but when you offer it up to the rear it won't line up, almost as off the prop is too long and is pushing the diff out of alignment with the bolt holes. Any ideas ?

[Brianmoooore]

I'm a little confused here, since the two cars should have diffs. which are quite a bit different from each other. The 320 should have a small case diff., with six bolts holding on the rear cover, while the 325 should sport a medium case version with eight rear cover bolts.
Either way, the two should be interchangeable, with a slight adjustment in rear propshaft length.
There is a sliding splined section just behind the centre bearing, which should be locked by a rubber lined collar over the splines, which tightens down on a taper as the collar is turned with a large open ended spanner, or water pump pliers.
It looks like someone may have left the collar loose on your friends car, and the splined section has adjusted in length without it being slackened, assuming that the two diffs are different.
It can also be difficult to get the upper two diff. mounting bolts to line up with the holes in the subframe. So much so that there is often a post on here asking if the position of the holes was moved at some time in production.

[Epyon85]

Upon further inspection both diffs have 8 bolts on the rear cover, I can only assume that my car has had a diff a swap at some point, I purchased it knowing it was originally a 320 auto which had been converted to a 325 manual, I was told that the rear diff had been left unchanged. I actually did tighten the adjustment nut to see if I could move the prop to diff flange to give me the clearance I need. It's as if the flange on the end of the prop is protruding from the centre of the rear subframe about 5/10mm more than it should, which is causing the 4 upper mouting bolts to be out of alignment when jacked up and offered to the car with the prop flange and diff flange bolted together.

Strange thing is on the 325, which is a genuine 325 with all untouched and original box, diff and prop, the prop to diff flange sits inside the rear subframe at the correct distance allowing my diff to mount up to it, in fact it has movement both inward and outward to a degree. What I can't understand is if they are both the same diff, why will one marry up and the other won't? Is there some difference in rear prop section length? To be honest I was under my car last night till about 9pm utterly confused and gave up as working in the dark on my drive covered in frost wasn't making the job any easier.

[Brianmoooore]

If they're both eight bolt, then the replacement should be an exact fit. Try unbolting the centre bearing and letting the centre hang down until the diff. is fitted.
Now we know that the 320 diff. has already been swapped at some time for a 325 one, is there any point in this swap?

[Epyon85]

Thats my plan for the weekend, hopefully it will be easier to get the prop to play ball than manuvering that bloody diff in a tight space under the car with t rex arms

The swap is down to two factors, one being i wanted to run some tests on my own drive train and two being his is a welded diff and i fancied trying that at least once in my E30 ownership. Probably regret it, i consider it a stepping stone to an LSD.

Really appreciate the help! ill post once ive had another crack and let you know how i got on.

[Brianmoooore]

his is a welded diff and i fancied trying that at least once in my E30 ownership. Probably regret it,

You will if you have an accident, and find that your insurance is void.

[Epyon85]

I'll keep that in mind...