Diffs... small...large.. and is there another?

[Jos]

ok e30's come with either a 'smallcase' diff or a 'largecase' diff. The largecase diff is also used on the Z3 and some of the 5series from that era correct?

There is also another diff yes? used on the e23 7 series and on the M535i?

This diff isn't compatible with the e30 as the backplate and output cups are different, I know I have one

The question.... is this diff physically bigger than the 'largecase' diff? If it isn't, does the backplate from the Z3M fit it ignoring the bolthole issue.

Reason for this line of questioning is as I have a 3.25 LSD from a 7series, instead of attempting to dismantle and retrofit it into a 'largecase' which it might not fit in why don't I just make the existing case e30 friendly by changing the backplate and cups. The cups I can weld and redrill, easy enough to do.. and if the backplate profile is the same the case can be redrilled and tapped to match.. simple eh..so will it work? Any takers or even a piccy of the rearbackplate would be handy. In the largest resolution possible, break out those digital cameras guys!

[Jhonno]

small case - m40's and most 320's

medium case - 325i's n sports

large case - e28 m535 e23 etc

pretty sure thats right...

[Toby_Unna]

the answer to your question is... yes. it is larger. front four bolt holes line up as you probably know, but as i remember the output cups sit further back and the whole rear of the diff is bulkier.

i think even a full custom back plate would be doing well to reach the e30 rear diff mount, probably possible though. i trial fitted my m535i diff, it was a while ago but i don't think it looked too promising

[Jos]

hmm, cheers for the replies guys, I sorta still want to use it somehow, only has 60k on in and the internals are mint. Plus with the supercharger plan the larger diff would be perfect on ratio(3.25) and be strong as hell.

Just lifted the diff out to compare, output cups are different sizes so would need 325 halfshafts I think, mod the shaft rather than the output cup on the diff. the cups are about 30mm further back and would straighten the halfshaft angle going to the wheels, might be better with 325 shafts but there is enough play in the CVs for it to fit ok. Front propmount is the same, slight shortening of the prop needed but nothing serious. Rear mount.. yeah that's a bugger.. the back of the casing is pretty much inline with the e30 mount bracket, only thing I can thing to do is cut off the e30 one and weld an L shaped bracket in it's place, then mount the diff bush to it, probably will work ok but might not be strong enough for 300+hp. It doesn't seem to foul on anything either which is a bonus. Custom plate I think its going to have to be with double bush mounts... God I am happy my dad has a machine shop..

Not a straight swap for sure, but not to mentally hard either.

Output flange diameters
small case: 86mm
medium case: 86mm
large case: 94mm

Howto mate flange to halfshaft..open to suggestion on this one...

edit: According to the ETK on RealOEM the Z3M uses 94mm output cups too.. so I suppose a Z3M halfshaft+ hub would be the best wayof doing it. Now to find a broken one..