I've got a 95 with an open diff. I know the car was rear-ended early on in its life as it's a salvage title and theres different color body parts in the rear (deck lid is blue inside and the the upper strut mount areas are red). back to the topic though, I'm trying to identify my diff. I have a finned housing but my third member has the four ribs on top like the one on the right in the top pic here:
http://www.miata.net/garage/diffguide/index.html

is it possible I have a 7 inch housing with a 6 inch third member shoved inside? I did some searching but I can't find any info on if this is even possible.
also my axle stubs are square, as in they don't have any tabs.

 

You can't combine 6" and 7" pieces. A 6" diff could have been swapped in though. The fins don't tell you anything, both came with fins (not all 6" are finned). The stubs with out extra tabs means its a 7" according to the page you linked.

 

Ah I geuss I should have been more clear. My stub shafts don't match any of the options on that site. They don't have any tabs at all, like a square with rounded off corners.

Thanks for the info though on the 6 inch in a 7 inch housing. All I could find was info saying it isn't possible to go the other way. Which means I have an early rear end :( was hoping I got lucky and could just buy a 3rd member and call it a day.

 

Sorry, I meant to say that a complete 6" w/carrier could have been swapped in.

If it doesn't have any tabs it should be a 7". If you look at the stubs for the 6" (1.6L) they all have extra tabs. So if yours has none you have a 7", which is what you need if you want to swap in a Torsen. You can use any of the axles (stub or single piece). The exception to that is if you get a MazdaSpeed dif you need the axles that go with it.

What are you trying to do?

 

So after doing some more research, I definitely have a 6" diff :( I have a rather annoying whine while engine braking which I am fairly sure is the rear end. I was hoping to just be able to pick up a torsen third member and fix two problems at once.

Just to be clear you said the 6" stuff can't be mixed with the 7" stuff, so that means I will need new half-shafts (or stubs) when I do the swap to 7"? Actually I should be able to find that answer else where. time to search

Edit: new axles too :P

 

Thats lame.

Yup, don't forget the drive shaft. I think there is a walk through at miata.net in the garage section.