Miata diff carrier swap - Help!

I have no idea why this is happening, but I can't put my old open diff in the new carrier I bought. It fits inside, but the gears lock up when I put the open diff assembly in the carrier. What is going on?

Long story short, I swapped out my open diff assembly for a VLSD a while back, using my original carrier. I never could get the drivers side stub axle to stay in the diff, and it always had a slow leak. last fall, I ordered a replacement circlip for that stub axle, and the circlip I got from mazda did not fit in the stub axle. I filed it down a little to get it to fit, and installed it, then couldn't get it back out. It still leaks, but now I can't get the stubshaft out. I ran it like this until the gears started whining horribly, and now I just want to swap my old, perfectly good open diff back into the car so I can drive it again. I can't get the original carrier back, because the stuck stubshaft has locked the original carrier to the VLSD I installed.

I bought a used carrier from a friend to use with my old open diff to get my car back together. The carrier I bought is from a VLSD car, but they're all the same, according to everybody on the internets. The open diff drops right into the used carrier, but if I put bolts in it and start to tighten, the diff locks right up. I think the ring gear is binding on the carrier, but how? Aren't the gears in open/vlsd 1.6 diffs identical?

Anyone have any insight? I'd like to get this car back on the road without spending a fortune. Maybe I should just buy a complete diff with carrier at this point, though I hate to spend any more when I should have all the parts I need right here.

To get that stub out - put a pry bar on each side of it so you're applying even pressure to both sides of the stub and not cocking it sideways. Now get a friend to give the side of the flange a good tap with a hammer as you apply pressure to the pry bars. The short, sharp shock will usually pop it free. It doesn't have to be in the same direction you're prying, just the shock does the job. You can do this by yourself, but it's a challenge to handle two pry bars and a hammer at the same time.

I've never come across the binding rear problem. As far as I know, the carriers and R&P gears are identical between the open and VLSD cars.

I know all about that trick. Tried it. Didn't work. I honestly don't think that stub is coming out without breaking something. But that's beside the point, I don't even want to mess with the vlsd unit until I get the car back on the road.

So is there any reason the diff would bind on the carrier? I think I can see marks on the inside where the ring gear is contacting the housing, how could this happen? The gears have never been apart, no damage at all to any of the gears, just normal wear, and they turn freely with no binding or resistance when out of the carrier. As far as I can tell, there's no wrong way to put these two parts together, right?

You could possibly put the whole diff/carrier assembly into the aluminum housing upside down, but I'm pretty sure that would be fairly obvious. Otherwise, I have no idea. There's no gasket missing, so that's not it.

I don't even think you can put the diff into the housing upside down, because the housing is notched to provide clearance for the ring gear, and if you flip it 180 degrees, I'm pretty sure it won't go in. This is really baffling, I'm going to take a closer look at everything tonight and try to dry fit it without RTV before I try it again for real.

If you want my old open diff + PPF to experiment with, let me know. I haven't touched them since we put my VLSD in, so they're still attached. Maybe my parts plus your "spare" carrier could at least get you back on the road.

I'll trade you for your SE-R sunroof deflector...

That might be an option, Justin. Probably couldn't swing that in time to make the event this weekend though.

Ok, so I did a little more investigation. I dry-fitted the diff without RTV, and once again as I torqued it down, the diff locked up. This time, however, I tried to turn it via the input flange, and it wasn't completely locked up. I was able to turn it a little, but t was really tight. After a few turns, it freed up somewhat, but still clearly wasn't right. Here's what I found when I opened the case back up:

So as I previously thought, the ring gear is rubbing against the aluminum housing. Have you ever seen something like this, Keith? The gear doesn't appear to be out of adjustment, the backlash does not seem excessive (as far as I know). There's less than 1mm of play in the input flange with the ring gear held in place. Since I already have this housing, and it's already nicked up, I'm inclined to just take a dremel to that ridge on the housing until the gear moves freely. I know that isn't ideal, but frankly the housing is already blemished, and 1.6 open diffs are almost worthless, so I won't be too broken up if something goes wrong later. Any glaring flaws in that logic?

Also just noticed that the ring gears on the VLSD I pilled out of this housing and my open diff are definitely different. The ring gear on the open diff doesn't have the same bevel as the VLSD gear. Weird.

VLSD:

Open:

May be a dumb question, but did you confirm the ODs of the ring gear is indeed 6"?

Like I said, the open diff pictured is the original diff from my '93 Miata. I have a 6" VLSD stuck in my original aluminum housing. The housing in the pics came from my friend's '90, (he went torsen when he blew up the 6" R&P). As you can see in the pics, the VLSD that came out of my friend's '90 has an extra bevel machined into the back of the ring gear - 3 angles on the VLSD gear, 2 angles on my old open gear. The peak on the back of the gear is what's contacting the aluminum housing, you can see the marks on the ring gear as well (much more faint, since the ring gear is much harder metal, so that's not obvious in the pics).

I guess this throws out the theory that all 6" miata R&Ps and aluminum housings are interchangeable. Just surprised I couldn't find ANY record of anyone else on the internet who's run into this issue, especially considering how many cars have the aluminum piece break in accidents and subsequently need it replaced.

There was a change to the rear end housing design in mid-1990, with the addition of cooling ribs. Could it be that the ring gear was changed at the same time with that extra bevel? Obviously, the rear with the extra bevel would fit inside one of the non-bevel housings, so there's only a problem if you try to go the other way.

This sounds reasonable. The original diff cover was finned, and this one I am trying to use isn't. Seems like they stopped cutting that bevel in the gear because the finned covers don't need it for clearance.

So given my desire to get all of this back together without spending any money, does taking a MM or so off that ridge with a dremel seem like a terrible idea?