Project TMB Build Diary 5: DIFF-appointment


While I had the engine and transmission out and the car sitting on jack stands, I decided to give the car a through going-over. Since it wasn’t going to be driven, I figure now was as good a time as any. One of the things that had bothered me on that initial drive home was the fact that the diff was welded. Now let me preface this by saying, I knew when I got it, that the diff was welded. It wasn’t something I was surprised with and the previous owner told me he did it, because drift car. He also provided another diff that was in the trunk, that I could swap out if I wasn’t down for living the welded diff life. No problem, I had a spare if I didn’t like the welded diff, sweet deal.

At this point in time, allow me to share a bit of history about the 10th Anniversary Edition Miata, that will shed some light about the discovery that I found when I checked the diff.

The 10th Anniversary Miata History Lesson

The Mazda 10th Anniversary Edition Miata Sales Brochure

In 1999, Mazda celebrated the anniversary of the release of the original NA Miata (1989) with a limited number of Sapphire Blue (a one year color by the way) painted NB1’s, all of them loaded options-wise (power steering, power windows, air conditioning, Torsen-II LSD 3.90 geared rear differential, chrome 15×7 wheels and a Bose stereo) and with a six-speed transmission. What made them unique was that the Torsen used in the 10th Anniversary cars was a 3.90 geared unit. Later Torsen-II’s are either geared 4.10 or 4.30. The 3.90 is very sought after, especially in boosted applications.

Now back to our story…

I distinctly remember the previous owner telling me that the rear diff was welded. I remember him saying that there was another diff in the trunk, in case I didn’t want to use the welded diff. Where this little tale starts going sideways is where what I ACTUALLY heard was not what I THOUGHT I heard. Allow me to explain.

What I THOUGHT I heard was that he removed the Torsen-II diff and replaced it with an open differential, that was then welded. The diff in the trunk, was in fact, the unmolested 3.90 geared Torsen-II diff, that I could just swap back in and everything would be sunshine and unicorns. That’s what I thought I heard, or rather I HOPED that was what I heard.

But it wasn’t. Because when I pulled the axles out of the diff and shined a flashlight to see inside the diff, I saw not the usual bar of an open differential, but instead the sight of a Torsen-II. A WELDED Torsen-II. For those not in the know, Torsen differentials for the Miata fetch a pretty sizeable amount. Since they can be bolted into either a NA (1.6L w/axles and 1.8L are a straight bolt-in) and a NB, a Torsen swap is a popular upgrade, especially if you are interested in racing or boosted applications. While an open differential (one tire fire) can be had for around $250 – $400 (including the housing), Torsen and Torsen-II diff carriers alone will fetch close to $1,000 (with the housing $1200 or more), with a 3.90 geared Torsen at the higher end of that spectrum (due to them only being used in the 10th Anniversary (’99) and Special Editions (’00-’02).

So yeah, my Torsen diff was welded. At this point, I should have just swapped in the open diff or went on eBay to hunt another Torsen. But thankfully, I already had a backup…

In Case of Emergency, Always Have A Backup

Strap in folks, this is about to be a long one…

Picture it: My garage, during the Covid Pandemic of 2020.

As I was at the beginning of the build, I was hoarding parts. The original diff in the NA was an open, but when I bought the turbo parts from a guy on FB Marketplace, he had a Torsen diff that he threw in the deal. What I got was this rusty, bright orange diff. I decided to tear it down and replace all the seals and apply some new sealant. What the ended up being was a nightmare.

Service your compenents, people!

The bolts were seized, covered in orange paint, or just outright snapped. Upon further inspection, the diff carrier itself had a crack, that the previous owner had used a lot of sealer on. But I eventually got it free and it is definitely a Torsen!

An open differential has a bar in the middle, see that round opening? That is what makes this a Torsen.

Fast forward: Months later.

Thanks to some luck and some savvy shopping, I was able to snag not only another Torsen-II differential but a clean housing with all the bolts as well. That new differential was then modified further with a set of polyurethane bushings and new seals.

Back to the present.

Since the NA is now gone, I had this 4.10 geared Torsen-II in need of a new home. There was nothing that needed to done to it (I already added new seals and sealant) so the swap was easy. I unbolted the welded differential from the car and replaced it with this one. It really is a simple remove and replace since it is basically the same part. As for the welded one, I still have the orange diff and I can transfer the diff to a new carrier. That would allow me to have a backup for the installed one just in case of a problem.

New differential going in!

The new differential is in and that is one less thing I will have to be concerned about when I finally get the car running. Thankfully I have a 4.10 speedometer gear for the transmission when it gets back in the car, so my dashboard should show the correct speed too. This project is starting to shape up!