I know that it would obviously be best practice to just rebuild the whole thing while its out and apart... but I'm a cheap bastard. I got a torsen differential core for a good price, but it's just the core. I know I would have to remove and disassemble my current open diff, and the transfer the old ring to the new differential core, put it all back together, and then set the backlash at the absolute minimum. But do I need to replace the side bearings and/or races, or go even further and replace the pinion bearing+crush tube? I know I could probably get away with not touching the pinion, but I wouldn't be surprised if I HAVE to do the side bearings and races. At some point down the line (when I finally boost the car) I plan on swapping in an OS giken, and I will do full rebuild when I do that, so I don't want to do 2 full rebuilds in a "short" span of time unless I absolutely have too.

 

If the core that you have already has the side bearings on it, they *should* be the same specifications of the bearings on the open diff. You could theoretically reuse the torsen’s bearings with the old open diff’s races (I’m assuming the torsen’s races were not provided to you, if they were I’d reuse those). Personally, I wouldn’t do this though. I don’t have a great reason, it just makes me feel icky lol

Don’t plan on being able to remove the bearings from your open differential center and reuse those on the torsen center. I had to damage my bearings to remove them.

Regarding the pinion, as long as you don’t affect the pinion nut, you should be able to reuse everything as it is.

This is all assuming that the bearings are healthy and that you are 100% sure you’ll rebuild it again in the future. If I were in your position, I’d at a bare minimum replace the side bearings. If you think there is even a small possibility that you might not ever make the jump to the OS Giken, I would go ahead and rebuild it all.

Why not just go ahead and completely rebuild it now, and then when the OS Giken comes in, then only replace the side bearings? You’d know everything is 100% fresh and good to go when it came time for the next rebuild, but you’d also be 100% fine if you decided to stay with the torsen.

 

Other than just being a cheap bastard, I also don't a have a press or decent set of bearing pullers, so not replacing those parts would also save me even more money because I don't have to buy more tools and/or pay someone else to do part of the job for me. If I already had the tools or didn't need them to replace the parts, then i'd probably just do the full rebuild because its only $300 in parts. but once I add the cost of buying new tools/equipment (or paying someone else to do the job for me) into the equation then the cost goes way up.

The core I bought does still have bearings on it, and they look to be in pretty good condition. And from my research, all 1994-2005 miatas use the same side bearings (same PN#), so I might not "need" to replace them. I have no idea what the races look like inside of my diff, I assume that they are probably fine, but I don't know if I can mix/match races. There could be something with them not being able to reuse/swap them because each bearing and race wears into each other a certain way (kinda like the camshaft caps in the engine).

And what you said about the pinion bearing does make a good argument for doing a full rebuild. I can do it all now, and when I put the OS Giken in, then I can just do the side bearings. Because the pinion bearing honestly looks something I'd leave to a professional, so I would hate to pay that cost twice, but if I can do jus the side bearings without having to mess with the pinion bearings, then it might just be best to rebuild it now. Especially since I have to get new side bearings for the OS Giken anyways.

 

I can’t comment on the bearing race/bearing mixing.

Last thing that I’ll add is I could’ve gotten away with only purchasing the press. I also purchased a medium sized bearing separator/puller before realizing that it wasn’t strong/big enough for the task. Realistically you could just buy the press and rent everything else. I was able to rent the larger bearing separator tool, a 3-jaw puller, and a bearing/seal driver tool. I think Autozone was where I got most of what I needed.

I bought the Harbor Freight 12-ton press, SKU 70604, $170 (I think I paid $150 when it was on sale)

edit: there’s also a really good video on the process on YouTube. I can’t remember the name and since I’m at work I don’t want to dig around for it, but he was rebuilding a diff (like a 4.7:1 ratio or something) for off-road use with a clutch-based limited slip diff other than an OS Giken. I remember the process he used for tightening down the pinion nut/crush washer being “tighten until snug and flush with anti-seize on the threads, back it off and clean everything, then reassemble and torque to 125 ft-lb”. It was like an hour or hour and a half long. He also didn’t measure the side bearing preload but instead got it “good and snug” using a tab of metal in a pair of vice grips. Really useful information and guide. I followed his instructions, but haven’t gotten a chance to drive the car with the new diff yet.