Miata 1.6 Limited Slip Differential

David, are you looking for the viscous limited slip that originally came with the 1.6 Miata? If so, there are some on eBay. I also see them pop-up on Craigslist. You might also try East Street Auto.
Joe, Mazda had an OEM limited slip for the 1.6 along with the open diff. and viscous limited slip. I'm trying to help a friend find an OEM limited slip.
David, as far as I have seen, the only OEM LSD that came with 1.6 Miatas was the viscous one. 1.8 cars had the option of a torsen LSD. This has a 7-inch ring gear, as opposed to the 6-inch that came with the 1.6 cars. The later diff can be installed in early cars, but a complete diff unit will be needed along with a driveshaft and axles from a 1.8 car.
Joe, I'll do my best to convince you that Mazda Comp 1.6 (6 inch ring gear) limited slip (NOT the viscus limited slip) does/did exist. From memory the first item one could no longer purchase from Mazda was the limited slip housing with the internal guts. The second item one could no longer purchase from Mazda was the limited slip internal guts. There should be Mazda Comp limited slip differentials out there. Why, because MANY folks who race 1.6 Spec Miata converted to the Miata 1995 plus torsen differential with a 7 inch ring gear.

Included are two videos which one shows and one talks about the Mazda Comp 1.6 limited slip differential with the second video showing the Mazda Comp limited slip at approx. 4 min. 34 seconds.

Video by long time racer Mark Bennett. https://www.youtube.com/watch?v=mZWFB9D7WQo

Video by long time Differential builder. https://www.youtube.com/watch?v=qlmkmrDbLPI

Maybe along the way Jesse Prather or someone else with knowledge will notice and confirm.
David, I sit corrected, but not OEM, just Mazda Comp. I just looked in a 1997 Mazda Comp catalog. It shows:
Part # T0Y1-27-200
Description: Mechanical L/S
Model/Engine Type: Miata 1.6 1990-93, 323 GTX 1988-89
Notes: Easy bolt-in fitment to non-viscous limited slip
Miatas. Replacement of viscous-type
differential requires changing output shafts
(Part #T005-27-270B).

I raced on an enduro team in a 1.6 spec Miata with a viscous LSD. The car had gone about 25,000 track miles before that LSD blew up. They can work. We should have better monitored the pinion play. The crush spacer likely crushed down over time allowing the pinion to float around. 8 hours into a 24 hour race it decided that it had enough. We had a spare center section, but when it blew up it took out the aluminum housing and that finished us. I'm not so sure that a clutch type would have held up for that many miles.
Big issue with the 1.6 differentials I've heard is shock loading them-- especially the Torsens-- but any of the 1.6 are a little weaker than the larger 1.8 diffs. if you have a driver that puts a wheel off or bounces off curbs and stays in the gas, the shock loading on that diff can take it out in relatively short time. Smooth drivers will of course make any car last longer, but the 1.6 is a known weak link in the Miata world.
All of the endurance cars we played with replaced with 1.8 hardware and never looked back.
Had my last 1.6 mazda comp limited slip years and years ago. Their like hen's teeth.
Odds of finding one are nearly nil. You never know though!
Made me look, have one complete with 4.30 gears and housing from my original spares.

maybe you need a spare...

Don Tucker
EP Miata