I am swapping an RX-7 GSL-SE rear end into an older Datsun Roadster, and I bought a used torsen differential off c/l. Looked fine, and I swapped it into an open 84-85 big axle carrier. Just got the axles narrowed and installed everything, and now when I turn the driveshaft, the axles reluctantly move but I can hold both axles by hand (with a helper) and still turn the driveshaft for many revs. This happens regardless of which direction I turn the driveshaft. I don't have any gear oil in yet, but I'm not sure it makes a difference. Not sure what could be wrong--I thought it was all mechanical. Any ideas?

 

There should be a bit more backlash than a standard diff but not what you describe. Did you set the crown and pinion tooth contact correctly? They are 26 spline axles?

Sounds stuffed.

 

I think we did, and yes, these are GSL-SE shortened axles at 26 spline. Since I had never set up a rear end before, I figured the worst case is it wouldn't be quite right and it might make some noise that I would see when I got the car running I'd have to take the pig out and take to a professional shop later. But I didn't see how there could be anything wrong mechanically that it wouldn't work at all.

"Sounds stuffed"--not sure what this means.

 

Gears or splines must be stripped for that to happen. I would open the centre up and check.

 

If you want I can check my stock torsen in my 94 miata when I have it in the air next. They are weird and act closer to open diffs than most think.

Depending on the one you bought, the torque bias can vary, that is why you will hear miata people talking about a "tight" torsen or similar terminology (that and gear wear).

Explains the whole thing quite well.
https://www.miata.net/garage/diffguide/

Some more useful info:
Solomiata : Drivetrain : Interchangeand also a video (not my own) to compare what you are seeing on yours.