I bought a GTUs 4.30 differential and speed sensor from a rotary shop a while back and my NA engine's coolant seals went before I had a chance to put it on the car. I went to sell it today and we had trouble distinguishing that it was a 4.30. What can I look for to show me that its indeed a 4.30? We spun it and my 4.10 and the ratios looked to be about the same. I also have the speed sensor and can show pictures of both it and the differential if that will help someone. It does have the "differential oil" tag on the case. I also know its an N/A casing, and if I can prove its an s5 viscous LSD, it must be a 4.30 because there weren't 4.10 s5 LSDs, right?

Source: FAQ

Did my 2nd Gen RX-7 come with a Limited Slip Differential?
Only the following USA model 2nd gen RX-7s had a LSD from the factory:
86-88 GXL
87-91 Turbo
88 GTU
89-90 GTUs (however not the 89-90 GTU)

 

Unless someone swapped it beforehand, it should be a 4:30.

 

mark it and spin it. it should be pretty obvious which on went 4.1 ( a little more than 4 turns) and which went 4.3 (more than 4 and a quarter turns) if they both look like they turn the same, you may have been ripped off.

 

What are the ways to tell if its an LSD again?

The diff oil tag on the LSD and what about which direction the axles freely spin?

 

Count the teeth on the ring and pinion, then divide ring number by pinion number and will be the ratio. I guess you can't do that if it's installed in the housing. Just mark the pinion and axle flange, give the pinion exactly one turn and see how far the flange is turned.

 

+1. counting the teeth and dividing will work. do both the 4.10 and 4.30