diff question
 

I am trying to buy a diff from a guy for my fd. Hes not sure if it is the 3.9 or the 4.1. Its not stock .it was rebuilt with the clutch style limited slip. Is there any way to tell the differences in the two?

spin the axle around in the diff once. If the driveshaft flange spins around just less than 4 times it's a 3.9, just more than 4 times it's a 4.1

^^thats probably the best way to find out

you can count the teeth on the ring gear and divide it by the number of teeth on the pinion and that will tell you what gear it is.