Diff
 

Wondering what other models & years came with a 4.1 LSD? I have one on my miata and was also wondering what the baseline pricing on a used is?

I understand its 84 thru 85 is that correct?

S4 - FC TII came with Clutch type LSD's that fit into the FD housing
S5 - FC TII came with the same type as the S4 but they dont fir into FD
not sure the gear ratios on the FC's

FD's came with 4.10 gears and torsen type diff

84-85 ( US ) is a 26 spline short nose"medium" diff

the FC NA diffs are also similar to the miata ( long nose medium, 26 spline )
auto is 3.9,, manual is 4.1

in some markets the original rx3 could be found with a long nose 4.111
( no typo,, these are different ring counts )

and short nose 4.1 ( open ,, 24 spline ) can be found in the piston cosmo/rx5 equiv,, the mazda 121

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the FC turbo diffs are long nose "maxi",, 30 spline diffs
s4 is clutch type with equal length stub shafts
s5 is viscous ,, with unequal stub shafts

s6/FD is equal length stub shaft torsen ( short nose )

again,, auto is 3.9,, manual is 4.1,, and upsecs can be 4.3

the s4 FC turbo clutch diff pumpkin hemi is usually fitted with the FD pinion and ring gear and put into FD carrier as a rebuild able upgrade

other sources for the short nose 30 spline 4.1 in some markets include the ford courier series and its brother bravo /b2000/2600
you need to be able to interpret the years and markets well to get both 4.1 and LSD
( easy to find the 4.1 in some early b2000,, and also a certain diesel b2500 will donate both LSD and 4.1 )