so the tranny in the SE is goin and quick so i have a s5 n/a trans layin around
iv read in the past about changing the tail shaft is this for speed-o reasons or length? i swear i read somthing about it for length but i set them side by side and the s5 is maybe a 1/2 inch longer not even

 

Tail housing, not shaft. I have an S5 NA trans in my FB. You have to swap so the shifter is in the correct spot for the car. The speedo is less important. I can't recall what I did with mine... I may have swapped it to the FC style so it would mesh correctly with the finer FC teeth.

As for the gear ratio, it's probably close enough.

 

^Like he said, its the tailshaft housing (along with all the shifter parts) that gets swapped. Makes the shifter line up and also makes the FB mount work like stock (the S5 mount is oddball)

 

These could be helpful

https://www.rx7club.com/showthread.p...t=transmission

https://www.rx7club.com/showthread.p...t=transmission

Just finished swapping my 84 tailhousing onto an 87 n/a trans for my GSL, the reason is for the shifter like these guys said. The trans is the same length but the shifter is ~2" further back on the FC trans.