nolimitmr2

For some reason, my transmission pops out of 4th gear when im driving... it doesnt do it to any other gear. I have an S13 SR20 swap in my 93 240 and it only happened a couple times with a stock clutch. Then i changed it to a stage 1 exedy and it rarely happened for the 3 months i had the clutch, then i just installed a spec stage 3 today and its happened like 3 times in one day. Could the tranny synchro be goin bad on me? Do they sell a tranny rebuild kit for the s13 tranny or do i need to buy a new one? thanx
Dustin