i just recently put a b16 into my 98 ek hatch and i bought brand new 99-00 civic si axles and the passenger one keeps popping out after smashing in 4th gear+ usually and the other day i was just cruising and it came out. i have the locking pin on it and im not missing anything any help will be much appreciated thank you.

 

something is exerting outward force on the axle, or the splines in the trans are worn. Check your ball joints and tie rod ends for play. If it's common that the 4th gear shift causes it, then that gear stack might be misaligned or worn as well, and could be popping it out.

 

possibly the "c" clip is no longer on the tranny side of the axle...make sure thats there, it helps the axle stay put.