I have a built b18 on 30 psi maybe around 400whp
my transmission is a 90-93 cable ls tranny and I've shredded 4th gear twice now
what could be causing this to happen a bad bearing, fork, or shim?
should I just run a 5th gear cuff?

 

Your problem is 400 whp... and the torque that comes with it when making that level of power on a non-VTEC B18. A cuff may help, but ultimately, you need stronger 3rd and 4th gears to slow down the frequency of failure.

 

You're making more power than the gears can hold. A cuff won't help, you'll need a 3/4 gear set. Is it the 90-91 trans or 92-93?