I have a b20b DOHC with an Competition Stage 1 Cluth and a Ultra lightweight flywheel. Everytime I switch in 3rd at a high rpm, it sounds like the gear is grinding. The rest of the gears are frin though. I was thinking that I should replace transmission to an ITR . Is it possible and what would be the best route.

 

B series transmissions have quite a reputation for developing a grind during the 2-3 upshift, so know that you're certainly not alone in your struggle.

An ITR tranny will fit and would probably be a very notable upgrade.