In a bmw header or exhaust manifold, A header for bmw e46 323I 1999   is  diifferent   for a header bmw 325I  2002-05 due to its flanges are in a different configuration.If you did put a performance header of a 325I to bmw 323i  the flanges are really way off the screw and the position of oxygen sensor are gonna hit the underbody of the car.   .As you can see this  is a bmw 323I e46 1999 car .With a wrong header for a different bmw model car.Always make sure the orientation  or the position of the flange are in the correct position or else this will be a nightmare for you.This is a real pics of a wrong header for bmw 323I due to the orientation of the flanges and oxygen sensor position is wrong for this BMW 323I 1999 or 328I   Study properly the orientation of the flanges as they will not fit your bmw 323I 1999 models.All bmw e46 model has 4 oxygen sensor ,so you need four hole position sensor in your header.