Posted this in the civic forum but had no luck

Well today I was driving and my car started making a super loud noise..i popped the hood and my header had a hole in it.(i have a b16a) Now I found the header I want but the problem is that my apex'i ws2 is 2.25". The header I want has a 2.5" collector..am i looking at this wrong or is there something I can do to use that header?

 

If the header has a 2.5" collector flange(like most of the JDM spec ones do), then you need a cat with a 2.5" flange.

 

yea but won't the other side of the cat...which bolts to the exhaust(the 3 hole part) not fit because the piping is 2.25"

 

Just because a cat has 2.5" flange to the header doesn't mean it has to have the same flange to the exhaust. IE, you can have a cat with a 2.5" inlet flange to match the header, and a 2.25" outlet flange to match your exhaust. You can do this by either having a custom cat welded up, welding a 2.5" inlet flange onto your stock cat, or some companies offer 2.5" to 2.25" flanged cats for JDM header applications.