my stock EX cat mounted up perfect with my mugen header (2.5 collector) correct me if I'm wrong, then to my apex WS catback for a 99-00 si. It needs to be 2 bolt flange at the header, then 3 bolt where it connects to the exhaust. Please help me find the correct one to purchase. All suggestions considered.

thanks guys

Where's a good place to buy?

 

IIRC the OEM ex/si cat is the same. Are you looking for OEM spec, or want to upgrade? I think the easiest way is to get a magnaflow Carsound cat that's 2.25 In and out and have a muffler shop weld on a new 3 hole flange and if possible re-use the flared flange from your original cat.

 

None.

 

If your stock cat mounted "perfectly" to a header with a 2.5" collector something is wrong.

Here is a photo of a stock civic sir cat next to a jdm itr cat which bolts to a 2.5" collector.