Like the thread states...

Working on gradually getting my R53 back to road worthiness and did a head gasket and lots of misc stuff over the past couple of weeks, but when I was tearing things down, I noticed that the exhaust manifold was loose at the bottom, so I removed it and the upper part of the precat (attached to the header) slid right off of the lower part (attached to the cat). The top part is just sort of a bell that overlaps the body with the flange at the back. Are these press-fit together and mine just separated? Did not look like there were any welds on the inside that were broken when I looked at it more closely - just looked like it was supposed to slide on - but it would leak like a sieve were that the case, so assuming it's broke... My phone was dead when I had it apart, so no pics...

 

It's a common breakage point - it should be welded solid.
If it's off the car it's easy to do but be sure the alignment is correct.
There are other threads here somewhere with photos.
A complete replacement is all one unit & around $1,000!

 

They must be tiny little welds, because I could see zero evidence of the thing having ever been welded! Think I'm just going to go with an OBX header then. The bottom part look like it's in pretty good shape, but I live in ND now, and there are pretty much zero regulations here on emissions, or anything else car-related. No inspections, no emissions... just don't tint your windows too dark! The back end is already a Borla sport, so should do well with the header. Thanks man.

Aside: I'm from MA as well - my car lived there for about 6 years before moving to southern CA and then up here. The salt really did a number on some of the undercarriage connectors. They don't salt the roads up here either!