This is a common problem for the exhaust joint between the centre pipe and backbox. It's exposed to water being close to the offside rear wheel so what I found happens it snaps/cracks off below the joint where it's welded onto the exhaust flange, see below.



The rest of the exhaust system on my car was solid, so this time rather than change the centre pipe I thought I'd try and fix it.
To do this I used a MIG welder and angle grinder with cutting, grinding and wire wheels.
Start by buying a 52mm bore exhaust pipe (Demon Tweaks supply a half a meter section), this will fit over the existing exaust pipe on the car.


With a wire wheel clean up the exhaust pipe to ensure the pipe is solid a few inches away from the joint.


Once you know the exhaust is solid you can continue. Clean the flange by grinding the rust and excess material so you have a good surface to weld with. Ideally it should be as flat and square as possible.


Trim off the centre pipe end by about an inch don't go too far down toward the bend.


Measure and cut a section of new pipe, this needs to fit over at least one inch of the old pipe to seal. Here I used a smaller tube to line up the pieces to get the length right.


Once you're happy with everything weld on the new short section, you'll see where we are going with this repair now.


Use a light to check for any holes in your welding, I have a few pin pricks - better to weld them up.




I cleaned up the spatter but didn't grind it back - it should last long enough like this.


The welding caused the pipe to shrink a bit so I cut some slots into the pipe - don't cut too deep or you'll leave a gap.


The back box fitted I really had to force the pipe to fit so the clamp is there purely to stop it coming lose..

A bit of VHT paint to protect it. I found removing the back box helps while welding and it doesn't take too long.

I did this spot of work 3 months ago and it's held up perfectly fine. I've bought 3 centre exhausts over the years I've owned a 323f and each one has broke in this spot -and each one has lasted exactly 3 years (the warranty period!).

I would say this could be done by most exhaust garages but my guess is they would rather sell you a new exhaust.