A cat-back that has been on a car and through heat cycles is no longer the shape it was in the box. Its front flange has taken a set, its hangers have stretched into a particular hang, and its pipe has grown and shrunk back into whatever position the mounts allowed. It will not return to the drawing.
A new downpipe, by contrast, is made to a nominal geometry on a bench. The two are being asked to meet at a point where one of them is defined by a drawing and the other by the car it has been living in. That mismatch is the whole problem, and it is not a quality fault in either part.
A flange does not warp evenly. The face nearest the weld and the pipe runs hotter than the outer edge, so the two regions expand by different amounts and the face cups slightly. When it cools the cup partly remains, which is why an old flange rarely sits flat against a straightedge.
Put a straightedge across the face and look for light at the middle or at the edges rather than measuring. A face that rocks on the straightedge is dished; one that shows a gap only at one edge is twisted. Both change how much the bolts have to pull to close the joint.
A gasket takes up surface roughness and a small amount of misalignment. It cannot take up a face that is out of flat by more than its compressed thickness, and it cannot correct two flanges that are not parallel, because closing one side opens the other.
This is where owners reach for a thicker gasket and make things worse. A thick soft gasket under a face that is not flat creeps, blows out at the thin side, and leaves the joint looser than it started. If the faces are not flat, the fix is at the faces, not in the gasket.
Only one end of the junction should be adjusted. The downpipe is the new part and the one you can still work on, so it is normally the end that gets fettled, whether that means relieving a hole, dressing a face or shortening the pipe a little. The old system stays where the car has settled it.
The exception is a cat-back whose front joint has visibly failed, with a dished face or a cracked weld at the flange. In that case the old part is the fault and dressing the new one just transfers the distortion. Judge by which face rocks on a straightedge, not by which part is newer.
Assemble from the front, leave the new junction loose, and let the rear of the system hang on its own hangers before anything is tightened. Then bring the junction faces together by hand and see whether they touch evenly. If they only touch after the bolts pull them, the faces are fighting.
Where a connecting pipe has enough slack, a small rotation at the slip can take out the twist and let the flange faces sit parallel. Tighten the rear mounts first, then the front, and recheck the faces after the first heat cycle rather than before it.
Some combinations are simply built to different geometry, particularly when the cat-back was designed for one version of the car and the downpipe for another. If the flange centres are offset, or the pipe runs at a different angle, no amount of dressing will make a joint that stays sealed.
The honest repair is a short transition section made to bridge the two, welded to the downpipe so the stainless system behind it keeps its own geometry. That keeps one joint, in a place you can reach, instead of three adaptations stacked where the pipe is hardest to get at.
