Be careful where you put those cats! Make sure there is ample heat shielding above where you integrate your cats (heat rises). Would hate to see you engulf your cabin in smoke from smoldering carpet backing. The flange area looks risky to me.

Re the studs, don't crank them in too tight or you risk cracking the head. Just crank them in until they don't move anymore under modest pressure. Visually gauge the length of engagement and don't force anything.

If your VW is smelling awful with a failing cat, it's running rich, and that's what's probably doing in the cat (cats don't have a service life on a properly calibrated engine). Remember, all a cat does is accelerate a naturally-occurring reaction (oxidation). That smell you refer to is unburned fuel going out the tailpipe. Complete, un-catalyzed combustion hardly smells.

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2012 xDrive35d
2001 BMW X5 3.0i 5MT
and two 1987 Corvettes - Callaway Twin Turbo and Guldstrand Grand Sport 80
Block Tester