Stock headers won't seal

[Jaxonalden]

He did say a little ol' lady was the last owner. Unless she knew how to properly use a clutch I can see it being replaced in 10,000 miles. I remember one being replaced on a VW in 3,000 miles! This lady rode the clutch so much and it wore it down to the rivets! The flywheel had turned color because of the heat from the slipping.

[Lil bastard]

They are likely leaking from heat warpage. This can happen even to factory collectors, especially if they're not properly torqued.

As mentioned, the solution is to have them decked. But, removing material from the flange will only increase their propensity to warp in the future.

To minimize this, ensure you're removing as little material as absolutely necessary.

Set up a dial gauge and take as many data points as you can, especially from around the ports and only machine the flange as deep as the largest variation. You can use a straight edge and feeler gauge too, but it won't be quite as accurate and you may remove more material than necessary.

[Blue-S]

Are you sure that it's leaking where the flanges seal to the heads? You might have a crack in one (or more) of the primary tubes at the flange. My 2000 S stock header/cat units recently developed this condition, and it's hard to see with the headers installed on the engine. I have no fault codes yet, but there is some noise.

[NoviBlown]

I'm pretty sure it's the headers where they meet the heads. The old gaskets were black all the way to the edges which means exhaust gas was leaking out there.