Hmmmm...that sort of looks like it's just a press fit. My F250 is like that.

 

Exactly...it was, anyway. The exhaust leaks over the years have scavenged cast iron off the manifold and wont seal against the bell on the pipe. Realize the exhaust is super hot CO2 and water vapor. That water vapor is hard on iron.

The steel can seals on tbe outside of the manifold outlet.

A closr look at the picture shows the exhaust pipe flange wasnt made correctly- the contact area around the manifold isnt even.

 

IMHO there should be a donut. It has the added benefit of accepting some out of angle between the manifold and the Y-pipe.
Had once sanded a new round onto such a pipe-end. piece of wooden plank, sewing and cutting to fit into the pipe with a hook kind of thingy with the proper shape and sanding paper tacked onto the side. just turning it round and re-tacking the sanding paper a couple of times did the job.

CAREFUL!
Whenever you do sanding or work in an exhaust manifold make sure you remove "ALL" the debris, grit, etc from it before starting up the engine. You risk debris and worse, abrasive particles being blown back into the cylinder(s) by the gas fluctuations.
No compressed air but vacuum with some smaller hose attached can work. Best is to remove the manifold.

 

YES. Absolutely never let sanding grit in there, it can be sucked in on reversion...Its a wonderful myth that air only flows one way thru an engine. Back- flow is why blowers explode on race cars
..

As fast as it destroyed the sandpaper, I just gave up trying to sand it I'm guessing the heat and carbon have hardened the surface. It acted like tool steel it was so hard.

NAPA didnt list a donut or gasket. Yes, it should have one. It may be possible to find one that fits up inside the manifold and has a dome on the pipe side but I have no way to determine what donut to use. An exhaust shop might figure it out.

As soft as the new donut was, it may be possible to cut it to fit the dome on the manifold. I assume the material hardens with heat?

 

Could be. That would depend on the material the manufacturer uses for the donut. But it may get some tempering with the heat treatment.

 

Yes, its hardening. Went from blue to blue-grey in maybe 20 miles. It's harder now, a screwdriver broke off a tiny bit of the material that crumbled away. So in theory, a new one could be remolded or cut into another shape.