I almost forgot to show the images of a
very important tool mentioned in the posts by Eric and Tyson.....
You won't find this modest little tool in any Porsche catalog, as it was custom made.
John Walker brought it along and used it when a (short) stud had to be replaced. Naturally, the stud refused all attemps to turn out. Last resort was to break it off, which Tyson did very neatly without much more twisting.
The drill jig was then bolted in place. You'll see from the images that it has a circular, stepped area that fits right into the exhaust port. This centers the jig precisely for drilling.
The jig also features a hard steel bore for the drill. It kept the drill bit running straight and true.
Tyson will be the first to tell you how hard the stud was to drill out. He used a sharp bit, air powered drill, and a *lot* of muscle.
But the final result was a perfectly straight and centered hole.
Although John had brought some Time-Serts, they were not needed. The remnants of the old stud threads were picked out of the hole, cleaned up with a tap, followed by the new stud.
If you ever do this job, remember your chance of drilling out a stud freehand would be pretty slim, so investigate having a similar jig made up by a machinist.