Thanks for pointing out some things I haven't considered. I like the idea of matching the internal and external threads of the insert. If I do a threaded insert that is definitely an idea to use. It wouldn't be hard to do I think, just use the same pitch for both threads and start the threading at the same point.
Nick,
I have read that post before, and Bob would be happy to know I agree with him. It would definitely be best to avoid machining out the threads if possible. I have personally never seen in person one of those magical Moto Guzzi exhaust rings, they must be made to a better fit then the Ducati ones. That's easy to believe because I've measured what looked like nice Ducati ones and they seemed pretty small. It could be that a major contributing factor to the exhaust thread problem is poor fitting rings. I was working on that as a solution to fixing one of my heads first in fact. As I said above " I do have a short piece of aluminum bronze, enough to make two inserts. I was going to make an oversized exhaust nut with it. One advantage with the CNC mill and lathe is I can make any thread I want."
I also considered plating as you mentioned, as a way to make the rings a better fit. Another option I considered is a custom thread pitch. With a CNC lathe I could do a 48x1.51 pitch for example. It would take some figuring but it would be possible to make a nut that was self locking, where, when bottomed out the difference in pitch was matched to the slop in the threads.
My cylinder heads threads were pretty bad almost totally flat on one side, but the rest of them were OK. My brother had already bought a 48x1.5 tap so I used it to clean up the threads as Bob suggests. They cleaned up ok, marginal overall but I think would have worked. I made one test piece first at the standard thread dimensions, It was a much better fit. This is where I screwed up, I was making another test piece at slightly oversized dimensions and I think I had dialed in the fit really well BUT that is when I got my aluminum test piece stuck in the cylinder head. It threaded in perfectly by hand, no slop nice and smooth. But in my rush to finish what I was doing I didn't think to coat the threads with any kind of anti seize!! Lesson learned
If I had been using the final bronze piece it would have been fine. So getting the piece unstuck from the head made the threads that were marginal no longer viable. Aaron