The last two threads or in some cases more have to be removed because it is not possible to thread the port all the way to the bottom with a complete thread. So by removing the first couple of threads it allows the spigot to be screwed in without the threads bottoming out where the internal thread stops and this lets the end bottom out at the base of the port.
The new spigot I bought today has maybe 1 thread machined off the end. After inspecting the threads in my port, it looks like I'll have to machine 3 threads from the spigot. I'll measure it up tomorrow to be sure before I chuck up the spigot in the lathe. Guess I should take some pictures for the resto thread so this is clear to everyone asking about this in the future.
By the way, I thought about the requirement to bottom out the spigot in the ports. I'd never given it much thought before and figured it was to prevent hot gasses from impinging on the threads and possibly erode the threads from behind. But when Mr. Pete said it would beat out the threads, it all made sense. If you tighten it until it binds just on the end of the threads. It is fully supported only by the thread alignment and really only held tight by the last thread. If the exhaust is subject to vibes, it can, and based on Mr. Pete's comments, will eventually move the spigot side to side and once that happens it will continue until the threads are obliterated. But, if you bottom the face of the spigot on the bottom of the port, you now have a firm square base against which the spigot is held tight by the entire length of threads. Plus, all the threads are loaded against the flank of their back side. This puts both opposing lateral forces and compressive force on the spigot by the port. So, the ability to loosen the spigot and cause the spigot to move side to side is greatly reduced and apparently totally eliminated. Since I have one bottomed spigot that is rock solid in the head and the other bottomed on the threads only and was found to be loose, I've got real life proof right in front of me.
Last edited by Snakeoil; 12-12-2012 at 10:22 PM.