I think that 1/4-32 is correct- I dug through a bunch of stuff and came up with my thread gauge- and that is what fits the closest.
One thing that led me to question is that I have one of those cheap 'glow plug wrench' things that looks like a little tire lug wrench- it has threaded spots to store spare glow plugs in the handle of it- I tried a 1/4-24 tap on those threads and it threaded right in with a good fit- felt 'right'- but when I threaded my glow plug adapter that I made with the 1/4-24 tap, the glow plug feels 'wrong' in it.

I think it's one of those situations where it is very close- but not exactly it. That was a trial adapter anyhow- I'm just experimenting with trying to come up with the best way to make a few of them- I'm gonna order a 1/4-32 tap and try again.

Anyone know where I can buy a 14mm die to thread the 'spark plug' end of an adapter? Or know the correct change gears for a mini lathe to turn the threads?

I haven't had the greatest luck starting with a spark plug- by the time I get the porcelain insulator cracked out of it- it looks like I beat it with a hammer- which I did.

Anyone know a good way to remove the porcelain?