I'm not sure exactly how accurate you need to be, but one time I needed to
do something like this, I turned a small piece of threaded rod down smooth
on one end in the lathe. I screwed this rod into the threaded hole and
indicated off the turned end.

To mount the threaded rod for turning, I placed a piece of bar stock in the
3-jaw chuck, and then bored and threaded it to the same thread. I then
tightened the threaded rod into this simple jig to hold it centered in the
chuck....or course doing all of this without unchucking anything.

I'm sure this isn't an exact method...but it worked very well in my
case...especially since the threaded hole I was working on was kinda small.
Not sure how well it would work for a larger hole like yours.

Todd

"Steve" wrote in message
om...
I need to indicate an internal thread. It is a 2 1/4-8 thread and it
is on a backplate. Someone bored the register off center and I need to
bore and resleeve the backplate but the register bore has got to be on
the same centerline as the thread to make this work. I have the back
plate set up in the 4 jaw now I just need to figure out how to dial it
in so the bore and the thread centerline line up. Any suggestions
greatly appreciated. Thanks, Steve