On Mar 3, 1:00*am, Don Foreman wrote:
...
You could measure it. *

Put a sharp V threading bit in the toolholder and stick a DI on the
bed with a magnet or whatever....

Or mount the DI in the toolpost, set the lathe to thread and see if it
tracks the spindle thread pitch. I would unplug the power cord first.

When you have it set to follow the spindle thread, clean up the
backplate thread. It may be correct but tight. I have an old South
Bend chuck that won't quite screw all the way onto my lathe.

jsw