On Jun 25, 3:59*pm, "J. Clarke" wrote:
Got the bright idea of using a 1/2" copper pipe union as a bushing and
then realized that I don't have a drill bit the right size. *Looks like
it's a tight 11/16, an 18mm, or a slightly loose 23/32. *

The drill has to match the exterior dimension of the union? That
dimension is NOT CONTROLLED, it can be different from one
to the next.
One I have handy measures 18.6 mm, a little under 47/64 inch...