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. Anybody know
offhand which one is right?

Will depend on whether it's M or L. Can't just is mic it???

--