"Louis Ohland" wrote in message
...
I'd like to bore a hole approx 2.75", I have a boring head from APT
which will do it, but cranking the slide over far enough to use either of
the vertical bar mounts leads to significant vibration on my Sieg X3.

Can I use a horizontally mounted boring bar in order to reduce the
rotating offset mass? .500 inch round shanks in a 2.5" head.

What cutter are you using? A HSS with the right grind and light cuts should
do it.