Peter Jason wrote:
...
http://www.mk-diamond-saws.com/image...tter_small.jpg
http://ignitiontools.com/auctionpic/...E-CUTTER-1.jpg
http://www.trilug.org/~chilcote/imag...e-cutter-s.jpg
http://zenstoves.net/Supplies/CircleHoleCutter.jpg

All of these tools are meant to be used in a drill press, for the
stability. I have used a circle cutter in a hand drill, but it was a
much smaller hole (4" diam) and it was tricky. If you do it, be very,
*very* careful. Use the very slowest speed that you can and keep a very
tight grip on the drill. The problem is that the bit will grab and
bind. Depending upon how flexible the boiler surround is, it may very
well be impossible with a hand drill.

Bob