Purchase a Metal hole cutter from screw fix with a centre drill (aprox £20). Look for the type designed for Metat/Cast Iron.
Use a variable Drill and after drilling a small pilot hole govern the drill speed to aprox 75 Revs and apply moderate pressure. You will find the hole cutter will cut a clean hole. If you are putting a collar onto the Stack pipe for waste feed I recomend a 64mm collar but this of course depends on what you want to achieve
If you got to Plumb Centre they would advise.
RayH, September 2009
