I am converting a HW Radiator into a one-pipe Steam Rad. I need to add
a vent valve. I'll need to drill and tap a hole. The tap wants a
21/64" hole.

I am wondering if someone can advise me on some cast iron drilling:


Is it better to drill a pilot (or several gradually larger pilots) or
go with the final size from the outset?

I use Cobalt bits - is this OK for cast iron?

I have a variable speed drill. What speed should I drill at?

Any advice is appreciated. Thanks!



Michael Stoic