Michael Stoic wrote:

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?

Cobalt is top of the line, no problem with soft cast iron. If they are
split point (4 cutting edges on tip) and you have a starting dimple you
should be able to drill easy enough if you use some kind of lubricant
and not too fast. Split points often drill better without a pilot hole
but you do need to keep the workpiece stable. Very curious, what size tap?

Frank