New Air Vents in Old Radiators

jjjessek

I have some old radiators in my house, and in the course of removing a bathroom I had to drain the system.  The problem is that they do not have air vents that I need to refill the system.  They do have plugs, but these have resisted all attempts at removal by impact drivers or by screw excractors. 

My question is whether I can tap out threads in a new place on the radiator to screw new air vents into it.  The plugs are in the end of the radiator on the left side, and I am thinking of tapping out threads on the same end on the right side. 

Is this safe?  The cast iron where the plugs are is a little thicker than the metal in other places on the radiators and I am worried that the pressure in the system might cause problems if the hole that I would drill in it compramised it.

Any thoughts?