Plug a smog hole

JBurch said
Go to the hardware store, get 8 1/4-20 set screws 1/4 to 3/8 long, buy a 1/4-20 tap while you are there unless you already have a tap and die set. Tap the holes not quite the full length of the set screw, if you like you could use a little J-B Weld on the set screws, run them in and make them tight. Grind or flat file anything that sticks up till it's smooth. Done!

That's what I did. Be sure to "NOT" tap all the way through. You want the set screw to bottom out in the threads so that it locks itself against the last thread in the hole. I don't think any kind of epoxy or JB weld will help because all those two part epoxies melt at higher temps. If you torque the set screws down hard against the last thread in the hole it really can't go anywhere. I tightened the hell out of mine knowing that I would never have to remove them.
Self tapping screws are usually meant of for thinner sheet metal or other (less than 3/16") metals. I've never tried using self tapping screws in cast iron.
Cast iron is pretty easy to tap because of it's metallic properties. Just do your best to keep the tap square and straight (use a "starter" tap as opposed to a "bottom" tap), but even it you get the hole a little crooked the screw will still lock in there if you go deep enough. It will also help if you get a larger drill bit and cut a slight taper at the entrance of the hole, this will make it easier to start the tap in the hole. I use a 45 degree chamfering tool to make the taper but a drill bit will work.
I can post pics of a "bottom" and "starter tap" if you need to see the difference.

treblig