Plug a smog hole

TrailBeast said
You can also tap a ball bearing in the hole and slightly peen the edge of the hole like they do in transmission castings where something need to be drilled but where the drill went in needs plugged like the green circle on this pump stator.

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I was going to suggest the same thing but was not sure if the cast iron head (area around a hole) would crack or break off (malleability). Cast iron is somewhat brittle and when you get near an edge it can sometime chip/break off. Of course it wouldn't hurt to try. He could also get a straight rod that's a 2 or 3 thousandths of an inch larger than the smog hole, grind a very slight taper on the short piece of rod and hammer it in till it gets stuck then grind off anything that's still protruding. But since the OP hasn't ever tapped a hole I figured that he wouldn't have the instruments to measure the hole or measure a piece of rod (as far as thousandths of an inch goes).

Treblig