Pin hole in cast iron head

Your math is spot on. I also think you are correct in the hole size guess. My concern with the epoxy is the long term. The force pushing on the epoxy would be 0.005 lbf if the epoxy stayed 100% adhered to the cast iron. If the epoxy lost contact with the cast iron, the 16 psi would be acting on a larger area. It is then a snowball until failure.

I like the thought of brazing/soldering but brazing especially would require high localized heat on a thin metal area. I have a buddy that's a great mechanic and fabricator, but even he is not that comfortable with cast iron.

I'm thinking of drilling a small diameter exploratory hole (in the existing hole) to try to decide if there is enough metal to tap/plug or braze. Seems like even brazing would want fresh metal.

Other thoughts - how big of a hole would radiator leak stop fix?

And it is a leak because my colored alcohol (that I use for cc'ing) slowly trickled from the port side into the water jacket.