Rather than drilling and tapping I used to cut short sections of 1/4 inch round stock and drill holes in the stock and drop them into the holes below the rocker shafts in my old iron heads. I don't know any reason why it wouldn't work on Eddies or other aluminum heads as well. The big advantage to this method is the ease of removal in the event you need either more or less oil to the heads. All you need is a magnet or a pick. It's very easy to make up several sizes, sort of like changing carb jets.