A friend with no internet access is building Sweet Violet (the 3.5" version of Sweet Pea) and has a question about the drawings. On the Header Flange (at the front of the boiler) the drawing shows the steam pipe holes as 3/16" dia counter bored 1/4" by 1/8" deep. But the steam pipes are designated as 3/16" OD - another part of the drawing shows 2 such pipes inside the super-heater tube which is 9/16" OD x 20 gauge, which tends to reinforce the notion that they are indeed 3/16" OD. If so, what is the
1/4" counter bore for in the Header Flange?