I a previous question I included a drawing for a strut. In this strut drawing it designates a 2 BA Hole in the bottom of the strut. I have seen this call out for many other parts that are part of Spitfire blueprints. I have also seen 4 BA Holes called out too. I was clueless initial as to what this meant.
In the following drawing it again calls for 2 BA bolt holes. However one was added in revision 4 and in the notes the wording is (with line breaks preserved)
4 MOD. No. SPITFIRE 1164 SEAFIRE 282 .25" R. & DIMS 1.21", .75", .30", .29", .38" & 2 B.A. BOLT HOLE ADDED. ONE 2 B.A. BOLT HOLE 3/8" DIA. L.M.H. 4.1.44.
Based on this I thought the hole size was 3/8" of a diameter. After that I did some googling on BA HOLEs and came across several charts detailing BA bolt sizes. While the 2 BA and 4 BA were part of the tables, nothing on the table come close to a 3/8 Dia. And not that I can find it now but I was reading on one web page that BA bolts are popular in modelling and aviation. I also have something in the back of my head that says sheet metal is normally too thin to tap for threads.
With all that in mind what diameter size should I be using for 2BA and 4 BA bolt holes?