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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.

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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?

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