Regarding a BA thread gauge, Michael Walters says:
So long as you could identify the pitch there would be no need to have a special threadform gauge.
Exactly, but how do I identify the pitch? If I had a full set of BA taps I could make a set of gauges, but indeed, if I had a set of taps I wouldn't need the gauges, would I?
BA is indeed a 'metric' thread but it is unique in having pitches, in millimetres:
1, 0.9, 0.81, 0.73, 0.66, 0.59, 0.53, 0.48, 0.43 and so on. Only the first (0BA) is common with a metric series.
Cheers, Tim