BA THREAD SYSTEM
E.M.G. Stevens
MPH 515
Page 36
The answer to Jacqueline Bickerstaffs query in the October issue regarding OBA dimensions is that all BA threads have
metric dimensions because the BA system is basically metric. Although tables usually give dimensions in inches (with a
generous number of decimals), conversion to millimetres readily reveals their metric origin. For instance:
OBA diameter is 0.2362 inches = 6.00 millimetres
1BA diameter is 0.2087 inches = 5.30 millimetres
2BA diameter is 0.1850 inches = 4.70 millimetres
3BA diameter is 0. 1614 inches = 4. 10 millimetres
4BA diameter is 0. 1417 inches = 3.60 millimetres
12BA diameter is 0.0511 inches = 1.30 millimetres
As might be expected, the thread pitches are also metric, but in a roundabout sort of way. Whereas in most thread systems a
given size is referred to by its overall diameter (eg, 1/4 inch BSF, 6 millimetres SI), in the BA system the diameter is
ignored and each thread size is identified by its pitch (albeit indirectly) in a very cunning way: 0.9 millimetre is taken as a
starting point and raised to the power of the BA thread number concerned, the result being the pitch (in millimetres, of
course!). Looking at it another way, 2BA means a thread with a pitch the square root of 0.9 millimetres; 3BA is a thread
with a pitch the cube root of 0.9 millimetres etc.
Thus, we have:
1BA pitch = 0.9' = 0.90 millimetres
2BA pitch = 0.92 = 0.81 millimetres
3BA pitch = 0.93 = 0.73 millimetres
4BA pitch = 0.94 = 0.66 millimetres
12BA pitch = 0.912 = 0.28 millimetres
Truly, engineers can be as devious as politicians