BA vs Metric threads

A lot of the drawings I have show BA screw sizes. My natural inclination would be to convert to the nearest metric size. Are there any practical reasons for investing in BA taps & ties in addition to my metric ones?


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Well Robin, the BA series are actually metrically defined.


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Andrew Mawson

from Robin

Only the fact that there are a greater number of sizes available in the BA series.

With metric, from 6mm down the standards are only 6, 5, 4, 3.5, 3, 2.5,

2, 1.6, 1.2 and 1 (and 3.5, 2, 1.6, 1.2 and 1 are not easily available) whereas the BA series (shown as their metric nominal diameters) are 6, 5.3, 4.7, 4.1, 3.6, 3.2, 2.8, 2.5, 2.2, 1.9, 1.7, 1.5, 1.3, 1.2, 1, 0.9 and 0.79 (again 1.2, 1, 0.9 and 0.79 are not readily available).

In other words 5 metric sizes compared to 13 BA.


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Depends whether you have a full set of the various metric fine taps and dies as well as the usual coarse ones.

Mark Rand RTFM

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Mark Rand

The other thing to consider is are the materials on your drawings that you are threading metric or imperial. eg 5BA is the common size used on

1/8 rod, 7BA on 3/32 and 2BA on 3/16.

If you were using these imperial sizes from the drawings the nearest metric thread would be a bit slack or tight. If you start changing threads to metric then you will need to alter the off the shelf stock sizes, which will entail buying metric fasteners, metric reamers, etc.


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BA threads in the standard sizes like 8,6,4 and 2 are generally available, and model engineering places still keep the odd sizes.

Longer-term they will die out through lack of use. We still keep some sizes at the factory as transformer laminations are still coming through with BA cleance holes.

We did a bulk buy of 2BA CH HD recently, and have previously bought bulk lots of

4BA as well.

The 3BA screws that I asked about recently, turned up this morning, many thanks to Colin Docherty!


-- Peter & Rita Forbes Email:

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Peter A Forbes

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