English or metric. How to tell?

[David TS]

The other very British" thing about BA threads is the size relationship:

To get the dimensions of a size say, 1 BA, multiply the previous larger size (Zero) buy 0.9. Ditto all the way down.

Bruce, that's not actually correct.

1BA is 88.333% of 0BA diameter, 2BA is 88.679% of 1BA diameter, etc - and it doesn't follow in a linear scale on pitch either.

BA is a weird thread, as despite the 'British Association' name, it is actually a Swiss thread, and metric thread form.

---------- Post added at 05:50 PM ---------- Previous post was at 05:46 PM ----------

Yes, but in practice the two threads will fit together fine, without needing to be forced together. When I last measured the external diameter of a 0 BA thread it was 6mm diameter.

M6 coarse and 0BA are indeed 6mm dia x 1mm pitch.

HOWEVER, interchange them at your peril, they couldn't be more different.

M6 coarse is 60 degree thread form, 0BA is 47.5 degree thread form.

Interchanging them is NOT a good idea!