English or metric. How to tell?

[Flying10uk]

For example, Zero BA has the same pitch as M6, but EVERYTHING else about it is different.

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.

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[Bruce_in_Oz]

Yep! That Zero BA diameter detail seems to have missed "publication" in my first post.

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.

Had to be something to do with manual gearbox lathes that could be adjusted in those multiples. For us mature chaps, the original Meccano screws and nuts were all BA, Apparently, when the French took over, they changed to the nearest metric thread.

And then, there are "Enfield" screws; bearing no resemblance to any other on the planet. From the Martini Henry to the last No1 Mk 6, and including the P13 / P14, all were held together with "Enfield Specials", except the Lee Enfield stock bolt, which is 7/16" Whitworth. Then, there are the VERY British "Admiralty" threads, as used by the entire Japanese civil and military ship-building industry until fairly recently.

Mausers up to the M98 and the Type 30 and 38 Arisakas are full of Imperial threads, not so much the Type 99.

There is also a range of "aerospace / aeronautical" screws that have a simi8lar "form" to BA; apparently a good thing when bolting together components made from Aluminium alloys.

[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!

[Flying10uk]

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

All depends on how much torque is put on the nut/thread.

If memory serves me correctly 2BA will fit 3/16 B.S.F.

---------- Post added at 12:51 AM ---------- Previous post was at 12:43 AM ----------

A story that I heard was that BSW dropped a spanner size on all it's threads, making it the same as B.S.F. spanner sizes, during WW2 in order to save steel. How true and how much steel was saved I don't know but it does give one explanation as to why one encounters 2 sizes of B.S.W. nuts/bolts.