Thread Size

I was hoping someone could help identify the following thread sizes. I have just been given a box with several hundred machine screws in three different threads. But there are no nuts, I would like to buy some but these are not standard M3 M4 M5 and I don't think they are Imperial

Major Dia - 4.6 mm ... Pitch 0.81

Major Dia - 3.48 mm ... pitch 0.69

Major Dia - 2.71 mm ... Pitch 0.52

(the 3.5 mm one is not an electric socket size, it don't fit! and I measure them at Major Dia 3.36 & pitch 0.61)

First one is 2BA

Second is probably 6-36

Last one is 6BA

Most likely 2BA, 4BA and 6BA

Looks like they could be specials, I converted them to 3/16 x 32, 9/64 x 26 and 7/64 x 20.

Not sure how you're measuring the pitch, and why you've given two different readings for the major diameter and pitch of the "3.5mm one", but, as others have said, these are most likely to be 2,4, and 6 BA screws.

4BA was used instead of M3.5 on older electrical sockets.

I didn't give 2 different dia for the 3.5 mm one, I made a comparison with an electrical screw which is smaller.

I basically measure the pitch by counting the threads (for the smallest I counted 37.5 over 20mm - giving a pich of 0.53 or 48 tpi - which would fit in very well with the 6BA

So Yes, and thank you all, they must be a collection of BA machine screws, they just looked very metric, normally I can tell just looking if they are imperial. And the one measuring just a bit below 3.5 confused me even more.

Any suppliers of BA nuts - not quite sure if it is worth bothering? just seems a shame not to use them

I get mine from EKP Supplies

https://www.ekpsupplies.com

Here.

Hi, they are also available from Poly Model Engineering and Reeves 2000

Regards Nick.

BA fasteners are metric, 0BA is 1mm pitch

regards Martin

Start with 1mm and successively multiply by 0.9 to get all the higher number pitches.

Edited By Martin Kyte on 24/02/2023 22:33:51

Thanks again for that, metric thread first proposed in Britain in 1884 !

Chronos sell a BA thread ptich gauge for not too much money which might help firm up the identification.

.

If you hone your ‘looking’ skills a little [with the help of some magnifying optics] you should be able to tell that the thread-form is sharper on BA than either imperial or metric.

47.5°

MichaelG.

As Michael says, BA thread form is 47.5 degrees.

Whitworth form is 55 degrees

Unified and Metric are 60 degrees.

Might be worth getting hread gauges for BA, Whitworth, Unified and Metric.

These will allow you to find thread form and pitch (or tpi for Imperial sized fasteners ).

Howard