Identify imperial bolt size

Hi

I need help identifying a thread size - created in the 1960s in England. Machined into brass - we don’t want to drill/retap the piece.

An M3 bolt is slightly too small, an M4 too big. Using an online converter we purchased 6-32 UNC and 4-40 UNC but neither fit.

Appreciate it’s not much to go on but any help would be gratefully received.

thank you

Margaret

It could well be a BA thread, if you can say what the item is then that would help.

5BA is just a little larger than M3 at about 3.2mm but was not a preferred commercial size though often used for models

4BA would fall mid way between M3 and M4 as it is approx 3.5mm diameter and was one of the preferred sizes in the BA range

Jason beat me. Agreed, 4BA likely. BA threads were used widely in UK, especially in electrical apparatus, millions of holes tapped in brass to receive them.

[ as usual ] Jason probably has the answer, Margaret

As a very helpful quick check, you might try screwing something like a wooden barbecue-skewer gently into the tapped hole. … when unscrewed this should reveal an impression of the thread, the pitch of which can then be measured.

MichaelG.

I have to object to the idea that something from the 60s is vintage

Martin C

You're not American.

If not a BA size (Pity we have no dimensions for the part that scrrews in ) might it be 5/32 BSW?

Same size as used by Meccano

0156" OD, (3.968 mm. if you must )

Howard

Thank you everyone.

the part is a puppet Undercontrol mechanism as used in tv shows and films of the era.

Like the others,as not metric or UN I would go for 4BA. At 3.97mm 5/32w is likely to be tooo big, and rather coarse. Good luck. Noel.

Thanks Noel. I’ll order some 4BA and keep you all posted. Appreciate everyone’s help very much.

Margaret

I'd bet the farm on it being a BA thread - extremely common for small fasteners, especially telecomms, electromechanical, instruments, electrical etc. Highly likely in a British made mechanism of that era. Although supplanted by metric in new products, BA is still widely available; the internet is your friend!

Dave

Margaret, The only thing if ordering in is to specify the head type. Hexagon, cheese head, countersunk, slot or allen key, socket head. These are the common types, then there is the length. Best wishes Noel.

4BA dome slot heads ordered. Finger’s crossed🤞 thanks everyone 🙏🏻

Could be worth trying m 3.5

lots of those in a UK home remove a screw from a switch plate or socket just to see if it fits

m 3.5 is about 42 tpi

60's is vintage, but not as vintage as me. I'm in my 9th decade when you count up the decades starting with 1949. Only the other day, I tapped two holes for 4BA screws as there were plenty of countersunk ones within easy reach. And they are in the new Atlas 12 x 24 cross slide which has now got threads of 1/2UNF, 4, 5 and 6mm plus the 4BA's in it.

Edited By old mart on 25/06/2023 19:53:31

The item may not be classed as ‘vintage’ but the thread may well be! How old might a vintage wine be? Classic cars are generally over 25 years and veteran cars are pre-1930?

M3.5 is interchangable with 4BA. This non-preferred size is used in the UK for commonality with older electrical fittings. Availble from any hardware or electrical supplier.

Robert.

Ouch! The pitch and diameter may be similar, but the angle of the thread is completely different.

Also far less choice of heads and material in M3.5

Pitch is not that similar, depends a bit on how long the female thread is, might be OK on an thin electrical box tab to mix the two but with M3.5 having a tpi of 42.3 (0.6mm) and BA one of 38.5 things will soon get tight

Edited By JasonB on 27/06/2023 07:14:29

Hi, 4 BA pitch = 0.66mm, M3.5 pitch = 0.6mm, so in ten threads, the 4 BA will be 0.6mm longer. I've just tried a 4 BA into an electrical box, and without forcing it with a screw driver, it won't even poke through the 2mm thick lug, likewise the threads of the M3.5 screw won't show through a standard 4 BA nut, so I don't class them as compatible.

Regards Nick.

Edited By Nicholas Farr on 27/06/2023 07:49:11