Identify imperial bolt size

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Identify imperial bolt size

This topic has 29 replies, 19 voices, and was last updated 28 June 2023 at 14:30 by Margaret Trelawny.

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24 June 2023 at 21:01 #649874
Margaret Trelawny
Participant
@margarettrelawny34058
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
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24 June 2023 at 21:01 #29250
Margaret Trelawny
Participant
@margarettrelawny34058
Vintage bolt thread
25 June 2023 at 06:57 #649912
JasonB
Moderator
@jasonb
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
25 June 2023 at 07:04 #649913
Kiwi Bloke
Participant
@kiwibloke62605
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.
25 June 2023 at 07:04 #649914
Michael Gilligan
Participant
@michaelgilligan61133
[ 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.
25 June 2023 at 07:59 #649920
Martin Connelly
Participant
@martinconnelly55370
I have to object to the idea that something from the 60s is vintage

Martin C
25 June 2023 at 08:10 #649921
Nick Wheeler
Participant
@nickwheeler
Posted by Martin Connelly on 25/06/2023 07:59:38:

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

Martin C

You're not American.
25 June 2023 at 09:15 #649924
Howard Lewis
Participant
@howardlewis46836
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
25 June 2023 at 09:26 #649926
Margaret Trelawny
Participant
@margarettrelawny34058
Thank you everyone.

the part is a puppet Undercontrol mechanism as used in tv shows and films of the era.
25 June 2023 at 09:40 #649928
noel shelley
Participant
@noelshelley55608
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.
25 June 2023 at 09:43 #649930
Margaret Trelawny
Participant
@margarettrelawny34058
Thanks Noel. I’ll order some 4BA and keep you all posted. Appreciate everyone’s help very much.

Margaret
25 June 2023 at 09:50 #649931
SillyOldDuffer
Moderator
@sillyoldduffer
Posted by Margaret Trelawny on 25/06/2023 09:26:52:

…

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

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
25 June 2023 at 10:05 #649933
noel shelley
Participant
@noelshelley55608
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.
25 June 2023 at 10:29 #649939
Margaret Trelawny
Participant
@margarettrelawny34058
4BA dome slot heads ordered. Finger’s crossed🤞 thanks everyone 🙏🏻
25 June 2023 at 13:27 #649949
Ian Parkin
Participant
@ianparkin39383
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
25 June 2023 at 19:50 #649991
old mart
Participant
@oldmart
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
26 June 2023 at 08:40 #650021
not done it yet
Participant
@notdoneityet
Posted by old mart on 25/06/2023 19:50:20:

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?
26 June 2023 at 08:47 #650023
Robert Atkinson 2
Participant
@robertatkinson2
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.
27 June 2023 at 06:51 #650110
Chris Pearson 1
Participant
@chrispearson1
Posted by Robert Atkinson 2 on 26/06/2023 08:47:11:

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.
27 June 2023 at 07:01 #650113
JasonB
Moderator
@jasonb
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
27 June 2023 at 07:47 #650118
Nicholas Farr
Participant
@nicholasfarr14254
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
27 June 2023 at 11:26 #650135
Howard Lewis
Participant
@howardlewis46836
Metric and BA threads should not be interchangeable.

Metric are 60 degree form while BA are 47.5 degrees, so there is bound to be a mismatch of form, as well as pitch.

Howard
27 June 2023 at 11:27 #650136
Macolm
Participant
@macolm
If you need to substitute M3.5 screws for 4BA in old electrical boxes, you can buy "rethreaders" from Screwfix and Toolstation amoung others. The resultant fit is not particularly good, and best not to over tighten!
27 June 2023 at 15:44 #650160
File Handle
Participant
@filehandle
I also tried a 4BA nut on a 3.5mm electrical screw. Jams after a few turns, but I wasn't surprised as they are dissimilar. I wonder if M3.5 are being sold as 4BA causing confusion.
27 June 2023 at 16:31 #650168
john halfpenny
Participant
@johnhalfpenny52803
I keep my metric and imperial electrical socket screws in separate tins since first encountering this 25 years ago. They are not interchangeable.
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