Almost like an M4 screw but smaller

[JohnD]

I have some English made Chubb locks. Some of them were made to the same design over many years and the parts are interchangeable, though sometimes hand-fitting is needed and you can often see that the method of manufacture has changed over time.

Many of the modern case covers are held on with M4 countersunk SS screws of 6mm length, and I keep a small stock.

I was recently working on an older lock of seemingly the same design. and as the screws were burred, I put new screws in. However they only went in a few turns before starting to bind. On examination they appear to be slightly fatter, and although, on such a short screw, I can't see a difference in thread, I think they must be a pre-Metric thread. They would have been assembled in the early 1970's, during Britain Metrication changeover. There is no great load on them and they have cross-point heads. Due to wear I can't see if they have PZ marks. The head is slightly smaller than an M4, this is visible to the eye.

Any ideas what they might be?

 

[big-all]

1/8" perhaps 3.175mm?

 

[SammyInnit]

.

 

[Burnerman]

The BA thread appeared there from time to time...hardy a load bearing design which showed up in electrical gear too.
John

 

[JohnD]

Thanks for the helpful replies.

I have some BA screws somewhere, I'll see if I have one like it.

Having had a good look round, I'm considering a UNF thread. Americans still use it for some non-load-bearing purposes.

The nomenclature is much less self explanatory, but 8-36 might be about right and is a shade bigger than 4mm.

But different industries used to use different thread families.

OOI, I see that there are only three non-SI (non-metric) countries left in the world. Can anyone guess which?

Spark plugs are always metric, even in the US.

Ordinary camera tripod screws are always 1/4" Whitworth

The EU did not force metrication on the UK.
"The United Kingdom avoided having to comply with the 1989 European Units of Measurement Directive (89/617/EEC), which required all member states to make the metric system compulsory, by negotiating derogations (delayed switchovers), including for miles on road signs and for pints for draught beer, cider, and milk sales"

 

[Burnerman]

UNF/UNC threads were pretty common over here too at one time....who knows if they made it into locks though!
Could easily be BSW or BSF of course.
John

 

[JohnD]

thanks!

I just had a look at the "equivalence" tables and BSF/BSW seem only to go down to 1/4" (about 6mm)


edit
just found another table that lists BSF 3/16" as needing a 4mm drill that you are going to thread. So I presume no smaller than 4mm, which seems a shade too big.

But 3/16" BSW (coarse) uses a 3.7mm drill, but has only 24 TPI

 

[rsgaz]

seem only to go down to 1/4"

BS84:1956 specified BSW down to 1/8th and BSF down to 3/16ths.

 

[Notch7]

Metric fine?

2BA?

 

[Tigercubrider]

Try owning a mid sixties British bike
They slowly switched all sorts of imperial threads but stayed imperial