Old imperial conduit thread sizes

[DanDIY]

I have brought some old Coughtrie type lights from ebay. I was going to recondition them and fit them in my house. I am struggling to find out the thread sized used on them.

The inlet/outlet is close to a Modern 20mm conduit thread, but the 20mm is a bit small. Do any old timers know what the standard conduit sizes would have been?

Internally I have got to replace the lamp holders and earthing straps. Likewise a modern M4 conduit screw is loose in the hole. Id guess it would be about M5, but obviously its going to be some kind of imperial thread.

I have a rusty conduit screw and a (plastic!) plug that I can measure, but I always find sizing imperial fittings confusing. For example 3/4 BSP as used in plumbing is 1" diameter!!!

 

[Notch7]

I would guess its 1/2" BSP

Old electrical threads I think were BA,

BA1 or BA2?

 

[DanDIY]

Thanks for the reply. I can take them to work tomorrow and measure them and compare with those sizes. At least that gives me a starting point for which imperial size charts to go through!

 

[Notch7]

1/2" BSP is 19mm

 

[EFLImpudence]

And 19mm is 3/4".

 

[AdrianUK]

And 19mm is 3/4".

Is it not because metric threads are OUTSIDE diameter & BSP threads are INSIDE diameter?? ie. 1/2" BSP would be thread on a galv iron pipe that had a 1/2" internal diameter.

 

[winston1]

I don't think conduit used BSP threads, but rather one of their own. I may be wrong.

BA threads were indeed used for screws. Interestingly BA threads are actually metric based.

 

[333rocky333]

20mm was always known just as 3/4, most wholesalers sell 20mm female to 3/4 male and vise versa adapters
20mm is larger than 3/4, next size was 1 inch
Though some stuff used and still do use Pg sizes

Quote
"DIN 40430 Pg thread usage will eventually be phased out in continental Europe.
A new metric thread standard (EN 50262) will be phased in as mandatory beginning on March 1, 2001 replacing the Pg thread.
Metric electrical threads are already in use in the UK where they have replaced the (now obsolete for electrical use) BSP threads."

https://www.ebay.co.uk/i/2321452227...=9044964&device=c&campaignid=856333651&crdt=0

Metpro make them
http://www.metpro.co.uk/downloads/MetproCatalogue.pdf

Metric to Imperial Adaptor Galv
20mm(M) to ¾”(F) FA1G
25mm(M) to 1”(F) FA3G

Imperial to Metric Adaptor Galv
¾”(M) to 20mm (F) FA2G
1”(M) to 25mm (F) FA4G

 

[securespark]

Imperial conduit sizes were:

5/8", 3/4", 1", 1 1/4", 1 1/2", 2".

 

[SUNRAY]

Imperial conduit sizes were:

5/8", 3/4", 1", 1 1/4", 1 1/2", 2".

and smaller

 

[DanDIY]

Thanks for all the replies. I think the mystery is now solved.

The screws inside the fitting which will take the lamp holder were:
Major diameter 4.74mm
TPI: 34 approx

I reckon this is a BA2 - my TPI measurement is a bit out, but the screw I had was only about 5mm long, so there was probably error in my measurement.

BA2 specs:
4.7mm major diameter
TPI 31.36
Here is where the screws will go.

The conduit thread was:
Major diameter 20.5mm
TPI: 14 approx

This is definitely 1/2" BSP, as I have tried a plumbing fitting.

the specs for 1/2" BSP are:
Major diameter 20.955mm
TPI: 14

I could have used an adaptor and replaced the conduit and box with 20mm metric, however I would like to keep the old conduit as it has a nice patina. I only wanted the conduit thread sizes to plug the entry / exit of the existing conduit which currently have some plastic 20mm forced in there! As this is going to be a domestic ceiling light I will drill a 20mm hole in the back of the old box and turn it into a rear entry.

 

[Jackrae]

Interestingly BA threads are actually metric based.

The only 'metric' thing about BA threads is that the OD of 0BA is 6mm x 1mm pitch. See https://en.wikipedia.org/wiki/British_Association_screw_threads which also states the box screw as 2BA

 

[winston1]

The only 'metric' thing about BA threads is that the OD of 0BA is 6mm x 1mm pitch. See https://en.wikipedia.org/wiki/British_Association_screw_threads which also states the box screw as 2BA

That's metric then isn't it? If they were imperial they'd be 1/4" diameter 0.04" pitch.

 

[DanDIY]

The only 'metric' thing about BA threads is that the OD of 0BA is 6mm x 1mm pitch. See https://en.wikipedia.org/wiki/British_Association_screw_threads which also states the box screw as 2BA

I had looked on that Wikipedia page to get the BA dimensions. I'd missed the last paragraph though!

 

[SimonH2]

I don't think conduit used BSP threads, but rather one of their own.

I thought that as well - and I still have dad's conduit thread taps and dies.