Thread size on old coughtrie lights

[DanDIY]

This forum was able to help me a couple of years back identifying the thread size of some old BA screws in an old light fitting I had brought. Now I've been buying more old reclaimed lights....

This time I have some old Coughtrie bulkhead lights. I am trying to find the size of the inlet thread which would take a cable gland or conduit. The lights have a female thread integral to the outer casing. One of them had an old male adaptor screwed in, which I have removed and measured.

I measured the OD of the male thread at 18.75mm, which is pretty close to 3/4"

I only had about 3 threads on this adaptor, but with a bit of extrapolation I've calculated about 19TPI. Or a pitch of 1.3mm if its metric.


Threads I can find which are close are 3/4 UNEF - which seems a pretty niche thread from America and I cant find much about it in relation to electrical conduit. I was expecting something UK.

Any ideas?

 

[Robin0577]

I don't know what the thread spec actually is on old 3/4" imperial electrical conduit, but these will fit it.

https://www.edwardes.co.uk/en/categories/adaptors-steel---die-cast

 

[rsgaz]

thread spec actually is on old 3/4" imperial electrical conduit

Imperial conduit was 3/4 inch nominal OD, but will under-read when measured as the threads have rounded tips. But 16tpi.

BS 31

 

[JohnD]

The heavyweight galvanised iron stuff? Sometimes called "gas-barrel conduit"

I've never checked, but I always believed it to be a BSP thread.

The notional size used to be measured by the internal bore, so half-inch would be about three-quarters external, where it was threaded.

Subsequently the thickness of the metal was reduced, so none of the diameters matched the name.

 

[DanDIY]

I don't know what the thread spec actually is on old 3/4" imperial electrical conduit, but these will fit it.

https://www.edwardes.co.uk/en/categories/adaptors-steel---die-cast

Thanks - that seems so obvious. I had tried googling "imperial conduit fittings" "imperial conduit gland" etc.

I've ordered a few, I'm fairly confident that they will fit.

 

[Edddd]

I had a few of these and, if you don't mind harming the 'heritage' of the fitting, they tap out pretty easily to 20mm to accept modern fittings. Later and current models have a metric thread anyway.