Anglepoise 1227 and 75 screw threads
 

This refers to older Herbert Terry lamps, c.1948-1973. A thread on this forum a couple of years ago concluded that the smaller hinge nuts and bolts are threaded 6BA and the larger tensioning bolt and it's (frequently missing) knurled thumb-wheel are 5BA. I recently had to find a replacement nut for the bolt which connects the fork with the base and found by experience that I had been mistaken in saying it was 1/4" Whitworth. The nut fits a 1/4" Whitworth spanner alright but the thread is 5/16" 26tpi which is not Whitworth or BSF but BSC (cycle thread). M8 Fine thread is so close to 5/16" BSC as to be interchangeable with it. The base nut will rarely be missing but as a Google search for "Anglepoise threads" points to this site I thought it was worth putting the record straight.

Re: Anglepoise 1227 and 75 screw threads
 

Hi, I think you will find the thread is 5/16th BSB (British Standard Brass) and not BSCy. The BSB thread is 55 degrees as is the BSW angle and the BSCy is 60 degrees so therefore they are not interchanable. BSB nuts sizes are the same as whitworth ones.

All BSB threads are 26tpi regardless of their diameter.

Regards
Trevor.

https://en.wikipedia.org/wiki/British_Standard_Brass

Re: Anglepoise 1227 and 75 screw threads
 

The thread appears to me to be 60 degrees (from measuring of an enlarged photograph of the shank). M8 fine (1mm pitch, 25.4 tpi 60 degrees) fits very well indeed. In practice, for this particulàr purpose, 5/16 BSB, 5/16 BSCy, or M8 fine would all work.

Re: Anglepoise 1227 and 75 screw threads
 

And I can indeed confirm that 5BA works for the knurled nut, as you've said.