Radiator Air Bleed valve size

Hi All, I want to replace the air bleed valves in my rads which are standard types albeit probably about 25 years old.
Picture attached of an old valve. The nut fits a 1/2inch socket but if you measure the diameter of the threaded part (as shown in the picture) it comes out at a fraction under 3/8inch.
Been down to my local plumbers shop a couple of times but he must be on hols as it's all locked up. Can anyone tell me what the correct size is, will a standard 1/2inch replacement fit? Has the thread size changed over the last 25 years?
Thanks in advance.

 

Thanks All, will let you know how I get on.