steve 
Member
  
  
   | Joined:  | 11-06-2007 |  
   | Location:  | Perth, Australia |  
   | Posts:  | 2 |  
   | Status:  | 
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   Ron, 
 
The thread is 5/8" BSP (British Standard Pipe) which is extremely similar size & pitch as 16mm.  BSP threads are usually tapered however the JH tank uses a parallel thread with a fibre washer. 
 
I just replaced the drain plug in my tank as it was brass and had fractured, I had no luck in getting an off the shelf plug to fit exactly, so I bought a standard 16mm sump plug (very close fit) and got an engineering shop to run a 5/8" BSP dye nut over it, you then use a fibre or copper washer and your done. 
 
Cheers Steve 
 
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