XJ6 water drain bolt thread size , in block ?
I noticed no one has responded. I don’t know, but it’s either 1/2-20 UNF or 1/4-19 BSP. Hard to detect the thread pitch difference on such a short length but possible I suppose. The OD is very slightly larger than 1/2" for the pipe thread–maybe 0.52" (measured near the bolt head). If you get >1/2" it’s likely the BSP.
The similar holes into the oil gallery on the intake side are generally agreed to be 1/2-20. OTOH, this hole was originally fitted with a spigot, most of which have tapered pipe thread.
I’m curious myself.
I have searched and some say 1/2 and others 1/4 
All the early blocks are 1/4 BSP.
Peter B.
Early blocks BSP the spanner size on the Eynots drain tap is whitworth. Later blocks it’s a UNF thread identical to the oil gallery plug thread. The plug used in the later blocks simply had a slightly longer head.
Hence the different part no.