Edit - looking at Minispares website -the rocker adjusters are listed as being 5/16" as I suspected..... and nuts are 16p each.

http://www.minispares.com/shop/classic/ ... .aspx?1004
As it's part of the engine, I would think that the thread form is UNF, which also has a 60° thread angle.angle of the thread is 60 degrees opposed to 55 degrees on a British thread