I bought what I thought was a bargain at a recent model engineering show - a boxed and still wrapped pair of Moore and Wright 123 blocks at £10. Trouble is they have imperial threads, 5/16" something or other! I needed to use these the other day so I had to drill out one of the holes to pass some 8mm studding through. Although quite hard they couldn't resist the 8mm glass drill I have, but I was wondering how hard these things are? If possible I'd like to replace the 5/16" threads with M10 ones if possible. Vic.