Hi Simon
As I've replied to a previous thread of yours on this subject, I could advise exactly what I bought for the same problem. Trouble is I'm working abroad and all my records are back in the UK.
The threads were definitely not metric though as I had to order the rosejoints, the agent didn't have them in stock. You could always call in at your supplier, explain the situation and ask for a selection of nuts to try on the threads. When you find the right one to fit, you know the thread!!
Regards
Dave Tickle