Handlebar Knobs- Thread Size?

Er..... very sorry for any confusion folks. My mistake - simple fact is I misread the number on my 6mm and 8mm dies.
To put the matter right both my z50 and ST70 definitely have 8mm threads on the handlebar knobs. End of story.
Senility creeps up quicker than I thought!
Brian

No worries Brian, I too need me reading glasses nowadays I'm afraid, but after all these years I can distinguish an M-6 from 1/4 UNF or UNC and a 5/16 Unified from an M-8 at a few paces fortunately
Cheers,
Dave

Success!!!

I managed to drill out the sheared bolts and retap them M8. Borrowed a ratchet tap wrench from work. They now have temporary M8 screws in until I buy some NOS replacements.

Thanks for all the info and advice chaps .