Thanks guys,
No1 / Stormin, this is a very old vice that's why I thought 8TPI acme from my measurements and inspection of the thread, think it's just single start.
Thanks Stew, that's what I found on the internet too teh 29 deg included angle. I think because it's quite course, and deep it maybe needs quite a bit of side clearance - maybe that's where I'm going wrong.
Bernd, yes I could do with a thread gauge like that. I was trying to grind it using the screw but it may be a bit worn. Whatever I did when I tried to grind my tool - it didn't work. Think I may do a couple of practice runs on something soft like nylon.
Jason, cheers, I looked for a tap and you can buy it from Tracy tools but £22 each which sort of defeats the point a bit! Having said that, it, if need be and I can't screw cut one, it could put an otherwise good vice back into service and still save me a few quid.
Even if I can't get it to work, it'll still be an interesting exercise for me, something new and if it does work - another string to my bow!
Thanks for the help.
Nick