There are two sizes that I know of one is 8.65 mm and the other is a full nut 10.98mm in depth. I always use the 8.65mm which leaves the correct 1-2 threads showing after torquing up 16lbs
I have adapted a huge sash clamp for this very work and re-adapted it for a TR6. You know where I am if you want to borrow.
Huge thank you to Stuart for showing the pictures.Everyday is a school day!