Thread is 1,25 size is m12
But these are called studbolts and they come in different spline sizes.
For datsun 260z it is m12x1,25 with 13mm spline.
I bought mine at Biesheuvel Autosport
ps. Hoi henry ik ben dezelfde als van de mazdaclub. mocht je nog meer vragen hebben pm me of voeg mijn...