Once you have the wheel studs I would measure the shoulder just below the knurl and the knurl itself and split the difference. With the studs I used, it was 5/8"
This one. Part No. 610-283
I had to machine the holes in the rotor to accept the shoulder behind the head.
This one. Part No. 610-283

I had to machine the holes in the rotor to accept the shoulder behind the head.