Ofna 8 spider would be stronger than the Ofna or HB 6 spider setup, more spider gears equals less load on each gear, so less wear & greater durability; seems like around here 6 spider gears just isn't enough for some loonies & they still manage to destroy them
Not sure on the screw question, but this should answer it:
http://www.unlimitedengineering.com/...structions.pdf