A3 = 10mm
A3N = 12mm
A35(early sprint) = 10mm
A35(stock) = 12mm
It's ferkin crazy - this, there ain't no real way to tell without taking the piston out, but as a general rule, the "old" Tomos used 10mm and the "new" Tomos used 12mm, tho exceptions exist.
I have an A3 piston with a 10mm pin (whereas I need an A3 piston with a 12mm pin) - so if anyone DOES wind up stick with a 12mm pin piston set and needs a 10mm, I will gladly trade em.