John,
They are 7/16 - 20 with a special head for t he radius rod pin to sit in.
Bratton's has the whole kit for $27.65 - all the parts you need except the felt pad. (.95 cents)
The radius ball is 1 1/2". Don't worry about a little wear on the ball as long as the new metal cups can maintain an air gap of 1/16" or more. Hopefully it will be 1/8" or more gap.