That is not completely true. The very early 1909 had dippers and believe there some late 27 with them.
To answer your question, 1/8 inch in about the center. You then should X the rod and cap.
For what it is worth Mr. Carnut, we used 3/16 holes. Three of them just like the Model A's. I will try to Post some Pictures, to get a better idea. thanks Herm.