Thanks for the drawing! I'm still not sure I totally get it. Does the o-ring go around the rudder post (functioning like packing material)--or is it between the top and base plates, functioning like a gasket? I need to verify while I'm underway, but I'm pretty sure my leak is due to a lack of packing effectiveness...
Sorry, that drawing was created quick, down and dirty as we say on the spot to give you a faster answer.
The “O” ring closely fits the diameter of the rudder post and slides down over it to then nest critically BETWEEN THE TWO BRONZE PLATES. It is then simply compressed by the tightening of the upper plate against the lower plate.
Between ample/generous amounts of waterproof grease below in the shaft post and the well greased rubber seal, water intrusion problems were resolved for us.
...Thinking back now, I actually had two rubber seals made up. One was the thicker 3/8”(??) an a 1/8”. We actually slid the both down on the rudder shaft at the time. (Smaller one first of memory serves me.) ...It HAS been a long while back!!!