Likely a cam not staying locked is a cam not actually locking completely. The new cam may be a mismatch to the old drilled rudders. Lock the cam down by itself and then sight into the front of the casting (area under where the the tiller arm would be) there is a small "window" where you can see a feature on the cam that you can reference for the "full locked cam" position. Mark the casting at this feature point on the casting. Open the cam and then lock the rudder / tiller arm down and check again. Likely the cam is not rotating fully closed. You may have to wallow the upper casting bolt hole open a little. Remove the upper bolt. Lock the rudder and tiller arm down once again. Be sure to slide the tiller arm forward into the cam recess completely, but not so far as to force the cam open any. Re-check the cam "full lock position" again. Look into the bolt hole and you will likely see a slight misalignment. If so... just drill for proper position.