Later OMC motors had the pin hole in the tube to fill the inner exhaust housing with water. This helped to keep the noise down, especially from those three cylinder loop charged motors.
If you look at the ’58 QD exhaust housing, you will see that the drive shaft and its spline area are in a sealed tube which is separate from the exhaust chamber.
Before doing anything drastic like drilling a hole into the cooling water passage, please try to find the service bulletin describing this issue. It has to be out there somewhere. Wedgie might even have it in his stash. If you do decide to drill a small hole, I’m guessing it would go in the area marked by the red arrows.
