I am doing a little engine work on the bird and I am thinking that the water bypass contributes to the slow overheating that the car experiences on hot days.  Assuming that the bipass is in the range of 9/16" dia, a 1/4" hole in a plate at the water pump would reduce the flow area by 80% and a 3/16" hole by nearly 90%.  My understanding of this bypass is that it decreases the time it takes the engine to achieve operating temp.  This isn't much of an issue to me since I don't drive the car on cold days anyhow. So I think I will go with a 3/16" orifice.  That will insure that the passage isn't an air pocket and give a little circulation. 

Doug T

The Highlands, Louisville, Ky.