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.

I don’t think engine warm up times will change with the bypass restricted but the heater will be slower in coming up to temperature in cooler weather.  A 3/16” hole in the restriction is just large enough to keep the air bled from the water pump and timing cover and and also not get stopped up with some of the smaller crud that floats around in the system.  At 3/16”, the hole is small enough to actually present a noticable restriction thus forcing more water to go to the radiator rather than recirculating at the engine.

Lorena, Texas (South of Waco)