But the radiator is designed for that scenario. The upper tank (the copper/aluminum part that has the cap on it) contains a built in accumulator of sorts... if you service the cooling system properly and fill the radiator to the proper level the air gap in the radiator tank contains more than enough volume to to mitigate this initial start up pressure surge as you describe.
Also t-stats are not water/air tight seals, and the check valve for air is also going to equalize pressure.
Your right, it's not hard to understand.
What I would like to understand is how drilling holes in the t-stat is an acceptable solution. For the cost of a tow, or the possible damage caused by an overheated motor I would want to know why I had to drill holes and bypass the t-stat. Is there a crack in a cylinder/head pressuring the water jacket? Is the t-stat housing damaged? Is the water pump impeller worn (happens more than you think)? Is there a family of mice now entombed in a watery sarcophagus?
if OP is happy, then Yay!. But it would be interesting to know the root cause... for science and what not.
