There are several ways to modify the bypass valve to prevent it from opening and allowing the coolant to bypass the radiator. You can remove the cotter key and remove all of the assembly except for the center brass rod. Most people cut the rod. Another option is to use a section of metal fuel line over the brass rod to prevent the bypass assembly from compressing. In my 1938 Buick Century restoration discussion on this site, I covered how I did the bypass conversion on that car. It would take me