I'm working on a big bolt, but I also have a 57 Buick. I had a similar problem for years with it. In the summer I would be ok, but in winter the thermostat would not open until the water in the block was too hot, then when it did open the water would circulate through the radiator, and immediately the thermostat would close and the cycle would start over again. I think this was because I was using a thermostat with an opening temperature that was too high. It is possible the same thing would happen to your bolt if you increase the thermostat temperature too much.
I finally gave up, took the thermostat out completely. In the summer the car runs fine. It gets up to normal operating temperature just fine without a thermostat, and in winter I put a piece of cardboard over part of the radiator, and everything works like a charm. I'm sure this is not the way it should be done, but it works, so I'm happy. Allen
