I just finished upgrading the Cooling System on my 56. Nice new Water Pump, 5.5 Inch Water Pump Pulley, Aluminum Rad, 6 Blade Fan, Plug in the Bypass Hose, and a Genuine NOS Robert Shaw Thermostat. Everything works and no Leaks( hopefully) EXCEPT the Thermostat. Its like no Stat at all. Water is flowing into Rad as Engine warms up. It maxes out at about 120 Degrees.
This is the same thing I experienced a few Years ago with 2 Mr. Gasket Stats. Put a conventional 160 Degree Stat back in and everything is good. I did test it in Boiling Water and then in Cold to make sure it closed. Worked as it should. I have heard of these sticking closed but not open. Any Ideas?