<VV> Thermostat damper door

[Bruce]

just finished putting my 63 monza 110 engine back together after resealing everything that could leak oil, rebuilding both carbs,checking everything else. (all OK) converting to electric fuel pump, , gas tank float/ sending unit, rebuilding transaxle. Runs good (except I trashed the speedo cable when pulling the engine)Now, the right side thermostat door does not open when up to temp.  worked OK before !  I thought that when the thermostat failed, the door would stay open!  Any off hand ideas as to what could be wrong??
...,Bruce