Illinois thermostatic disc for 1/2" 1MG and 3/4" 3MGH series radiator traps in all patterns, as well as, all 50 pound pressure MG series F&T traps. Thermostatic discs are a sealed liquid filled capsule that closes to keep the steam contained in the heating apparatus, and open to allow the discharge of air and condensate when it cools. They are used in either radiator or fin tube heating applications, as well as, F&T traps used for steam return mains.