The Rockwood 550 Lever Extension Flush Bolt Is For Fire Rated Hollow Metal Swinging Doors measuring up to 4'w x 10'h rated up to and including 3 Hours. 550 Lever Extension Flush Bolt 3/4" bolt throw, 3/4" rod backset. 12" rod length (center of the face to bolt end retracted 1-1/2" adjustable bolt head. Call For Pricing For Other size rods available are 18", 24", 36", and 48" Use #570 Dust Proof Strike to prevent dirt blocking the bottom strike.