Fully compatible: the Chain Drive Rail Extension Kit for 8'-High Garage Doors is required for reliable, everyday operation of Chamberlain chain drive models in 8-ft. garage door applications..Note: Measure your door's backset, cross bore and thickness to ensure you find the right fit. Easy Setup: featuring a quick-install extension rail and replacement chain, the kit includes all necessary hardware and instructions for a quick and easy installation. Includes one-year warranty, rail extension, replacement chain, all necessary hardware and an instruction manual for easy setup. Proven quality: rigorous quality testing ensures lasting performance. About Chamberlain: the largest and most dependable garage door opener manufacturer in North America.