This is NOT a go-forward ASIN, please see pack of 1 ASIN. Norton Speed-Lok fasteners quick, tool-free disc changes onto TR back-up pads allow for more grinding time. Twist on and twist off in 2-1/2" turns. Disc locks securely to back-up pad, but is easily removed. Speed-Lok TR back-up pads and discs have rubber threaded hubs. Speed-Lok TR discs and back-up pads are used on vertical shaft grinders.