Combination cylinder socket torques the upper cylinder head nuts and the rocker arm studs without having to constanlty change sockets from 9/16" to 13/16" The 9/16" portion slides into the 13/16" socket to allow it to be used to torque rocker arm stud. 13/16" socket is a 6 point Williams socket and the 9/16" 6 point portion is manufactured by Blue Chip Tools and hardened to 45 to 50 RC as well as being nickel plated. A fine addition to your Corvair toolbox.