The SD55 Sigmadrive is designed to fit a 4″ diameter output flange.  However there are instances where a transmission with a 5″ flange can still be in the torque range of the SD55.  To accommodate this setup an adapter flange is used on the SD55 to convert the bolt pattern to the larger flange.  The adapter flange #1 comes with hex bolts and a pilot to allow it to be fitted to the Sigmadrive.