The clamshell large (CSL) is an aircraft aluminum highly machined sprocket adapter for your gas powered motorized bicycle, which goes on the hub between the left and right spokes. This unique, custom part will save you on time, energy, and frustration -- it is one of top parts recommended by long-time bike motorizing hobbyists. When you go to clamp it on...make sure the 9 holes for the sprocket bolts are not blocked by the spokes. Also, the rim or lip on the adapter will face or point in the direction of the sprocket you will be mounting. Next comes the round disk....flat on one side and with a rim on the other side. Position the round disk so the flat side is out and the rim is pointed towards the clamshell. Next, place the gas powered motorized bicycle sprocket on the flat side of the round disk, and bolt it on. You will need to use the bent wire centering technique for centering the sprocket as seen in our assembly instructions. It is acceptable to file out the holes on the sprocket if necessary to center the sprocket.