Capt,n Flipflop is referring to the area between the back of your machined flywheel and the seal in the front of your engine. Basically all the forces or loads are on the threaded portion of the crank. Not too stable and can break off. Ideally the hubs should fit closely over the area behind the threads on the crank and be an 1/8th inch in front of the seal as in this picture. This is a 16cc Echo I did some time ago. You don't need the key. Just use a good Loctite type sealer. I use 620 and it is tough stuff. The taper on the crank and hub will lock together even without Loctite.