What's with the screw hole in the bell / additional bell screw?
The hardware bag inside your Tengu motor box (sizes 40xx and up) contains a screw that will fit the screw hole in the motor bell. This screw serves 2 purposes
1) You can use it to additionally secure your bell and shaft together. The shaft sits in the bell very tightly and is secured by two grub screws visible through the elliptical cutouts, so in most cases, this additional connection can be omitted, but feel free to use it for extra safety.
2) In the rare case that you need to exchange the shaft in your motor (in case of a model swap or bad crash), you can insert the screw only halfway into the thread, and use it to press/hammer the shaft out of the bell without damaging the bell surface. (Make sure to remove the grub screws, clips and washers first)