FXXD-S 後波箱安裝問題

what about the 8x14 bearing? focusing on the 1 suppose in the casing, i believe 5x10 bearing for the other purpose.

u seems right, check this out:
MST FXX Aluminum Rear Gear Box
send it back to where u bought.

Hey mate, I believe MST comes with 2 different version of gear case, 1 is as shown in your construction guide, the other 1 is the 1 you have at the moment, but it should comes with the metal collar (silver color), check this out MST gear gear) and try to find this guy in your kit, if you can't find it, they really need to replace your parts.