skytrooper

Hi,

I have the following question, i have replaced all my plastic bearings in my tamiya thundershot with ball bearings, i noticed there are two plastic bearings (the bearing or whatever it is, is called E1 on page 9 of the manual illustration nr 17 shows it more clearly)
each with a flat spot on them, can those plastic bearings or whatever it is be replaced with ball bearings, why do they have the flat spot on them?? (it's like a circle that is cut slightly straight down on the side of the plastic bearing?

Thanks