Savage spur gear question

[lincpimp]

Ok, I have the ss kit with the steel spur. I have the 52t plastic spur and see that some other parts are required. This looks to be them: http://www3.towerhobbies.com/cgi-bin...?&I=LXDLJ0&P=7

Now how is that assembled? The steel spur has one of the washers with the hex center, the spring and nut. The pic shows a pair of hex washers, why?

I see that the pastic spur uses a bushing, but where does the second washer go? Also could I use a 5x11x4 ball bearing instead of the bushing?