Edited: I just found about the best Tutorial from start to finish on building a motorized Bike. But this video
and most youtube videos I've seen show the flat washers on the back with the nut, and not on the heads of the bolts. ?? So I don't know ...
This Sequence of videos start from un-boxing the engine to final assembly and premixing oil and gas, to starting for the first time. I copied the URL's to share. I'll look for a place on these forums to post them to.