You should have used the two shims that you had plus you're missing two. there should be two on the top shaft one on each end. one on each end of The Idler gear. The only thing is that I do is when you put all the transmission screws in tighten them up until I just bought them out and then turn them back a whole turn or turn and a half. Then spin the transmission. That will allow for some warpage in some of the cases the Traxxas sends out.
