TRA3984X is the output gear that comes stock on the Forward Only Shaft in the ERevo's tranny. It has 33 teeth on it just like the output gear of the Traxxas CD. I used that gear because the output gear on the Traxxas CD is not sold separately. I found it was a lot of money to buy the whole Traxxas CD just to use the output gear so I modified the output gear TRA3984X instead. I am also working on fitting a
different gear from Traxxas that has the same tooth count and that is inexpensive and may require less modifications.