i know this is super old, but the HPI rs4 drift transmission uses a drive cup that is recessed and sits flush with the end of the transmission. it is a part unique to the drift chassis, so you need the actual part. i am making my own drift conversion as well, i used a standard single speed transmission but opened i opened it up and shortened everything by about 1/4-1/2 inch. this includes the housing, and the shaft. i would upload a picture but this site wont let me.