Check to be sure there are not any burrs on either of the parts, take a hone stone to any spot that is raised and causing the interference. I have always taken and stoned the back face of the ring gear just to be sure there are no knicks from handling, and also the same for the TL unit mating face. Be sure to then clean surfaces thoroughly. These surfaces are precion machined so fits are tight, if you don't get them going together squarely they will bind