Admittedly I have not tried to replace the core yet, but I need to on my SM (as it's causing air balls) and have been holding off after reading on pinside of all the trouble people had replacing theirs. Supposedly Stern used some form of loctite (at least on SM) and even people in our pinside community who I know have ten times the skill set I do, agreed it became a bigger project than expected. Maybe it's a non-issue on the newer games.