I did a bbr big bore kit back in the day and never removed the motor. Or at least I don't remember ever removing it.
Ive never done them in the frame cause I hate squatting down. But it certainly can. Just have to remove the seat, tank, coil, and ignition box. Only takes a few mins to drop it out of the frame unless you have one of the bbr cradles.
XR100's don't come apart in the frame? I have to do top ends on both of my XR100's next week. I had no idea the motor had to come out. Glad I read this Lol
