Had an awful time trying to insert the headstock into the front frame and not mess up the bearings. Tried to rotate bike upside down, put it on its side, twisted my body odd ways and still managed to mess it up. Half hour later succeeded, but I rather forget the whole thing!
I remember doing some once and found best to give a good coating of grease to hold them in place so they don't fall out. not easy.
I believe that is what you mean.