>>10939069Belville heads, Technic heads, and Fabuland heads (and maybe more) all share the same size balljoint. All you need to do is take a Fabuland grey balljoint out of the head which may or may not require a lot of force (Ive had somenthat came off as easy as a brick and had to use pliers on some) and both connections are the standard stud size. For some reason Belville heads can fit 2x2 bricks and plates for custom hats of hair but I haven't really done anything with this knowledge yet. And the arms are connected by a standard bar size.
