I recently started experimenting with GBC machines build out of LEGO.
Ball bearings are ideal as balls (have a lot of momentum due to their weight), but are also impacting on the LEGO bricks hard enough that (eventually) they make the plastic crack. (This limits what you can do in the GBC design. You need to avoid large drops or high-speed bumps into end-stops.)
So I'm currently using glass marbles for balls at the moment.
Ideally I would like to use LEGO for the balls.
I realize they would be very lightweight which poses some design issues for some of the mechanisms, but finding solutions to that is part of the fun.
Unfortunately I don't seem to be able to find either single piece LEGO balls or sets of multiple LEGO pieces (like 2 half-domes) that together form a proper ball.
I'm looking for a ball shape with a diameter of at least 2 studs. (Between 2 and 3 studs would be perfect for my purpose, but somewhat larger would be fine too.)
I'm OK with something that has a hole into which an axle, pin or stud can be fitted.
I'm NOT OK with a stud sticking out, because that would affect the rolling behavior too much. A recessed stud that doesn't stick out beyond the outline of the sphere would be OK.
If a true ball/sphere is possible in LEGO, what piece(s) do I need ?