Gridfinity-modules with a center pin that can hold anything with a hole, for example rings, gears, facemills...

An M3x12 countersunk screw is required to attach the center pin to the base. If you want to make your own center pin sizes, note that in addition to the M3 hole, a 6,5mm counterbore is used to center the pin on the base.

The 1x1 base can hold a part up to ~40mm, the 2x2 base can hold a part up to ~80mm, and the 3x3 base can hold either a single 120mm part or four 60mm parts.

Because of the centering boss, only the 2x2 base allows stacking other gridfinity modules on top, but the bases will all stack with themselves.