Updates: The FHZ Stack Kit adapts a Freehand Zero, and many other yo-yos that use a 4mm bolt to use YoYoFactory Hubstacks. It does not come with bearings, plastic stacks, or o-rings.

The kit comes with two threaded axles of different lengths, two FHZ Stack nuts and four shims. Since plastic yo-yos are slightly inconsistent, depending on the yo-yo you have, you may need to use one of the shims under each of the nuts to allow the stacks to spin properly.

The Standard kit does not come with stacks, o-rings, or the two size A bearings that the stacks need to work. The Deluxe Kit comes with two merc bearings at 50% off ($10.00 for those two mercs, not $20.00) The YYF plastic stacks and o-rings are available separately or can be scavenged from an existing YoYoFactory yo-yo you already own. This is a perfect way to use the parts you have left when you upgraded the stacks on your YoYoFactory yo-yo.

(Note: the standard short stack works best, the long stacks are just too long for this yo-yo).

This kit was manufactured with permission of YoYoFactory. (Thank you Ben and Hans)

Weight rings not included.

Update:
These stack kits work great on Buzzon element X's and on Phantoms.