Replacement YoyoFactory bearings. Sold individually.
Bearings comes in two sizes Small (A type), Large (C-type).
Size C is the most common bearing size.
Center Trac Bearings are an upgrade that allow the string to remain trapped in the center away from hitting the side of the yo-yo during instense string trick play. This will make your yo-yo as nearly unresponsive as possible.
HALF-SPEC is half the width of the normal size C bearing. This allows it to be used in responsive (tug-return) yoyos as the yoyo halves are closer together, bringing the yoyo string closer to the response system (ie. response pads).
Some normal-width Size C yoyos can have one of these HALF-SPEC bearings installed to convert an unresponsive yoyo to a responsive one, however, this does not work on all models, and may require a shorter axle.