This mounts into the center of an appropriately bored(90mm) fan hub. It allows you to mount the fan with a P1 bushing. Bolts to the hub with 9 bolts on a 110mm bolt circle. Bushing not included. There is also a AS/BS style here, or lighter duty 5 bolt type. Both Multiwing & Wingfan versions are interchangeable, but the Wingfan looks slightly more robust.