The hub inner bore is milled for a 1/4" axle key. The most common style for go-karts features a 1" inner (or inboard) shaft diameter with the axle turned down to a 3/4" outer diameter. (It is incompatible with Yerf-dog karts.)
Specifications:
Diameter: 4"
Length: 2-3/4"
Keyway & key size: 1/4"
Inboard shaft diameter: 1"
Outboard shaft diameter: 3/4"