The loose fitting hole is there to allow leeway in the assembly. The tolerances in the spacing between the hole in the axle where the spring perch sits and the hole where the king pin sits, the angle of these holes, the tolerance in where the hole in the perch is drilled to accept the bake actuator bolt, these all stack up and if that hole were made a tight fit those tolerances would have to be minuscule. Shove the end of the actuator into the kingpin cavity, insert the bolt in the actuator into the perch hole, hold the other end tight to the kingpin and crank down on the nut to hold it in place. The kingpin might need to be loose to allow it rotate to do this.