The king pin itself is what is oversize (0.010 or 0.020 wider diameter pin). That is why the axle needs to be reamed to fit the over-size king pin.
The1/2 ton bushings "float" in the spindles/knuckles. The bushings have over-size holes in them for the over-size kin pin. The 1/2 ton over-size bushings do not need to be reamed to fit the 1/2 ton over-size king pin.
If the bushings are real loose (loose side-to-side, not loose up/down) in the spindle/knuckle, the holes/seats in the spindle/knuckle might be "wallowed out" - this is a difficult problem to deal with.
