What about hitch pins on older racks?

Older threaded hitch pin racks (before 2004) use a ½ x 20 Threads Per Inch hitch pin. Visual Clue: Look at the 45-degree angle brace between the horizontal and vertical tubes that make up the receiver tube.  If there is a small hole right in the middle of that brace, the rack is threaded ½ x 13.  If there is no hole in the brace, the rack is threaded ½ x 20 Threads Per Inch.