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.