Depicted in the photos below are 2 bifold metal doors to my laundry area; age probably 1974. One bottom pin cap is missing(broke off when man removed doors to repair washer). It appears to fit over a 1/4" threaded pin. The cap itself is about 1/2" tall, with about a 5/8" flange (is that the right word?) diameter. It drops into the track shown on the floor, which has an opening approx. 3/8" wide and 1/2" deep. The doors are back in the track, and I'm a little disabled, so very hard for me to measure precisely.
Since these parts are getting scarce, if you have the entire assemblies for each door, top and bottom, I would purchase 1 of each, plus some extra plastic caps.