I replaced mine when I refinished the door. They are a Bakelite construction, had a flat spot that made the for not roll on the track.
Picked up nylon washers at hardware store and glued a stack with 5200. Rolls like roller skates, so door has to latch open now.
My shaft was 1/2" diameter was 1" over all I think. Since it sits on a 1/8" stainless rail, used three, the outer two 1 1/2" to keep it tight.
Total cost under $6.00, cause I built a set of spares. Will post pics from laptop or go see on blog, grandbankschoices.