As others have said the cap is just there to seal the grease in and dirt out. The cross pin secures the king pin in the axle. There are caps on botht eh top and bottom. I found the easiest way to remove thr king pin was the remove both the top and bottom caps, the cross pin, and then use a large diameter punch to drive the king pin out the bottom. Once I cleaned everything up the king pins would slide into the bushing in the spindle and the bore in the axle with little resistance.
For my 1950 3600 the dust caps were not 1 1/8" and I ended up ordering 1 1/16" caps from NAPA as my king pins were okay and I did not need to order the kit just oget the caps.
