The pump o ring is around the pump body, the pump has to be totally disassemble to change it, when you re installed the pump make sure you can turn the input shaft. you will need a 30mm or 1 3/16” deep thin wall chrome socket to remove the rear yolk. The thin shim goes between the yolk and bearing. I use lock tight anaerobic sealer around the threads of the bolts, ½ inch is all you need. If the o ring is not leaking do not change it. If you do not assemble it right the Trans will fail.