Factory seal, 2002 f250 Superduty. Thank you Revan I bought the tools to install the infill. I'm not sure how you would have done without it unless you have a lot of experience or I've thought about it too much. A thin film of red grease helped with this, I did this after putting the shaft back into the diff. I couldn't find any information on how to do this. It seems to have worked. made sure i started with a square shape, a small brass rod will do here,