I just had the rear differential serviced on my 74 F-100 (Ford 9 inch rear) and I noticed before that it was leaking at the drain plug. I know you are thinking that there is no drain plug on the Ford 9 inch, well someone added one at the bottom of the differential and it is leaking. After the service it is still leaking, right from the plug itself. Do I need a new plug or is there something else I should be looking for? Can this be welded up or what would be the best solution?

 

The drain plug can be real handy. I would pull it, clean it, and give it a good coat of teflon pipe dope or several turns of teflon pipe tape and re-install. That should stop the leak if the thread job was reasonable.

 

If there is a head on the plug a little teflon tape on the threads and a copper crush washer under the head should help? I hope so any way...