Stick a plug in it!

One of the reasons I dropped the differential from the 280z was the fact that all of my attempts to remove the fill plug were unsuccessful.  My thought was that I would be able to gain better access and more leverage on the plug with the unit out of the car.  I even gave it a bath with some simple green to remove the years of accumulated gunk.

After draining the oil, I removed the cover from the housing.  

I then soaked the plug for a week in a solution of acetone and automatic transmission fluid (said to be many times more effective than PB Blaster).

After the week of soaking the plug, I bolted the cover to something more substantial, in this case two 2x10's secured side-by-side.

I found that no combination of heating, hammering, impact wrench, and breaker bar could remove the fill plug. 

I even broke a wrench that I'd had since I was 12.  Ok, so it was a K-Mart branded wrench and maybe not the best, but my grandfather bought me that set...

My next trick was to weld a 4 foot bar onto the plug.

I did this twice.  The first weld wasn't very strong, but the second was pretty sturdy.  Unfortunately the plug still didn't budge.  Time for a diff-erent approach.

I started by grinding the plug flat.

I then used a drill press to create a 1/2" hole in the middle of the plug.

Then I used a 14 x 1.5 tap to thread the newly created hole.  Note that at this point I had soaked the cover in Simple Green (to clean the sludge from the back side) and used acetone to remove the paint.

 

I picked this plug because it was a bit shorter as well as the fact that it had an integrated rubber gasket.  The length of the plug is important since, when reassembled, the crossmember gives little room with which to remove the new plug.