Trann Filler plug
 

What size socket will fit around that massive POS? I have a 83' 12A stock tranny.

14 mm open end. Socket will not work on the square plug. I have a large, heavy crescent wrench that I use for the stubborn ones. Be careful not to round the edges of the plug.

ditto.

an eight point socket would work, if you have a set handy. that is what they are made for, and I would assume a 12 point socket would work too, but dont quote me on that. anything but a 6 point socket really, I know mine was a big pain to take off, as was the filler plug in the rear end, Good Luck

12 points usually don't work. The edges are usually rounded or chamfered already and a 12 point doesn't have enough contact surface. Good way to round it off unfortunately.