Hello fellow L322 owners, today I've been well and truly boneheaded. While installing a new inhibitor switch, I thought it would be efficient to do a tranny fluid change, and in the process totally stripped my filler plug.

The drain plug came out smoothly with no issues whatsoever, using an 8mm hex head. Without proper visibility I assumed the fill plug would be the same, so just went at it with a breaker bar until I realized I wasn't turning the bolt but stripping it. The plug is now completely smooth, and as a result, I'm now the proud owner of an unfillable transmission. I've narrowed my options by stupidly removing the fluid first, so I've got two questions:

1. What did I do wrong? Assuming the fill and drain plugs require differently-sized hex heads from factory, what size should I have used on the fill plug? Otherwise was it perhaps replaced with an aftermarket, tightened beyond spec, etc.
2. What on Earth do I do now? Given how little clearance there is to the fill plug, what's the best way to go about removing it? I don't think there is enough room to drill it out. The bottom 1/4 of the plug still has some hex grooves in it, so I suppose I could potentially tap a hex in well enough to turn it. But that seems like a long shot.

Any help is much appreciated, as my RR is too big for the garage door to close. I'm pretty sure it was built for a Prius.

Edit: For clarification, I have a 2004, so it's a BMW model year, but I don't have the ZF transmission.

 

long shot , but worth a try . if you get one of the "spline" bars that's a bit bigger than the hole in the plug , splines cut into and grip the sides of the hole, as you say , not a lot of room .bottom 1/4 still their , could you put an Allan key in fill with chemical metal ,let it cure to max strength before trying to undo (some of the chemical metals can be drilled and taped ) as would stop the Allen key twisting . personally i would not drill it out ,for fear of getting metal slithers into gearbox.

 

Maybe a little late on the draw, but at this point I would try heating the area that surrounds the plug with a torch and then try loosening it.

 

Can you pound a torx bit into it? I have had luck doing that to salvage a day on a messed up hex

 

Thanks you guys for the replies, I definitely should have hit it with some heat prior to wailing on it. I ended up taking it to a buddy's shop and he welded a bolt onto it. It eventually came off with some convincing. I would recommend heat and Kroil to anybody facing this issue as these plugs (mine at least) are incredibly soft.