AMC 20 fill plug question

[1981CJ5]

What size allen wrench do I use for the fill plug on the AMC 20 rear axle?

I tried 2 different sizes and it starts to round it out.

Thanks.

[jeepin916]

HMMM GO TO THE NEXT SIZE UP AND SMASH IT IN WITH A HAMMER ITS BORBABLY RUSTED TOGETHER ON THE OUTSIDE AND GET A NEW FILL PLUG ALSO THE OLD ONE IS GOING TO LOOK LIKEHELL WHEN IT COMES OUT HAD TO DO THAT WITH MINE