I'm down to taking all of the junk off of my block for taking it into the machinist (the man's hair stylist

). I have plugs (2 of them take a 1/4 inch drive, and the other one I THOUGHT took a 3/8 inch drive, but the 3/8 is too large and the 1/4 is too small. I know, find something in between, but I haven't found any drives that are inbetween. Does anybody know what size this is?
Between the beating of the freeze plugs my head is ringing and it's really hard to think. Just kidding, but this is really quite a stumper for me.