Measure it to be sure what size it is. If it is 5/8 square then get a short piece of 5/8 bar stock and use a big wrench. I doubt that it is actually 5/8 square which is why you need to measure it to be sure. Typically a 1/2 inch drive fits in there.
I understand, that is why I was so suprized it is very close to 5/8. I tried a 3/4 and it was too big, 1/2 goes swimming in it. My best guess is 5/8. I found a 17mm square wrench on eBay, I'm thinkin' that is too big. It is 0.67" I know it's close, but if it's too big it won't work. 5/8" is 0.625, I don't know if the 0.045 would keep it from fitting. Seems like it would work.
