New answers tagged bolts
There are no standard sizes. Braces of the type you are considering most commonly have holes that are less than 1/4" and more than 1/8", somewhere around 3/16". They are usually intended for #6, #8 or #10 screws. You could probably use 1/8" bolts, or maybe 3/16" depending on the brace. 1/4" bolts probably won't fit unless you ream out the holes. Which ...
Ultimately, as you tighten down the nut, the washer (that's an EXTERNAL star washer in the photo, by the way) will distort and its teeth will grip both surfaces. The manufacturer just got lazy and didn't make it flat, with the teeth splaying out on both sides.
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