1/2 inch.
But! Leave your board out in the sun to get warm
and expand. Drill your holes and when the boards cool down
the holes should be a tiny bit tighter than 1/2.
then take a 7/16 dowel and wrap super fine sandpaper around it to smooth
the holes. don't try to enlarge the hole, just get it smooth.. after this,
coat the inside of the holes with your favorite bullet lube.
this will sort of season the wood and also expand it a little as the lube soaks in.
after a couple of days it should be usable with a smooth but snug fit
