Ozzy
its something to do with fine threads are stronger and less likely to shear
its one of those clever/silly/nerdy techno equation things like shear stress and tensile strength stuff.
But 7/16 unf is not compulsory for iva you just have to have alternative fixings of adequate/ equal strength to 7/16 unf
But it is really easy to get hold a tap and the right drill bit for 7/16 unf
choice as they say is yours
HTH les g