5/16 wrenching header bolts

If and when you do find a 3/8 bolt with a 5/16 head I wanna see that too. Never have before. Standard for 3/8 dia. is 9/16 hex. I have some GM header bolts from and ol ElMeano I had that have 7/16 hex heads. Thats 2 sizes under standard. But thinking about it... a 5/16 head smaller than the 3/8 shank wouldn't hold anything.