gas tank strap hardware question

Can someone tell me the size bolt (diameter and length) carriage bolt for the gas tank straps, and why there are two different sized washers? thanks

well i just checked mine and it's 3/8 by 3 3/8 long.
i cant say on the washer cause mine has always just had a lock-washer on each side.

My washers are the same size.

Thank you guys :yes: