Missing screw can somebody eyeball the proper size?

a 7/16 head is 1/4 thread size , 1/2 hex head is 5/16 thread , ect...... no one call a bolt or screw by its head size its the threads thats the part that threads in . i would say 1/4-20 x 1/2 to 5/8 in length will do , or a valve cover bolt .