what fastener is used for #516 valve cover

Studs are not stock for holding the valve covers on. The threaded hole in the head is 5/16-18 UNC. Quite often studs will be coarse on one end and fine on the other end so that would be 5/16-24 UNF. But if they are aftermarket who knows. I would just pull the studs and use bolts. I have 5/16 stainless bolts from the hardware store holding my valve covers on.