what fastener is used for #516 valve cover

I have determined that the thread count is neither 20 per inch or 24 per inch , it seems to be 5/16", this head (516)has 5 studs ea holding rocker cover, I have a tremendous asst of fasteners but nothing works, would this be some kind of acorn nut or sheet metal type fastener, please help I am putting on both heads today and would now be held up because of rocker covers. thanx