Well let me apologize for the stupid question, because I just now realized that those studs are indeed 1/4-20. I’m a big dummy. BUT, thankfully they’re already tapped, because all I need to do is pay a visit to the hardware store and pick up some set screws.
Here’s a question that’s unrelated to the holes, but related to the heads. I noticed most of the valves are pretty sooty looking. Does that mean I’m running rich?