Good question Bruce. I will check on my Genesis Silver whenever I get around to stripping and painting it. I cannot remember where I found the thread size, I bought, but I believe it was a Weber grill diagram for the burners.
I like to remove the burner screws because they are always rusted and worn often time, I have to remove them anyways because the original owners make the mistake of tightening them all the way down.
Edit:
Bruce if you like I can send you some and you can test them. I believe i just ordered a 100 pack.