I just picked up a greddy blow off valve for free. However there was one catch the screw or pin or whatever it is on the top is missing. Is there anything special to that part? Or can i just thread a screw into it. The one i have is in the pic below.

 

It's an Allen head screw. It's used to rotate the stud to adjust the BOV's tension relief.

I'm hypothesizing here, but provided a substitute screw Is there In place (or the like), you could double up on a few nuts to make the whole stud rotate - simulating the effect of the absent screw. Eitherwise, substitute the missing screw and take vicecrips to the stud and adjust from there. The latter example being a joke.