I just assembled my first upper.  Everything went great except the front sight post installation.  The threads are too tight and I cant seem to get it to work in all the way.  Anyone know what the thread measurements are for it so i can run a tap through it?   thanks

Where did you buy the upper?  Do you know if the sight assembly is forged or cast.  I'm not sure of the exact  
dimension of the threaded post but would avoid running a tap through.  Try screwing the post in without the spring and detent first to be sure that it will line up and thread correctly.

If that doesn't work grab a .22 bore brush and run it through the front sight base.  There could be a little bur or something not allowing it to thread.