You can take them out with a pair of hemostats and cut a screwdriver slot in them and re-install. Problem solved. You can also flatten one side of a piece of brake line to make a D tool. As far as the thread, I never measured it. You can also get thumb adjustable mixture screws
here. 
If you were looking top repair the threads due to a boo boo can't help you at the moment I would have to go out and find the extra screws I have and measure the threads.
