For a screw in a tapped hole you will need a minimum of three full threads to get the full strength of the screw. Also fine pitch threads are stronger than course threads. So figure the thickness of the aluminum and the size screw and number of threads per inch and see if you get three full threads. If not don't use a screw. It is also standard practive to use a helicoil insert in aluminum holes for screws that need to be removed along with the zinc chromate primer I noted above. The helical coil insert stays in the hole and the screw goes in and out of the insert.
When figuring how many full threads will be engaged make sure you allow for the imperfect threads at the start and finish of the hole. You need three full 100% circumference threads to get the proper strength.
When figuring how many full threads will be engaged make sure you allow for the imperfect threads at the start and finish of the hole. You need three full 100% circumference threads to get the proper strength.