My guess, knowing the application would be 6-32 or 8-32 with 10-32 as an outside chance. 10-24 is another possibility as the coarser thread is a better choice in aluminum if the mould is aluminum.
If you have calipers, measure the ID of the hole. That will give you the tap drill size. Now, look at tap drills applications and that should give you the screw size. My drill index has all the standard tap and body drill sizes by screw thread size stamped into the first row of the index.