OK. I've done some research on this, and according to my copy of Southerland's Handbook for Bicycle Mechanics, Third Edition, page 15-2, the threading for "mud guard eyes" on fork ends (dropouts) is 5mm x 0.8mm for Campagnolo, Shimano and Suntour brand dropouts. So, based on that, Miamijim et al are correct in stating that the correct screw size is M5x0.8 (there is also M5x0.9). Now, why I can't insert my M5x0.8 tap into the threaded eyelets in my Bridgestone with Suntour dropouts (the threads are free of paint and rust) I do not know, and I have used #10-32 screws for years without issue for water bottle cages and fender mounts. Therefore, I would suggest that you buy both types of screws, and if the larger M5 screws can be installed without binding, then use them, otherwise use the #10-32 screws.