There were a variety of different screws used on the various sights mounted on U.S. Krag rifles from the Model 1892 through the Model 1901. It is difficult to find information on the exact thread size and pitch used on the original rifles, but we have seen it reported as 8-26 and an M4 (diameter .159”) with a 1.0 mm pitch.
Screws sized in 8-26 screws are obsolete, and therefore difficult to find. M4 screws with a 1.0 mm pitch (as distinct from 0.7 mm pitch) are even more rare. We hope someday to make our own screws, but that day has not yet come.
In the meantime, people have used screws salvaged from one sight or rifle to attach another:
“It is not unusual to see incorrect screws used to attach rear sight to [Krag] rifles and carbines. The differences were so small in most cases, with the exception of the Model 1901 rear sight front screw, that they could be used interchangeably by armorers, hunters and collectors alike.”
Poyer, The American Krag Rifle and Carbine, 2ND Ed. Revised, at p. 175.
Although interchanging original Krag sight screws works out, many people have attempted to fit “sporterized” rifles with screws from another old rifle that looked right, but were not original Krag. Using screws that were “close” often forced the threads. Although the substitution would initially appear to have worked, damage to the threads would nevertheless result, and the next time the “almost right” screws were removed and replaced they would not hold. This is one reason so many U.S. Krag rifles are found to have thread damage in the rear sight holes.
If, your rifle has been drilled out and re-tapped, the Aluminum Kraghaüs rails can easily be modified to fit whatever screws you happen to have. It will also work with factory screws.
If the threads in your barrel have been stripped, Krag screws will simply no longer work. In that case, we recommend having a gunsmith bore out screw holes and re-tap them to accommodate modern screws. We believe this will save a lot of time and trouble in the long run. (Of course, one should not do this on an all-original, “collectable” arm, as it will diminish the collector value of the rifle.)
If you find this to be a little technical, show it to your gunsmith. He will know what we are talking about.