Krag Rifle
Side plate screw: 26 TPI .186 Diam Size 9(?)+
Drill Bits sized for butt plate screws:
Large screw: 1/4" Small screw: 5/32"
Side Plate screw revisited:
This is a very Odd screw with an old time thread that is difficult to measure accurately.
It might be 26 TPI - But is also close to 24 TPI. It is also close to being size 10. A 10-24 screw will enter the sideplate hole and turn about 3 turns before binding.
The problem with the screw is in the installation of "no gunsmithing Lyman and Redfield receiver peep sights as manufactured many years ago Specifically for Krag rifles. The Original sideplate's screw hole along with a longer replacement screw is used to secure the sight to the side of the receiver thru the sideplate. The original shorter screw is relatively common and easy to obtain, but the longer replacement screw is often not found with the sight when purchased used, as almost all of them are.
I have found that an ordinary 10-24 screw may be made to work by running a 10-24 tap into the side plate hole. After doing this operation both the original screw and the 10-24 screw will screw down tightly. One can see that the threads inside the hole are not as clean cut and continuous as they appeared before the tap was used, but apparently there remains enough of the new thread for either screw to grip.
I have performed this thread/hole conversion on 4 rifles, and all turned out well.
I used a new sharp tap. No clearance drill was necessary. I do not know how well the new screw would continue to hold if the sight was repeatedly removed and reinstalled to permit removal of the the bolt for bore cleaning from the breech end of the receiver.
Better to remove the elevation bar from the sight to permit bolt removal after carefully noting the elevation setting of the bar, locktite is also an option.