I have been doing some careful measuring of the krag screws this is what I have found up to now .. The screw pitch it 26 threads per inch witch is a British standard for tubing and bicycles the thread diameter is .180 that doesn't match anything British american or inerpalnitary If you know where this particular diameter originated  I would like to know ... the thread high from the root to the crown is .018 witch translate to a pitch diameter of  .5 this means the threads are short and square on top ... this was done to save ware on the taps and speed up production it is only used on threads where high tensile loading will not be expected so who came up with the bolt specification ??? 

---

Never mind my teenager came up with the answer I asked who would make a screw like this and in a blank monotone voice they said    " NASA "

This question has been asked before. I asked it, quite a number of years ago. I don't believe there is a simple answer.

I know the thread size, profile, and pitch are shared on some 1903 Springfield, U.S. Krag, and 'trap-door' Springfield screws.

I would not be surprised if some of our arsenal dies copied the components of French Arms that our Federal Arsenals first copied, in the early days of our Country.

Years ago, '5MadFarmers' shared a list of the contents of a 1902 - Standard Frankford Arsenal tap and die set. They called them "United States Form". The set had taps & dies for "U.S.F. Special" and "W.&S. Standard" threads.

The items that caught my eye, were: .187" X 26 tpi ( which is probably Krag side-plate and barrel-band screws) and .156" X 30 tpi (which is probably 1873 Springfield and Krag rear-sight screws).

Carbon Outlaw,
  The .180 are you are measuring is probably close to right for a 3/16 screw. (today's #10 screw is more correctly .189) The major diameter you can measure with a mike or calipers is always smaller than the nominal screw size.
Back in the day when they were using the rounded crests and valleys on those screws. The taps and dies were made from tool steel, very much inferior to todays high speed steels. Sharper peaks on the tap would have been more prone to breaking off and sharpening with a hone would have been more difficult.
The plus side of a screw with the large radii in the threads is they will be less likely to fracture, the down side is there was less bearing surface so in use they would wear faster.
The Unified system of screws was instituted in 1949 and used here till present. There was an American National system the came after the Krags and Springfields that predated the Unified system, my reference book did not give any dates for it.
Changes and improvements were coming fast and furious when these guns were produced.
  Making a replacement screw for any of these old timers is relatively easy if one has a lathe,  bench grinder, and a good drawing. I think the company that makes the scope makes for Krags also stocks screws.

Fred meant the company that makes the scope mounts, Kraghaus, in his last sentence.

thank you FredC I am getting a lot of good information With the peeks and valleys being rounded it more closely matches the British standard for fine thread and I did look at Kraghaus and it dose seem to be a good source for sight screws all this good information helps thanks ....

FWIW - S&S Firearms, Glendale, N.Y., has reproduction Krag and Springfield Screws.

I have purchased reproduction 1903 Springfield sling-swivel screws on ebay that have worked well for Krags.

I just took out the Redfield peep sight screw and need the side plate screw to replace it. Krag 1898 Carbine . Sight screw is too long. Thanks, Charles

My pet theory, touched on by Chuck (B/R) is that 26tpi is a Yankee-ization of what was probably a (nominal) 1mm pitch on the French arms, which we basically copied verbatim when setting up our "standards" in the late 1790s.

Wouldn't have taken much, given the non-interchangeable (pre-John Hall) standards, and measuring capabilities of the time to have either misread the dimension, or simply decided that an even 26 was "close enough" to 25.4 to work just fine for our new patterns.

Anyhow, that's my story (for which I charge nothing) and I'm sticken' to it until definite proof of another source is shown. :)

RCW wrote on Aug 13^th, 2017 at 7:19pm:

I just took out the Redfield peep sight screw and need the side plate screw to replace it. Krag 1898 Carbine . Sight screw is too long. Thanks, Charles

I would gladly trade a side plate screw for the Redfield version as I have a sporter and the sight but no screw. Just let me know! Thanks.

RCW, sent you a PM.