Trying to locate correct thread-tap.

Ammunition, reloading, shooting, etc
User avatar
butlersrangers
Posts: 6876
Joined: Wed Oct 07, 2009 11:35 pm
Location: Below the Bridge, Michigan

Re: Trying to locate correct thread-tap.

Post by butlersrangers » Mon Dec 28, 2020 8:22 pm

Fred - That is a model 1901 Krag carbine rear-sight that someone put on your rifle, at some point of time.

It is an 'incorrect' replacement on a U.S. Krag rifle, because it would be incorrectly sighted.

The rifle version is graduated to "20", for 2,000 yards, and has a built in correction for bullet drift. The rifle 1901 sight uses a .413" blade. The carbine blade is shorter.

(The carbine barrels did not show the same degree of "rotational bullet-drift", as the rifles. The carbine sight-leaf does not shift the slide's eye-piece to the left, as range is increased).

The 1901 carbine sight is worth about 3 to 4 times the value of a rifle sight, in comparable condition.
(I would estimate your carbine sight to be worth about $160, because condition is a tad rough)

You have the correct screws for the Krag 1901 sight. Rifles and Carbines use the same sight screws.

FredC
Posts: 882
Joined: Fri May 31, 2013 4:38 pm
Location: Dewees Texas

Re: Trying to locate correct thread-tap.

Post by FredC » Mon Dec 28, 2020 10:26 pm

BR, You burst my bubble again, not worth all that and change. The close ups were rather extreme in person it actually looks better, so maybe $161.00?

Trying to understand this, the best I can figure is the rifle was stored at my other grandfather's house while my dad was in Korea. This is the grandfather that my 35 Krag came from. It was a faux carbine when I got it, maybe a 24 inch barrel. If my grandfather cleaned them both at the same time maybe he mixed them up. I can think of no other possibility. If this is the case it means I traded the sight that belongs on my dad's rifle to Dan Lowry for the trigger guard. Nuts!

Back on track, the radii on the threads measure .008 plus and minus a little, also the thread flanks are 60 degree. I will measure the pitch diameter and make a detailed drawing. I know I measured the pitch diameter and it was almost exactly the same as 8-32. Pitch diameter is the theoretical measurement of the distance between the opposite 60 degree angles, measure with thread wires or a special micrometer.
The tap or thread former should have a couple thousanths larger Pitch Diameter as well as OD, with a couple thousanths smaller radius on the peaks. That way the screw will have a little clearance when threaded in.
Spamhaus got me again. Will have to unplug the router and get another IP address.

GBertolet
Posts: 21
Joined: Fri Aug 09, 2013 4:01 pm
Location: Pennsylvania

Re: Trying to locate correct thread-tap.

Post by GBertolet » Tue Dec 29, 2020 6:42 pm


User avatar
butlersrangers
Posts: 6876
Joined: Wed Oct 07, 2009 11:35 pm
Location: Below the Bridge, Michigan

Re: Trying to locate correct thread-tap.

Post by butlersrangers » Tue Dec 29, 2020 8:22 pm

Thanks, 'GBertolet' for the information !

I've just ordered three 8-30TPI taps (@ $9.50, each) from Victor Machining.
Unfortunately, the one-inch 8-30 Die is out of stock; I tried to order the Die, (normally $11.50), too.

Minimum order they accept is $25, before shipping costs.

Victor Machining is located in Warren, Michigan, about 45 to 60 minutes from me. :lol:

FredC
Posts: 882
Joined: Fri May 31, 2013 4:38 pm
Location: Dewees Texas

Re: Trying to locate correct thread-tap.

Post by FredC » Wed Dec 30, 2020 9:13 pm

BR, before you tap and possibly ruin your dovetail piece, do drill and tap a piece of scrap to make sure things work out. I can see 2 potential problems both would make the screws fit too tight. One is the new Unified thread form will have a smaller PD (pitch diameter) that the old Krag screw form. The other is the root of the screw may run into the pointy internal crest of a properly drilled Unified hole, the cure is to drill a slightly larger hole than the math would call for.
For your application these taps could work fine with no problems. Some one else trying to clean up threads in a barrel will have 2 problems, Krag barrels are tapped with a bottoming tap to get full threads to near the bottom. The tap in the Victor photo is standard tapered and would barely start in a Krag barrel. Grinding off almost all the taper could get you a tap that resembles a bottoming tap. Hand grinding clearance is possible to help the tap cut.
Not being careful when tapping a barrel could ruin the barrel with pressure from the bore blowing out the sight screw.
Be careful, think and be smart.

User avatar
butlersrangers
Posts: 6876
Joined: Wed Oct 07, 2009 11:35 pm
Location: Below the Bridge, Michigan

Re: Trying to locate correct thread-tap.

Post by butlersrangers » Wed Dec 30, 2020 11:46 pm

Thanks, Fred. I am not rushing the process and will follow your suggestions. Experimenting with hole sizes and thread engagement makes sense.

I plan to send one of the 8-30 taps to you. (I don't need three).

Coincidentally, I was thinking about grinding the tip of one tap, to improvise a 'bottoming tap'.

Your 'tool' idea of a hardened (drill-rod) 8-30 screw of proper Form, is still the best idea to approximate original threads.

FredC
Posts: 882
Joined: Fri May 31, 2013 4:38 pm
Location: Dewees Texas

Re: Trying to locate correct thread-tap.

Post by FredC » Thu Dec 31, 2020 2:09 am

Still planning on that this week. Have made a number of measurements already. Need to make a drawing to put the information together. I will look at the specs for Unified threads. By using the formulas you can make a proper screw of any reasonable threads per inch. With the formulas the correct tap drill size can be determined. One could also just interpolate from similar threads.
These Krag threads do not have much contact on the 60 degree angles compared to the Unified threads.

FredC
Posts: 882
Joined: Fri May 31, 2013 4:38 pm
Location: Dewees Texas

Re: Trying to locate correct thread-tap.

Post by FredC » Fri Jan 08, 2021 5:25 pm

Doing this in stages so i do not loose all the typing if Spamhaus gets me.
The screen shot of the Krag screw drawing shows how I arrived at the final size for the screws on hand and the thread former. Left hand side is an 8-32 screw from Machinery's Handbook. I did the math on the 8-32 to make sure I was understanding all the details correctly. Math worked out exactly. The krag side shows the details of the finished thread on the far right. Pointy threads were for measurements and details. Both screws on hand measured the same so I have no clue what the armories tolerance range is. These 2 screws were relatively tight in the barrels on hand so I am going with the idea they are near maximum.

Pitch diameter as measured with .024 wires is .14197 Drawing .1419 (I used the measurement over the .024 wires as .185)
major diameter (OD) as measured with mike .158 drawing using .006 radius .1587
Minor diameter as measured with the comparator .129 drawing using .008 radius .129

Earlier I said the threads have an approximate .008 radius in the root and crests. That is hard to measure exactly with the comparator. Diameters are easier and more exact. So the crest of the screw measured has an .006 radius and the root (Minor diameter) has a .006 radius.
The 2 formers pictured are a starter and finisher. Starter is about the same as my screws and the finisher is about .0015 larger to make clearance for a plus size screw. They formers are in the oven for final tempering and all looks good. I will test them on an 8-32 hole in brass then in steel. hopefully they would be capable of rearranging the treads in a Criterion barrel.
Attachments
KragScrewSreenShotRS.jpg
KragScrewSreenShotRS.jpg (454.6 KiB) Viewed 67 times
L&GThreadFormersRS.jpg
L&GThreadFormersRS.jpg (302.36 KiB) Viewed 67 times

User avatar
butlersrangers
Posts: 6876
Joined: Wed Oct 07, 2009 11:35 pm
Location: Below the Bridge, Michigan

Re: Trying to locate correct thread-tap.

Post by butlersrangers » Fri Jan 08, 2021 7:14 pm

That is awesome looking work, Fred!

I did receive the (made in China) 8-30-t.p.i. Taps, from Victor Machinery Exchange, that "GBertolet" located.

These Taps will cut a useable 8-30 thread in a 'through-hole', but, they are not Bottoming Taps.

They would not give many threads in a 'blind-hole' in a barrel or reform the threads in Criterion-barrel rear-sight holes (8-32) to accept original 8-30 screws.
Attachments
a new Chinese 8-30 t.p.i. tap.jpg
a new Chinese 8-30 t.p.i. tap.jpg (111.03 KiB) Viewed 61 times
a new Chinese 8-30 tap.jpg
a new Chinese 8-30 tap.jpg (119.15 KiB) Viewed 61 times

User avatar
Parashooter
Posts: 571
Joined: Thu Feb 04, 2010 5:14 am
Location: Kragmudgeon House, CT

Re: Trying to locate correct thread-tap.

Post by Parashooter » Fri Jan 08, 2021 8:39 pm

A few minutes grinder work will convert it to a bottoming tap. Decent video demonstration at https://www.youtube.com/watch?v=ngEancc8XJw

Post Reply