Questions re: drilling and tapping a Yugo SKS

[Matt Dillon]

Folks, I have finally purchased a Choate scope mount, after trying several different type of receiver cover mounts(and wasting a ton of good ammo in vain).
This is the first gun that I will attempt to drill and tap myself (I have paid a gunsmith in the past, but the budget won't allow it these days).
I have purchased the drill and taps from MidwayUSA, and had a couple of questions while I wait for the Choate mount to come in.
I was wondering, how do you accurately mark the holes in order to center punch them? I have read the article from surplusrifle.com, and he reccomends clamping the choate mount to the rifle while drilling the holes. I was wondering, how do you accurately center punch the starting hole through the choate mount? Is there some type of bushing to use, to get the center punch centered in the scope mount holes?
After I successfully drill the holes, should I place the tap into the drill press chuck (Delta Radial drill press), in order to start the tap square to the receiver? If I do that, how do I turn the tap to start it in the hole?
How hard is the SKS receiver? I will be mounting the Choate mount on to a 59/66 Yugoslavian SKS.
Any advice and help regarding mounting this scope mount will be greatly appreciated, thanks so much in advance!

 

[asknight]

Q. I was wondering, how do you accurately mark the holes in order to center punch them?
A. You use a receiver jig specifically for this mounting scopes to the SKS. If you don't have this jig, which is in all probability one-off custom made, you will require the services of a gunsmith, or machinist who can make one. Or you can risk it and c-clamp the mount to the receiver.

Q. I was wondering, how do you accurately center punch the starting hole through the choate mount?
A. You don't, you use the reciever jig.

Q. After I successfully drill the holes, should I place the tap into the drill press chuck (Delta Radial drill press), in order to start the tap square to the receiver?
A. Absolutely not, unless you are well versed in broken-tap extraction.

Q. If I do that, how do I turn the tap to start it in the hole?
A. With a T-handle tap-wrench.

Q. How hard is the SKS receiver?
A. 45-65Rc depending on the temperature of the oven on the date of manufacture in Yugoslavia.

From http://www.surplusrifle.com/reviews/choatesksscope/index.asp
Manufacturer's Mounting Tips: We suggest you employ a real gunsmith to install this scope. The threads are standard 8-40 fine. The tap drill bit is #28 (.140 Dia). Every real gunsmith will have these drill bits and taps. We suggest you clamp the mount on the side of the receiver. Make sure you can see the open sights and make sure you can remove the dust cover & bolt. Align the top or bottom of the mount parallel with the bore. After that it is the usual task of spotting, drilling and tapping four holes. We strongly recommend that you use thread locking compound on the screws and also between the mount and the receiver.

 

[bobsmith]

I would have it done by a gunsmith especially since the receiver is in the 45-65RC range. It'll be like trying to drill and tap a knife blade.

 

[brickeyee]

"Q. After I successfully drill the holes, should I place the tap into the drill press chuck (Delta Radial drill press), in order to start the tap square to the receiver?
A. Absolutely not, unless you are well versed in broken-tap extraction."

Actually, this is a common method and works well. You do not power up the drill press, just use it to hold everything perpendicular.
Turn the chuck by hand until the tap has a good start, then loosen the chuck and use a tap wrench to finish.
It is about the same as using a tap holder.
The trick to getting threads in hard receivers is to soften the spot to be threaded. One method is to chuck up a plain old nail with a blunt tip. Run it into the spot until the metal is red hot and then allow to cool. The heated area will now be soft enough to tap.

 

[Powderman]

Actually, this is a common method and works well. You do not power up the drill press, just use it to hold everything perpendicular.

Incidentally, Brownell's sells a tap handle that can be clamped in your drill press for exactly this purpose.

Moreover, don't forget to lube your tap well. Brownell's also sells a tap lube called Do-Drill.

Work slow. Measure 2-3 times, cut once. Make sure your mount is square to the receiver; measure on both horizontal and vertical axes. When you tap, lube first, then go in 1/2 turn at a time. Back the tap out, then start in again. Do NOT use any sideways or lateral force to turn the tap handle--it should go straight in.

 

[Clark]

Here is my Yugo SKS with Choate scope mount drilled and tapped.

I think I put one on a Chineese SKS 10 years ago, before I had a mill, and it was harder to do, becuase I had to drill right on the edge.

The SKS is soft to drill and tap.
I like to put epoxy between the mount and receiver and Lok-Tite on the screw theads.

 

[john kay]

Matt,
A lot of good advice. Here's another 2 cents worth. If that thing tests on the Rockwell C scale at 45 to 65, I would not even attempt it. If I remember, an ordinary file is around 80 to 90 Rc. Annealing it may or may not help because you may even make it harder than it is now, not knowing what type of steel you're working with. Most can be annealed as mentioned with no trouble at all. Putting it on an EDM would be too costly. I approach this problem ONLY from the hardness point of view.
Quote for the day: Happiness is not the station you get off at. It's the train you come in on.

 

[brickeyee]

"Annealing it may or may not help because you may even make it harder than it is now, not knowing what type of steel you're working with."

No type of steel hardens if heated and then allowed to slowly cool.
Annealing is by definition softening metal by heating and cooling.
Friction heating tends to produce a more localized anneal than using a torch, unless you have a very small torch (jewelers type) to do the job.

 

[RyanM]

No type of steel hardens if heated and then allowed to slowly cool.

Apparently you've never heard of air-hardening steels. It's all a matter of how slow "slow" is. Some steels will anneal if allowed to air cool, but some alloys need to be very well insulated to cool slowly enough to anneal. Although, the chances of an old Yugoslavian gun being made out of an alloy like that is very slim.

 

[john kay]

Try this. Bring up to temperature some air hardening tool steel such as the 401's and Vega and let it cool without your "annealing" process. Then put the part on the Rockwell c scale and read what you got.

 

[john kay]

"NO type of steel when heated and allowed to cool hardens." Broad statement. Should I recommend some mettalurgical books on the subject. They can be very enlightening.

 

[dfaugh]

OK, I've successfully drilled and tapped all kinds of "hard" gun parts (including many receivers). I just use GOOD quality drill bit (not the junk you get from Harbor Freight, although I use these, too, for "regular" projects). Use a good, sharp 'high speed steel" bit. AND USE CUTTING OIL (WD-40 will work in a pinch, but "real" cutting oil is better). DO NOT put alot of pressure on the bit, let it cut naturally. Keep area flooded with cutting oil. IF you over heat the bit(w/ too much pressure, or not enough oil), it lose its hardness, and won't work any more.

Chucking the tap in a drill press is a good idea (My father was a master machinist, and taught me to do this) to make sure you start out "square"...BUT, you're gonna turn the spindle BY HAND to get a few threads started. AGAIN USE CUTTING OIL, and very light pressure, as the tap will do the work by itself. After you've got the tap started square, then use a standard tap wrench (again with oil) to finish. When cutting threads, back off ever 1-2 turns to clear the hole(or risk a broken tap, which is a REALLY bad thing). Take your time!
Now, you really don't need to do the drill press thing, mostly, if you're careful, you can start the tap just as well by hand, but I do do it sometimes, just to be sure of my alignment.

 

[john kay]

Dfaugh,
You are perfectly correct in what you say, especially about starting the tap square and putting the vertical quill in nuetral and using it as a pre tapping station. I refer your meaning that some common knowlege and patience is about the ingredients you need to do a job like this. You were lucky. You had a Dad that was a Master machinist. I did not have a luxury as this but had to start from scratch. I earned my degree in Metallugy in 1956 and then was a graduated modelmaker/toolmaker/die maker for 33 years. I also ran my gunsmith shop congruently. The knowledge wasn't too great, but the experience was, and what I have learned I will gladly bestow on anyone that asks for it. I hate to see an in-experienced person start off half cocked, or be taken by an unscrupulous person who owns about three pieces and then he's an authority on guns. Give the same person a dremel and he just graduated to a "Gunsmith." I'm sure you know there's plenty of these people out there and have the good sense to stay way or see through the BS It never is shameful for a person to ask a question, whatever it is, and I will do my best to help anyone that I see can going on the wrong track. Thanx
Quote for the day: Las Vegas was not built on winners. It was built on losers.

 

[brickeyee]

"Broad statement. Should I recommend some mettalurgical books on the subject. They can be very enlightening."

Find a gun made of a steel that will harden when air cooled.
It will be a long search.
Even 'air hardening' only applies to sections 2 inches or more in diameter.
It does not work with smaller pieces ver effectively.
Air hardening is also one of teh softest steels, even after being hardened.
It is very difficult to keep a smaller cross section in martensitic structure with the slow cooling in air.
Cooling with compressed air helps some.

 

[Clark]

I have drilled and tapped 50 ~100 rilfles over the last 10 years.

They were all easy except one 1896 Swede.
Thinking back, maybe the tap was dull.