I recently acquired a 1950 Win 70 in 375 H&H that was a very late serial number to have not been factory Drilled and Tapped. It had one hole not through drilled in the rear bridge in the proper location to match a Redfield JR70-Mag base. My friend, an accomplished, longtime gunsmith through drilled that hole and drilled the second hole .435" back from the first hole. He tapped them almost all the way through, but not quite. He said it was a super hard receiver and that his drill bit was carbide but his tap would just not bust through the entire depths of the holes.

I tried my Hanson 6-48 Plug tap. I did not dare force it. I did not get through either. I got it to protrude about 1/16". I am thinking that a quality Taper Tap may get the job done. I am leary of HSS Taps of this size as if they break they are a bear to get removed. Carbon taps are what is recommended but some let go so easily with no warning. I use about a 3 inch long Starrett Tap Handle.

What advice, brand, style of Tap, etc do the knowledgeable have to offer?

Sounds like you need a competent gunsmith more than a tap....

rear of the action you said?

This is a FARMER fix, not recommended to be used by anyone


get a bolt that fits, heat it till just not melting, screw it in as far as it can go, leave it there an hour .. repeat 2 to 3 times

run you oiled tap in and see if that "fixes" it

My competent gunsmith has built guns that have won many National Championships and has been featured in books and magazines since the 1960's. I don't need to find a competent gunsmith.

I will give the heated bolt a try. There are plenty of threads cut into the rear bridge. It is probably lacking the last two. If I can't get it tapped through it will not be the end of the world.

How about some Tap recommendations? HSS, Carbon, Tapered, 2 Flute, 3 Flute, Etc.

When I was doing quite a bit of scope mounting I found the Ruger Mini 14's were bitch hard and likely to easy break taps if not careful. I purchased a set of B Square drills and 3 flute taps covering the various scope base holding screw sizes. The lube I found best was CRC 5-56 which is similar to WD40 (I never tried WD40). Generally got a feel for when the tap was starting to bind up, reversed and gave another spray of 5-56 before proceeding again. Didn't have any more breakages with judicious use of the B Square taps and 5-56.

Why don't you run 6-48 screws in each hole and see how many threads they catch before stopping. If it is three or more, just adjust your base screws length so the base is held tight without the screw bottoming out. No sense risking breaking taps if you already have enough threads to hold the base.

For many years (40+) I have used OSG taps, they seem to me to cut better than anything else I have used although once I started buying them I quit trying anything else.

Leave it alone. There is no problem to solve here.
The problem statement is: How do I mount sights/scope/etc on this rifle?
Not, how do I tap a hole.
But I get it; I want holes tapped all the way through too.
But here is a better question; How many threads does it take to hold a scope base on?
Answer; not many.

Hard to find quality 6-48 taps. I just threw half a dozen 8-40s away (Hanson). Would never have ordered them, think they came unbranded from Brownells. The best option I've found lately is the R&N brand from Brownells, and it appears they are phasing them out. As they are with most gunsmith tools....If anybody has suggestions please speak up. Oh, and definitely use HSS, carbon just won't be up to the task for the hard ones.

And M-70s can be a bitch! The method given for heating the rear bridge with a hot bolt is a good idea. I would add some insulation of some kind as well.

Yeah I was thinking of spot annealing like I used to do to M1903 and 03-A3 Springfields, but hate to do that to a nice model 70 original finish, plus the rear bridge is a lot thicker on a Model 70. The hot bolt may do the trick.

It isn't like my friend isn't capable, it was a quick favor. I drove down he finished drilling the first hole, which had also been tapped, drilled the second hole and then tried to tap the holes with the tap on hand. He is long retired from active gunsmithing for the public. It is not like I sent him the barreled action and he had to do it.

As I said earlier it won't be the end of the world if it stays as it is; not worth the hassle of a broken tap. It has plenty of threads in that thick rear bridge. We ran bottom taps as far as they would go, which is plenty.

The R&F or R&N, or whatever at Brownells have reviews all over the place - China made, Hanson marked, etc?

I have a multitude of bases for the gun. I don't have rings for them yet. I am running my old Redfield JR-70A Mag and medium rings. I have an extended Leupold VX-III 1.75-6 Heavy Duplex on the gun. I have options now.

Sorry you are correct it is R&N. I do have taps marked as such and they cut better.....

mcmaster offers a TiN coated "chip clearing" 6x48 - i have no idea who makes them, and don't have a recommendation for them.. but DANG 24 bucks for a tap?

https://www.mcmaster.com/produ...ps/thread-size~6-48/

Anyone have experience with these type of spiral cobalt taps in very hard gun steel?

https://encrypted-tbn3.gstatic...WDA-9FWnPNoflRfEb52Q

I have been using these lately https://www.ebay.com/itm/126036253006 and have no complaints with either their 6-48s or 8-40s. As for the taps like Bobster just posted, I have not used any in the wire gauge sizes, but we bought a package of 1/4-20 made by a name brand and that small diameter neck hasn't worked out very well, even in 1018.

Funny you bring this up. I am drilling/tapping a Remington made 1903A3 to put a Lyman peep sight on the rear like on a classic sporter. It's a recovered drill rifle receiver so not concerned with "ruining" the receiver.

The back hole drilled and tapped fine. The front hole drilled, but I'm on my second tap. First one bought from Midway, second from Brownells - like you said jeffeosso, 24 bucks!

Still haven't gotten it to tap all the way through. Super, super hard. Been considering spot heating.

just a thought, but since i don't trust no body -- you might, just holding in your fingers, try the proper drill bit and see if it's actually a through hole .. grasping for straws, but i'd try it, might first try to put shank end in first, to not bugger up the threads

Years ago a friend of mine who works at Earnhart/Childress racing engines came up with some 6-48 powdered metal taps that were head and shoulders above anything else I've ever used. They would cut through a 1903 receiver like it was butter. They were expensive, like $25 each, but I'd buy them again in a heartbeat. You might give these people a call and see if they have anything https://taylortool.com/high-pe...powdered-metal-taps/ but 6-48 and 8-40 taps are hard to find.

I use Cobalt drills and Tin Coated taps on Model 70’s and $24 a tap is a lot cheaper than correcting a train wreck

A carbide threadmill cares very little how hard your action is.

Taps are so 1890's.

Neer saw so much to do over a blooming tapped hole.

Many years ago I was taught to just find a scrap of steel about 3/4" inch square, heat it up cherry red and place it on top of the spot you want to anneal.


Leave it there until everything is cool..drill and tap!

Have no idea how many times I've done this without drama

quote:

Originally posted by Duane Wiebe (CG&R):
Neer saw so much to do over a blooming tapped hole.

Many years ago I was taught to just find a scrap of steel about 3/4" inch square, heat it up cherry red and place it on top of the spot you want to anneal.


Leave it there until everything is cool..drill and tap!

Have no idea how many times I've done this without drama

This is a great technique!

quote:

Originally posted by Duane Wiebe (CG&R):
Neer saw so much to do over a blooming tapped hole.

Many years ago I was taught to just find a scrap of steel about 3/4" inch square, heat it up cherry red and place it on top of the spot you want to anneal.



Leave it there until everything is cool..drill and tap!

Have no idea how many times I've done this without drama

That is how I used to do the M1903 and 03-A3 Springfields except I used to use a 3/8" bolt.

DW is spot on!
I still have not understood the actual problem; this is pontification and useless discussion over something I would have solved on minute two.

quote:

Originally posted by dpcd:
DW is spot on!
I still have not understood the actual problem; this is pontification and useless discussion over something I would have solved on minute two.

water cooler chat is fun ...

Besides Tom, i don't think you or DW were born knowing all the things, right?

Use Tap-Magic and HSS taps.

I have not broken a tap since I started using bear grease for tap lube.

Good job Jeff; you caught me again.

quote:

Originally posted by dpcd:
Good job Jeff; you caught me again.

But I do still like you and will always respect you

quote:

Originally posted by SDH:
I have not broken a tap since I started using bear grease for tap lube.

Interesting. I wonder how natural lanolin would work?

quote:

Originally posted by SDH:
I have not broken a tap since I started using bear grease for tap lube.

I was always told by old timers that bacon grease (specifically, bacon oil, the part that remains liquid if you put it out in a dish on the counter to cool) is the best cutting fluid. I tried it for awhile, threading on the lathe, but eventually went back to using high sulfur tapping fluid; too hard to source the bacon oil in enough quantity, and the tap fluid is easier to store and use. The bacon oil did work well, however.

quote:

Originally posted by ssdave:
[QUOTE]Originally posted by SDH:
I have not broken a tap since I started using bear grease for tap lube.

I was always told by old timers that bacon grease (specifically, bacon oil, the part that remains liquid if you put it out in a dish on the counter to cool) is the best cutting fluid. I tried it for awhile, threading on the lathe, but eventually went back to using high sulfur tapping fluid; too hard to source the bacon oil in enough quantity, and the tap fluid is easier to store and use. The bacon oil did work well, however.[/QUOTE

Even more difficult to source bear fat for grease but I used to have a houndsman bear hunter friend in Western Oregon before they outlawed hounds. I'm still using the 8 oz bottle I rendered more than 30 years ago. A little goes a long way and tapping isn't exactly a daily process. It does smell a bit gamey though...

As I understood the OP's dilemma, what do you do when you need to D&T a receiver where you don't want to damage the finish? So, spot heating is out. What's left are super hard drills and taps. So, what's best?

Have used the Brownell's R&N 2 flute taps with the ground threads for years
Why 2 flute, a lot less flex than a 3 flute tap

Had to open up a newer Win M70 receiver for 8x40 screws a few months ago, DANG those are tough, I could tap one hole, then shorten the tap for the next hole
I used one tap up on 4 holes.

Just a few weeks ago opened up the holes in an older Rem M700 receiver, new tap would go about half way in and Squeek, OK back it out and then use a reground Bottoming tap and get the last 3 threads thru the hole

Years ago ( now about 35 - 45 years ) we used to take Carbon, yes Carbon taps, and heat them up to red and Quench them in Mercury
Don't dare do that in todays world.
The taps had been used and shortened to only about 6 - 8 threads, so nice and stiff
The tap would come out hard as Glass.
Mind you this was a LAST DITCH effort on those super hard receivers

Bear or Bacon Grease, yes have done that as well, on Stainless it was needed more than anything else

My Grandparents saved all their Bacon Grease, I still have about 3 gallons of it, that I heated up and strained all the little bits out of to make it cleaner and put it in a good plastic container

Had a GOOD laugh years ago when I had a Labor and Industries Inspection, I asked about help getting a MSDS
I told him the Oringial maker had not put anything down in writing in Centuries.
The Primary Maker of this grease was dead.
So I needed help getting a MSDS sheet
He had no CLUE on how to help me, I then opened up the container, and took a bit of the grease on my finger and ate it.
OMG he just about CRAPPED his pants, even after I told him it was only Bacon Grease he kept looking over his shoulder at me until he left, LOL that was the LAST Inspection I ever got

I don't use Bacon Grease much anymore as the machine clean up was many times to much

Jim Wisner

Mercury! There's still underwater "pools" in the Americcan River from the gold rush. Years ago, I was given about a pint of mercury and thought I'd won the jackpot.


Soon enough noticed the level was dropping even in a tightly sealer jar...Where's it going?.


Tried to give it away..no takers! In fact, nobody wnted anything to do with it, Finally found a chemical supply outfit in Berkeley CA that took it off my hands...Might be the reason for all the nut cases in Berkeley?

PS...wouldn't lard work? pork fat is pork fat..and comes pre packaged..

OP,

If you get to where you're stuck on this, I think I can help. I gave up on drilling/tapping base/guard screw holes in actions a long time ago.

Instead, I ID bore the hole with an endmill and follow up with thread milling. This way it avoids broken taps and I can control everything much better.

The only issue here is your stuff is already 6-48. Trying to pick that up with a thread mill would be a nightmare. The base holes would have to be increased to 8-40.

Good luck.

Thanks to all for the replies. It was a good thread in that it gave a lot of techniques; some new to us and some reminders of the forgotten past.

I got both holes tapped all the way through. Approx 10-12 threads each. The new hole was tapped 6 threads and the older hole was tapped 8 threads.



I used a HSS 3 Flute taper tap and my probably 15-20 year old home grown 3 Flute Hanson bottom tap back and forth, cleaning the taps and threads each switch, while using lots of Tap Magic cutting oil. That "bottom" tap is a good one! It actually did cut the final thread on the second hole and probably would have on the first (new) hole.

I then switched to another tap new from Brownell's, on the new hole, it was a 3 spiral flute carbon taper tap marked Hanson 6-48NS USA. That tap cut through like butter. I think it was the R&N branded tap, but not sure.

Drill the hole slightly oversize.
One loses very little strength (or perhaps it would be better to say that one can go surprisingly large before the strength of the joint is reduced) and it massively reduces the amount of torque required to drive the tap through.

Peter Connan, no offense but that is the worse advice I have heard on this. Specs are specs usually for a darn good reason. If I didn't want it done right I would have just settled for the almost through holes, or tightened the base with one screw and drilling the other hole with a drill press or even hand drill. I may be a little OCD at times.

quote:

Originally posted by airgun1:
Peter Connan, no offense but that is the worse advice I have heard on this. Specs are specs usually for a darn good reason. If I didn't want it done right I would have just settled for the almost through holes, or tightened the base with one screw and drilling the other hole with a drill press or even hand drill. I may be a little OCD at times.

There are published specs for it. Not common, and it needs some searching, but I have found them for metric threads.
It's pretty common practice in the tool and die industry...

Oh, don't take my word for it: https://www.brownesales.com/ne...-what-does-it-matter