Drilling/tapping base holes
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  1. #1
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    Default Drilling/tapping base holes

    I recently picked up a T/C Contender Super 14 barrel in .30-30. These barrels use 4x 6-48 tapped holes in the rear for a scope base, and when this one arrived, two of the holes were bare and two had filler screws in place. This should have been a hint. The first filler screw was a bear to remove, even after soaking with penetrant and liberal amounts of heat. The rearmost filler screw ended up breaking off below the surface, and no method I had available would extract. I was able to get the majority of the screw drilled out, with the only remaining portion being the pieces still wedged in the threads themselves. I tried chasing the threads with a bottoming tap, to no avail. The tap will thread maybe 1-1.5 complete threads, except it binds enough that tap breakage is a concern. I've tried backing out and running forward a couple times, it still binds at roughly the same position.

    I think at this point the only solution is to drill/retap to the next larger size, which would be 8-40. I've never drilled/tapped anything this small or this precise, so I am just a little intimidated. My main concern is ensuring everything is lined up correctly, which shouldn't be an issue as I currently have the hole to guide me and the flat portion off the barrel boss/hinge to serve as a flat base to locate in the mill vise.

    Any tips or suggestions on how to move forward? Do I even need to use the mill, or could this just as easily be accomplished with a drill press?





  2. #2
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    Don't panic. This is a common problem. My motto is, "I can fix any thing that is broken. Unless someone has already tried to fix it."
    Consider the Brownells oversized screws.
    6X48 Oversize at Brownells

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    That's an interesting idea, but not sure it would work with these threads as they don't go all the way through...unless you modified the tap and converted it to a bottoming style.

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    Anyone else see the distortion in the reflections around the tapped hoes? Makes me think something else is amiss. If something real ugly happened to those holes and raised the metal around them unaltered bases will not fit well and will have a tendency to rock.

    I would check with a good straight edge and try some Prussian blue on a new factory base to figure it out.

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    The other holes seem to be fine, and have no issues locking the base in place when torqued to spec.

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    Mill. And an endmill, not a drill.

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    Consider a one pc base that is like a pic rail ad 2 more holes in it so you have a total of 6 screws. See TSOB scope mount for reference.

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    STOP. Get a loupe or other suitable magnifier and look in those holes for the tops of threads. They should be visible somewhere by now. If so, pick up the hole and move a few thou, like .003 toward the side where they are not visible. you should have them visible all the way around with very little movement. If they are not loose then suspect a threadlocker and apply heat. Use a sharp pick to remove the remains of the screw threads. This willl require a lot of patience so take your time. You should be able to remove the remains of the male threads this way.

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    Quote Originally Posted by wesg View Post
    Mill. And an endmill, not a drill.
    This. You can peck it out with a 3/32 or 7/64 center cutting end mill. Use the knee, not the quill. After that the hole should clean up with a bottom tap. Wont hurt to hit it with a mini torch also to break down any thread locker. Push come to shove the oversize Brownells stuff is handy to save your hide, and they have both taper and bottoming taps as well as fillister and Weaver oval screws.

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    Quote Originally Posted by FredC View Post
    Anyone else see the distortion in the reflections around the tapped hoes? Makes me think something else is amiss. If something real ugly happened to those holes and raised the metal around them unaltered bases will not fit well and will have a tendency to rock.

    I would check with a good straight edge and try some Prussian blue on a new factory base to figure it out.
    I was just thinking that was a crappy polishing job. At one point in time Thompson barrels weren't made very well. I heard some were reamed with a drill press and had a pretty bad bore to chamber misalignment.

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    It was, indeed a crappy polishing job. Too much pressure or dwell time in that area. Filler screws in the holes would have lessened the damage.

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    Thanks for the suggestions.

    I've tried heat, more heat, and then a little more heat (propane torch), so I feel pretty certain any thread locker is gone. I still haven't been able to clean up the hole with a #6 tap, so the plan is to go ahead and bump up to #8-40. I'll likely go ahead and order a 9/64 carbide 4-flute end mill, and then tap. In this situation, would I be better off tapping with a plug or bottom tap?

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    Quote Originally Posted by Forestgnome View Post
    I was just thinking that was a crappy polishing job. At one point in time Thompson barrels weren't made very well. I heard some were reamed with a drill press and had a pretty bad bore to chamber misalignment.
    you got that right . when someone ask me who dose a nice re blue job ? i reply good luck as for the most part no one ! as its hard to duplicate what the factory did if they did it right in the first place . ok so now back to the topic if there are some threads in the hole the over size screw may work with some 680 loctite and if that won't do then there's 8-40 [but why]off set and re drill the holes in the barrel or weld up the holes a re drill [ good luck ] the ez ist way would be the over sized screw and a good loctite but whatever way you go good luck and i hope it works out for you

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    You may start the threads with a plug tap but you will have to finish with a bottom tap. If the plug tap has a male center on the end you may have to grind the end back, just shy of the beginning of the thread.

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    Makes sense.

    Should I go with HSS or Hi-carbon taps? I've read that the benefit of the hi-carbon taps is they can be easily shattered if they break off in a hole...but how long do they last? I found a 3 tap set of Viking Norseman that includes a taper, plug, and bottom tap, except its an import (China). I may end up doing at least one other barrel, so the possibility of having to tap 4-8x more holes is there. Will a hi-carbon tap hold up?

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    but how long do they last?

    Carbon taps are cheap; I treat them as consumables, and use a new one on every job. It's cost is figured in my job charge.

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    That's sort of the question, how long does a "job" normally last...at least with regards to high carbon taps (and proper lubricant)? 4-6 holes? 8 holes?
    Last edited by Jason280; 09-14-2020 at 02:09 AM.

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    A typical job is 4 holes. The tap is not worthless at that point but it is not worth the chance of breaking a dull one in a hole to save the used tap.

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    I would try drilling the screws out with a left-hand drill. I see no reason to believe the original threads will not be okay


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